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48 / Chapter 2 • Foundations

P i le D r iv i ng

P i le h amm er s are massive weights lifted

by the energy of steam, compressed

air, compressed hydraulic fluid, or a

diesel explosion, then dropped

against a block that isin firm contact

with the top of the pile. Single-acting

hammers fall by gravity alone, while

double-acting hammers are forced

downward by reverse application of

the energy source that l if ts the ham-

mer. The hammer travels on tallverti-

cal rai ls cal led leads (pronounced

" leeds") at the f ront of a piledriver

(Figure 2.41). It isfirst hoisted up the

leads tothe top ofeach pile asdriving

commences , t hen f ollows t he pi le

down as i t penet ra tes the ear th. The

piledriver mechanism includes lifting

machinery to raise each pile into

position before driving.

FIGURE 2.41

Asteam piledriver hammers a precast

concrete pil e into the ground. The pil e i s

supported by the vertical structure

( le ads) of the pil edrive r and driven bya

heavy piston mechanism that follows it

down the leads as itpenetra tes deeper

into the soil. ( G o ur / e, y o f L o n e S t a r/ S a n- V e l

Goncre te )

I n cer ta in t ypes of so il s, p ile s

c an be d riven more e ffi ci en tl y by

vibration than by a heavy hammer,

using a special vibratory hammer

mechanism. When piles must be dri-

ven beneath an exist ing bui lding to

increase the capacity of i ts founda-

t ions , as is o ft en nece ssa ry when

increasing the height of a building,

they are forced into the soil by

hydraulic jacks that push downward

on the pile and upward on the

building.

Pi l e Mat e r i al s

Piles may be made of t imber, s teel ,

suecast or precast concrete, and vari-

ous combinations of these materials

( Fi gu re 2. 42 ). T imber pi le s have

been used since Roman times, at

which time they were driven bylarge

mechanical hammers hoisted bymus-

cle power . Their main advantage is

that they are economical for l ight1y

loaded foundat ions . On the minus

s ide, they cannot be spl iced dur ing

driving and are, therefore, limited inlength to the length of available tree

trunks, approximately 60feet (18 m)

maximum. Unless pressure treated

with a wood pres er va ti ve , o r com-

plete ly submerged below the water

table, they will decay. Relatively small

hammers must be used in driving

timber piles to avoid splitting them.

Capacit ies of t imber piles l ie in the

range of 10 to 35 tons each

(9000-32,000 kg).

Two forms ofsteel piles are used,

H-piles and p i p e p i le s . H-piles are spe-

cial hot-rolled, wide-flange sections, 8

to 14 inches deep (200-360 mm),

that are approximately square in

cross section. They are used mostly in

end bearing applications. H-piles dis-

place relatively little soil during driv-

ing. This minimizes the upward

displacement of adjacent soil, called

heaving, that sometimes occurs when

many piles are driven close together.

Heaving can be a particular problem

on urban sites , where i t can l if t adja-

cent buildings.

H-piles can be brought to the site

in any convenien t lengths , welded

t ogether a s d riv ing progr es se s t o

fo rm any necess ar y l ength o f p ile,

and cut off with an oxyacetylenetorch when the required depth is

reached. The cutof f ends can then be

welded onto other piles to avoid

waste. Corrosion can be a problem in

S T E EL H · PI L E S TE E L P IP E P IL E PRECAST

C O NC R ET E P IL E

WOODP ILE

Foundat io ns / 49

some soils, however, and unlike

closed pipe piles and hollow precast

concr ete pi le s, H-p il es canno t be

inspected after driving tobe sure they

are s traight and undamaged. Allow-

able loads on H-piles run from 30 to

120tons (27,000-110,000 kg).

Steel pipe piles have diameters of

8 t o 16 inches (200-400 mm). They

may be driven with the lower end

ei ther open o r c lo sed wit h a heavy

steel plate. An open pi le is ea si er to

drive than a closed one, but i ts inte-

rior must be cleaned of soil and

in spect ed be fo re be ing f ill ed wit h

concrete, whereas a closed pile can

be inspected and concreted immedi-

ately after driving. Pipe piles are stiff

and can ca rr y heavy loads ( 50- 150

tons, or 45,000-125,000 kg) . They

displace relatively large amounts of

soil during driving, which can lead to

upward heavi ng o f near by soil and

buil di ngs. The l ar ge r s iz es of pi pe

piles require a veryheavyhammer for

driving.

P r ec a st c o nc r et e p il es are square,octagonal , or round in section, and

inlarge sizes often have open cores to

allow inspection (Figures 2.42-2.44).

Most a re pr es tr es sed, but some fo r

FIGURE 2.42

Cross sections of conunon types of piles .

Pre ca st concrete pil es may be square or

round instead of the octagonal cross sec-

t ion shown, and may be hol low in the

larger sizes.




smal le r bu ilding s a re merely rein-

forced. Cross-sectional dimensions

range typ ical ly from 10 to 16 inches

(250-400 mm), and bea ring capac i-

ties from 60 to 120 tons

(55,000-110,000 kg). Advantages of

precast piles include high load capac-

ities, an absence of corrosion or

decay problems, and , in mos t s itua -

tions, a relative economy of cost. Pre-

cas t p iles mus t be handled careful ly

to avoid bending and cracking before

installation. Splices between lengths

of precast p il ing can be made effec -

t iv ely w ith mech anical f ast en in g

dev ices tha t a re cas t into the ends of

the sections.

A s it ec a st c o nc re te p il e i s m ade by

driving a hollow s teel she ll into the

ground and fil ling i t with concrete.

FIGURE ~.43Precast, prestressed concrete piles . Lifting loops are cast into the sides of the piles as

crane attachments for hoisting them into avertical position. ( C ou r te s y o j L on e S t a r/ S an-

j ,tl Concret e)

The she ll is sometimes corruga ted to

increase its stiffness; if the corruga-

t ion s are ci rc umferen ti al , a h ea vy

steel mandrel (a s ti ff , t ight-f it ting

l iner) is inserted in the she ll during

driving to pro tect the she ll from col-

l aps e, t he n w ithdrawn be fo re c on -

creting. Some shellswith longitudinal

c or rug at io ns a re st if f e nough tha t

they do not require mandrels. Some

FIGURE ~'44

A driven cluster of six precast concrete

piles , ready for cutting off and capping.

(Photo by A l v in E r ic s o n)

lypeSofmandrel-driven piles are lim-

i ted in length, and the large r d iame-

tersof sitecast piles (up to 16inches,

o r 400 mrn) can cause ground heav-

ing.Load capacities range from 50to

120 tons (45 ,000-110 ,000 kg). The

primary reason to use s itecas t con-

crete piles is their economy.

There a re many proprie ta ry sys -

tems of sitecast concrete piles, each

Stee l

Poin:

1,2,3,4

Open

Ended

2

Concrete

Plug

3

CASEDP1LES

with various advantages and disadvan-

tages (Figures 2.45 and 2.46). Pressure-

i n je c te d f o o ti n g s (Figure 2 .46) sha re

ch arac ter is ti cs of pi le s, p ier s, a nd

footings. They are highly resistant to

uplift forces, a property that isuseful

for t al l, sl en de r b ui ld in gs in wh ic h

the re isa poten tial for ove rturning of

the building, and for tensile anchors

for tent and pneumatic structures.

Fluted

Tapered

2

Comp ressed

Base

1

Foundat io ns / 51

Comp ressed

Concrete

1

Stee l

Poin:

2

Pedesta l

Pi le

1

UNCASED P1 L ES

FIGURE ~-45

Some proprie tary types of sitecast concrete piles . All are cast into steel casings that

have been driven into the ground; the uncased pil es a re made bywithdrawing the cas -

ing a s the concrete i spoured and saving i t for subsequent r euse . The numbe rs r efer to

the methods of driving tha t may be used with e ach: I.Mandrel driven. 2.Driven from

the top of the tube. 3 .Driven f rom the bot tom of the tube toavoid buckl ing i t. 4 . Je t.

ted. Jetting isaccomplished byadvancing a high-pressure water nozzle ahead of the

pil e towash the soi l back a longside the pil e to the sur fa ce . J et ting has a tendency todis rupt the soi l a round the pil e, so i t i s not a favored me thod ofdr iv ing under mos t

circumstances.




(a)

(d)

(b)

(e) (j)

FIGURE 2.46

Steps in the construction of a proprie tary pressure-injected, bottom-driven concrete pile footing. (a) Acharge of ave ry low

moisture concrete mis i s ins er te d into the bot tom of the s te el drive tnbe a t the sur fa ce of the ground and compacted into a

sealing plug with repeated blows of a drop hanuuer. (b) As the drop hammer drive s the sea ling plug into the ground, the

drive tnbe i spul le d a long by the f ri ct ion between the plug and the tnbe. (c) When the des ir ed depth i s r ea ched, the tnbe i s

held and a bulb of concrete i s formed byadding sma ll cha rges of concrete and driving the concrete out into the soi l w ith the

drop hammer. The bulb provide s anincrea sed bea ring a re a for the pil e and s tr engthens the bea ring s tr atnm bycompact ion.

(d, e) The sha ft i s formed of addit iona l compacted concrete a s the tnbe i sw ithdrawn. Ij) Charges of concrete are dropped

into the tnbe f rom aspec ia l bucke t snpported on the leads of the driving equipment . ( C ou r te s y o f F nmh i F oun dat i o n Com j J an y)

SEISMIC BASE

ISOLATION

In areas where s trong ear thquakes

ar e common, build ings a re some-

times p la ced on b a se i so la t or s that

a llow the ground to move lateral ly

back and forth beneath the building

wh ile the subst ructu re and supe r-

s tructure remain more or less at res t

and free from damage, A frequentlyused type of base i sola tor i s a mul ti -

layered sandwich of rubber and steel

plates (Figure 2.47). The rubber lay-

ers deform in shear to allow the rec-

tangular i sola tor to become a paral -

lelogram in cross section in response

to relative motion between the

ground and the bui lding. Alead core

deforms enough to allow this motion

to occur , provides damping act ion,

and keeps the layers of the sandwich

aligned.

UNDERPINNING

Underpinning is the process of

s tr engthen ing and s tabil iz ing the

foundations of an existing building.

lD isPlacemen t o f bui ld ing

' n = = = = = ~ ~ = = = = = n = = = = = = = n

FIGURE 2.47

Base isolation.

Unde rp inni ng / 53

I tmay be requ ired f or any o f a num-

ber of reasons: The existing founda-

tions may never have been adequate

to carry their loads, leading to exces-

s ive set tl ement of the bui lding over

time. A change in building use or

addit ions to the bui lding may over-

load the exist ing foundat ions . New

construction near a building may dis-

turb the soi l around i ts foundat ions

or require that its foundations be car-

ried deeper. Whatever the cause,

underpinning is a s low, expensive,

highly specialized task that isseldom

the same for any two buildings. Three

dif ferent a lternatives are available

Column base plate

Lead core

Multiple layers ofs teel

plates and rubber

Isolator base plate




when foundat ion capacity needs to

be increased: The foundat ions may

be enlarged; new, deep foundations

can be inserted under shallow ones to

car ry the load to a deeper, s tronger

A. ELEVAT ION SECT ION

stratum ofsoil;or the soil itselfcan be

strengthened bygrouting or bychem-

ical treatment. Figures 2.48 and 2.49

illustrate in diagrammatic form some

selected concepts of underpinning.

B. ELEVATION SECTION

FIG URE 248

Two methods of supporting a building while carrying out underpinuiug work beneath

its foundatiou, each shown in both elevation and sectiou. (a) Treuches are dug

beneath the existing foundation atintervals , leaving the majority of the foundation

supported by the soi l. When por tions of the new foundat ions have been completed in

the t renche s, using one of the types shown inFigure 2 .49, another s et of t renche s i s

dug between them and the remainder of the foundations is completed. (b) The foun-

dat ions of an ent ir e waI lCanbe exposed a tonce byneedl ing, inwhich the waI l i s sup-

ported temporarily on n e ed l e b e am s threaded through holes cut in the waIl. After

underpinuiug has been accompl ished, the jacks and needle beams a re removed and

the trench isbackfilled.

A. ELEVAT ION SECT ION B ELEVAT ION C SECT IONECT ION

FIG URE 249

Three types of underpinuiug. (a) A new foundation wall and footing are constructed

beneath the existing foundation. (b) New piles or caissons are constructed on either

side of the existing foundation. (c) Concrete mini-piles are cast into holes drilled diag-

onally through the existing foundation. Mini-piles do not generaIly require excavation

or temporary support of the building.

The structural design of a retain-

ing wal lmust take into account such

factors as the height of the wal l, the

character of the soi l behind the wal l,

the presence or absence of ground-

water behind the wall, any structures

whose foundations apply pressure to

the soilbehind the wall, and the char-

ac te r o f t he soi l benea th the base of

the wall, which must support the foot-

ing that keeps the wal l in place . Therate of structural failure in retaining

wallsis high relative to the rate offail-

ure in other types ofs t ructures . Fai l-

RETAINING

WALLS

A r e ta in i ng w a ll holds soil back to cre-

ate an abrupt change in the elevation

ofthe ground. A retaining wall must

res is t the pressure of the ear th that

bear s aga in st it on t he uphi ll s ide.

Retaining walls may be made of

masonry, preservative-treated wood,coated or galvanized steel , precast

concrete, or, most commonly, sitecast

concrete.

\atertable

.,

OVERTURNING SLIDING UNDERMINING

FIGURE 2.50

Three faIlure mechanisms in retaining waIls. The high water table shown in these illus-

tra tions creates pressure against the walls that contributes to their failures. The under-

mining failure is directly attributable to groundwater running beneath the base of the

wall,carrying soil with it.

S T ONEG R A V IT Y W A L L V ERT ICAL T IMBER

CA N TI LE V ER E D W A LLHOR I ZONTAL T IMBER

W A LL W I TH D E ADME N

FIGURE 2.51

Three types of simple retaining walls , usually used for heights not exceeding 3 feet

(900 mm), The deadmen in the hor izonta l t imbe r wal la r e t imbe rs embedded inthe soi l

behind the wall and connected to i tw ith t imbe rs inser te d into the wall at right angles.

The timbers, which should be pressure treated with awood preservative, are held

together with very large spikes or with steel reinforcing bars driven into drilled holes.

The crushed-stone drainage trench behind each wall i s impor tant a s a means of r el ie v-

ing water pressure against the wall to prevent wall failure. With proper engineering

design, any of these types of construction can also be used for taIler retaining walls .

Retaining WaI ls / 55

ure may occu r t hrough fr ac tu re of

the wal l, overturning of the wal ldue

to soi l fai lure , lateral s liding of the

wal l, o r undermin ing o f t he wa ll by

flowing groundwater (Figure 2.50).

Careful engineering design and site

supervision are crucial to the success

ofa retaining wall.

There are many waysof building

retaining wal ls . For wal ls less than 3

feet (900 mm) in height, simple,unreinforced wal ls of var ious types

are often appropriate (Figure 2.51).

For taller walls,and ones subjected to




FIGURE2.52

Cantilevered retaining walls of

concrete and concrete masonry. The

footing isshaped to resist s liding and

overturning, and drainage behind the

wall reduces the likelihood of undermin-

ing. The pattern of steel reinforcing

(broken lines) isdesigned to resist the

tensile forces in the wall.

FIGURE2.53

Aretaining wall made of concrete blocks

that are designed specifically for this

purpose. The blocks interlock to prevent

sliding. The wall leans back against the

soi l i t r e ta ins; thi s r educes the amonnt of

soil it must retain, and also,mal<esthe

wall much more stable against the lateral

push of the soi l. (Courte sy o jVERSA-LOK

Re t ai n in g W a l l S y s te m s )

RE INFORCED

CONC R E T E

M A S ON R YRE INFORCED

CONC R E T E

unusual loadings or groundwater, the

type most frequently employed today

isthe ca n ti l ev e re d co n c re t e r e ta i ni n g wa l l,

tW O examples of which are shown in

Figure 2.52. The shape and reinforc-

ing ofa canti levered wal lcan be cus-

tom designed to suit almost any

si tuation. P roprie ta ry syst ems o f

interlocking concrete blocks are also

used to const ruct s loping retaining

walls that need no stee! reinforcing

(Figure 2.53).

E a rt h r ei nf or c in g (Figure 2.54) i s

an economical alternative to conven-

Retaining Wal ls / 57

t ional retaining walls in many situa-

tions. Soilis compacted in thin layers,

each containing str ips or meshes of

galvanized steel , polymer f ibers, or

glass fibers, which stabilize the soil in

much the s ame manner a s the roots

of plants.

FIGURE2.54

Two examples of earth reinforcing.

The embankment in the top sec tion was

placed by alternating thin layers of earth

with layers of synthetic mesh fabric.

The retaining wall in the lower section

i smade of pre ca st concrete panel s f as -

tened to long galvanized steel straps

tha t run back into the soi l.




Geotextiles are f lexible fabrics made of chemically inert

plast ics that are highly res is tant to deter ioration in the

soil. They are used for a variety ofpurposes relating to the

foundations of buildings. One use issoil reinforcement.

The accompanying text describes soil reinforcement in

wh ich a p la sti c mesh f ab ri c o r gr id is us ed to suppo rt a

retaining wall. The same mesh may be utilized in layers to

stabilize engineered fill beneath a shallow footing (Figure

A). In a similar way,mesh geotextiles are employed to sta-

bilize marginal soils under driveways, roads, and airport

runways, acting verymuch asthe roots ofplants do in pre-

venting the movement of soil particles.

Another geotextile that isintroduced in this chapter

is drainage matting, an open matrix of plastic filaments

with a feltlike filter fabric laminated onto one side tokeep

soilparticles from entering the matrix. In addition topro-

viding free drainage around foundation walls, drainage

mat ting isoften used beneath the soi l in the bot toms of

planter boxes and under heavy paving tiles on rooftop ter-

races, where it maintains a free passage for the drainage

ofwater above a waterproof membrane.

Synthetic filter fabrics are wrapped over and around

subterranean crushed stone drainage layers such as the

one frequently used around a foundat ion drain. In thi s

posit ion, they keep the s tone and pipe f rom becoming

clogged with soil particles. Similar fabrics are supported

vertically above ground on stakes as temporary barriers to

f il ter soi l out of water that runs off a const ruct ion site,

thus preventing contamination oflakes and streams.

Special geotextiles are manufactured to stake down

on freshly cut slopes to prevent soil erosion and encour-

age revegetation; some ofthese are designed todecay and

disappear into the soi las plants take over the funct ion of

slope stabilization. Another type of geotextile isused for

weed control in landscaped beds, where it allowsrainwa-

ter to penetrate the soil but blocks sunlight, preventing

weeds from sprouting.

WATERPROOFING

AND DRAINAGE

The substructure of a building issub-

j ec t to in tru si on o f g roundwa te r,

especiallyif it is constructed on a site

with a h igh wat er table or soi l t ha t

drains poorly. Even i f the concrete

wal ls of a basement were per fect ly

constructed, groundwater could work

its way to the inside through the

microscopic pores in the concrete,

and basemen t wa ll s ar e usual ly f ar

from perfect, being riddled with

openings through which water can

move, including shr inkage cracks,

form tie holes, holes for entry ofutili-

ties, and joints between pours ofcon-

crete.

There are two fundamental

approaches to waterproofing:

dra inage and w a te rp r oo f m e m br a ne s.

Drainage reduces the amount of

water that reaches the foundat ion,

and a waterproof membrane actsas a

barrier to the passage of watert hr ough the bas ement wa ll . Most

buildings employ both approaches in

combination.

Some fo rm of dr ainage i s us ed

with almost every building substruc-

tu re . Dra inage is r el iabl e, ea sy to

ins ta ll , and prevents the bui ldup of

potentially destructive water pressure

against basement wal ls and slabs . A

slab, in particular, ismany times less

expensive if it does not have to be

BE LOW-SLAB DRA INAGE

Waterpl'oof'mgandDrainage / 59

FIGURE 2.55

Twomethods of relievingwaterpressure around a buildingsubstructure bydrainage.

Thegraveldrain (left)ishardto dowellbecause of thedifficultyof depositing the

crushed stoneand backfillsoilin neatly separated, alternating layers.The drainage

mat (right) is mucheasierand often more economicalto iustall. It isa manufactured

componentthatmaybe made of aloosematof stiff,inert fibers, aplasticegg-crate

structure, or someother veryopen, porous material. Itis faced onthe outside witha

filter fabric thatprevents finesoilparticles from enteringand cloggingthedrainage

passagesin themat. An y subterranean water thatapproaches thewallfallsthrough theporousmaterialof thematto thedrain pipe atthe footing.

reinforced against hydrostatic pres-

sure. Several schemes for basement

drainage are shown in Figures 2.55

and 2.56. The perforated pipes are at

least 4 inches (100 mm) in diameter

and p rovide an open channel i n the

c rushed st one bed t hrough which

water can f low by gravi ty either " to

daylight" below the building on a

sloping site or to a sump pit that is

automatically pumped dry whenever

i t fi ll s up. When extra pr ot ec ti on

against severe groundwater condi -

t ions is needed, d rai nage pi pe s ar e

A B OV E -S L AB D RA IN A G E

FIGURE~.56

Fora highdegree of securityagainstsubstructure flooding,drainage both around and

under thebasementis required, asseen here in a section view.Above-slabdrainage is

USednbuildingswithmat foundations.




in sta lled under the ba sement s lab.

The pipes should be laid at least 6

inches (150mm) below the top of the

basement f loor s lab to maintain the

water level safelybelow the slab level.

The perf or at ions i n the p ipes f ace

downward so that water i s drained

from the lowest possible level.

For small basements in relatively

dry locat ions , the only addit ional

moi st ur e p rot ec ti on requ ir ed i s aninexpensive coat of dampproofing, typ-

icallya liquid asphalt applied byspray

or roller to the outside of the founda-

t ion wa ll. Dampproo fi ng i s not an

effective barrier to water under pres-

sure, but i t wil l s top dif fuse ground

moisture f rom coming through the

wall.

Waterproof membranes are

applied to foundations in situations

where drainage and dampproofing

cannot provide sufficient protection.

The most commonly used mem-

branes are made from plastic,

asphalt, or synthetic rubber, applied

either in liquid form or aspreformed

Protection

FIGURE ~'57A diagrammatic representation of the

placement of sheet membrane water-

proofmg a round aba sement . Amnd s lab

of low-strength concrete was ponred to

serve a sa base for pla cement of the hor i-

zontal membrane. Notice that the verti-

c al and hor izonta l membranes join to

wrap the basement completely in a water-

proof enclosure.

sheets. Liquid membranes are

applied by spray gun, roller, or

squeegee and then allowed to cure in

place. They are ge.nerally considered

relat ively easy to Ins ta ll and easy to

form around difficult shapes. When

ful ly cured, the l iquid membrane is

seamless and fully bonded to the

underlying substrate. However, liquid

membranes are sensi tive to uneven

application, and most cannot be

applied over par tial ly cured, damp

concrete, or other surfaces to which

theycannot develop a good bond.

P re fo rmed shee t membr anes

may be adhered or mechanically fas-

tened to basement wal ls , or loosely

laid over horizontal surfaces (Figure

2 .57) . Their u se ensure s un if orm

mater ia l quali ti es and consistent

membrane thickness. However, sheet

membranes can be more dif ficult to

fo rm around unusua l shapes , and

seamscompleted on the construction

si te may be vul ne rab le t o laps es i n

quali ty. Sheet membranes that are

not adhered can be used over sub-

strates that willnot bond with liquid-

applied or adhered sheet mem-

branes. They are also a good choice

in situations where substrate cracking

or movement i s expected, because

such movement isless likely to stress

or split the membrane.

In a class byi tsel f i s bentonite c lay ,

a naturally occurring, highly expan-

siveclay.Bentonite can be sprayed on

as a slurry or mechanically attached

aspreformed sheets consisting of dry

clay sandwiched within corrugated

cardboard, geotextile fabric, or plas-

tic sheets. When the bentonite comes

in contact with moisture, i t swell s to

several times its dry volume and

forms an impe rv ious barri er to t he

f ur ther pas sage of moi st ur e. Ben-

tonite i s an eas ily applied, rel iable

waterproofing material, suitable for

application over diverse substrates.

For example, it can be applied in

sheet form directly to the soi lunder a

concrete s lab on grade or asa s lurry

to a hi gh ly irr egula r, r ough st one

wall. Itadjusts readily to cracking and

SE COND PO UR FIR ST POUR

,.<1

b-~

D

.. 0

o~ . co4 ,

cr

FIGURE ~'58

Asynthetic rubber waterstop isused 10

seal against.water penetra tion at move-ment joints and atjoints between ponrs

ofconc re te ina foundat ion. The type

shown here i s spl it on one s ide soi t s

halves can be placed fla t against the

formwork where another wall wi l l join

the one being poured. After the concrete

h as b een pon re d a nd cn red and t he

formwork has been removed f rom the

first wall, the split halves are folded back

logether before the next wall is poured,

Waterprooflog and Dra ioage / 61

movement in the substrate.

Other waterproofing materials

include cementi tious plasters and

admixtures for concrete or mor tar

t ha t r eact chemi ca ll y t o bl ock the

pores of these mater ia ls and render

them watertight. These products find

use in special ized applications, but

their effectiveness is limited by their

brittleness. They have no crack-

spann ing capab il ity , a s a re sul t of

which even minor cracking in the

concrete or masonry subst ra te can

cause failure of the waterproofing.

Joints in basement wall construc-

tion often require special attention to

ensure watertightness. Preformed

waterstops made of plastic, synthetic

rubber, or metal can be cas t into the

ma ti ng concr et e edges o f a j oi nt to

b lock t he pass age o f wa te r, Thes e

type s o f wa te rs t ops can be used i n

movement joints such as control

joints and expansion joints. They are

also effective in nonmovementjoints

(Figures 2.58 and 2.59). Waterstops

for nonmovement joints such as




FIGURE ~.59

Insta lling rubber waterstops in a building

foundation. (a)Workers unroll the water-

stop around aconcrete form, where it

wil l be inse rt ed in the top of the footing .

(b) The top half of the waterstop is left

projecting above the concrete of the

footing and will seal the joint between

the footing and the concrete wal l tha t w il l

be ponred atop the footing. (c) A water-

s to p r eady f or t he n ex t pon r o f a con-

crete wall, as diagrammed in Fignre 2.58.

( C o u r t es y o f V u l ca n Me t a lP r o d u c ts , I n c .,

B i rm in g h am , A l a bam a )

(c)

joints between pours of concrete can

a lsobemade ofs trips ofbenton ite o r

mastic that are temporarily adhered

to t he edge of one pour . Af ter the

adj acent pour i s compl et e, these

s tops remain embedded in the joint ,

where they form a watertight barrier

(Figure 2.60).

Shee ts , mast ics , c oa ti ng s, a nd

c lay membranes mus t general ly be

a pp li ed to the out side of the wal l s o

that t he y ca n b e suppo rt ed a ga in st

water pressure by the wall. Mem-

branes out side t he wall cannot be

reached for repair, so the installation

mus t be inspected with extreme care

before the excavation ar ound the

foundation wall is backfilled withsoil.

The membrane isa t r iskof being

punctured orto rn bys tones or equ ip -

ment during the backfil ling opera -

tion. Pro te c ti o n bo a rd s are applied over

the membran e to minimize thi s r isk

and t o shi eld the membrane from

ultraviolet deterio ra tion from sun-

l ight . The mos t common pro tect ion

board has an asphalt core sand-

wiched between plastic skins and is )\6

to V . i nch t hi ck (1.5- 6 mm) . Once

successfully installed and backfilled,

waterproofing membranes are

s hi elde d f rom wea th er and w il l l ast

Ben t on i le - - - -- - -- - ..walers lop

FIGURE ~.6o

Another type of waterstop utilizes a str ip

of bentonite claythat isplaced in the

formwork before the concrete isponred.

When groundwater wets the strip of ben-

tonite, the str ip swells and seals out the

water.

Waterproofing and Dra inage / 63

for a v ery long t ime, p ro vided the y

are not subsequently torn by founda-

tion cracking.

Whe re a b as emen t wal l i s bu il t

t ig h t t o a prop er ty l in e, t he ex ca va-

ti on cannot be enlarged to permit

workers to apply waterproofing to the

outside ofa basement wall.This prob-

lem isoften solved bythe installation

of b l in d - si d e wa t er p ro o f in g , in which a

d ra in age mat i s a pp li ed to the s lo pe

retention sheeting, which will be left

in p lace permanently , and the mem-

brane isapp lied to the dra inage mat.

F inal ly , the concrete wall is poured

against the membrane (Figure 2.61).




r------ WaleIpTOOfmembrane

polystyrene foam insulation

do not deter iorate in the soi l,

c an be used i n co ld c lima te s to

footings that l ie above the

frost line in the soil, resulting

excavation costs. Continuous

insulation board are placedthe perimeter of the building

such a way that heat f lowing into

soilin winter from the interior of

building maintains the soil

the footings at a tempera-

above freezing (Figure 2.62) .

beneath unheated bui ldings ,

installed thermal insulation

enough geothermal heat

shallow foundations to pre-

s lab

Wa te rp ro o f __J

memb ra ne

C ru s hed s to n e ...1

Filterfabric PVCdrainline

(perfora ted)

have been applied, drainage features

installed, and internal constructions

that support the basement walls,such

asinterior walls and floors, have been

comple ted, the a re a a round a sub -

s tructure in a benched excavat ion is

backfi l led to restore the level of the

ground. (Asubstructure built t ightly

against sheeted wallsof an excavation

needs l it tl e or no backf il ling.) The

backfilling operation involves placing

soil back agai ns t the ou tsi de of thebasemen t wal ls and compact ing it

there in layers, taking care not to

damage drainage or waterproofing

components or to exert excessive soil

pressure against the walls. An open,

fast-draining soil such as sand ispre-

ferred for backfilling because it

allows the perimeter drainage system

around the basement to do i ts work.

Compac ti on must be suffi ci en t to

minimize subsequent settling of the

backfilled area.

In some situations, especially in

backf il ling uti li ty t renches under

roadways and f loor s labs , set tl ing

can be vir tual ly eliminated by back-

f il ling with c o nt ro l le d l ow - st re n gt h

material (CLSM), which is made

from portland cement and/or fly

FIGURE 2.61

Blind-side waterproofmg isused where

the re i sno working space between a

sheeted excavat ion and the out side of

the foundation wall. The drainage mat Walls t ructu re ----~"'M:=:::_~and waterproof membrane a re appli ed to

the sbeet ing, then the basement wal l i s

poured against them.

Pro te ct iv e coa t ing

heat from basement rooms to the soil

outside . Thermal insulat ion may be

applied either ins ide or outside the

basement wall. Inside the wall, min-

eral bat t or plast ic foam insulat ion

may be in st al led between wood or

steel furring strips, asshown in Figure

23.5. Alternatively, polystyrene foam

or glass fiber insulation boards, typi-

cally 2 to 4 inches thick (50-100

mm) , may be p la ced on t he ou ts ide

of the wall, held there byeither adhe-

si ve o r the p re ssure o f the soi l. P ro -

prietary products are available that

combine insulation board and

drainage mat in a single assembly.

BASEMENT

INSULATION

Comfort requirements, heating fuel

efficiency, and building codes often

requi re that basement wal ls be ther-

mal ly insulated to retard the loss of

Backf il ling / 65

a sh ( a byproduct o f coal -bur ning

power plants), sand, and water.

CLSM, sometimes called "flowable

f il l," is br ought in conc re te mixer

tr ucks and pou red i nt o the excava-

ti on , wher e it compacts and level s

i ts el f, then har dens i nt o soil -li ke

mat er ia l. The st reng th o f CLSM is

matched to the s ituation: For a uti l-

it y tr ench , CLSM i s f ormul at ed so

that it is weak enough to be exca-

vat ed eas il y by ordinar y d iggingequipment when the pipe needs ser -

vicing, yet asstrong as a good-quality

compacted backfill. CLSMhas many

other uses in and around founda-

tions. It is often used to pour mud

slabs, which are weak concrete s labs

used t oc reat e a leve l, dr ybas e in an

irregular, often wet excavation. The

mud slab serves asa working surface

f or the r ei nfo rc ing and pou ring o f a

f oundat ion mat o r ba sement fl oo r

slab and is often the surface to

which a waterproofing membrane is

applied. CLSM is also used to

replace pockets of unstable soil that

may be encounte red benea th a sub-

s tr uct ur e o r to c re ate a s table vo l-

ume of backf il l around a basement

wall.

FIGURE 2.62

A typical detail for ashallow frost-

protected footing.




pancy isdesirable so that the building

willbegin generating income assoon

as possible. Normally, the substruc-

ture of a building is completed

before work begins on itssuperstruc-

ture. Ifthe building has several levels

of basements, however, substructure

work can take many months, even

yea rs . I n such a cas e, u p-d ow n c on -

str uc t i on is sometimes an economical

opt ion, even i f i ts f ir st cos t i ssome-

what more than that of the normal

procedure , because i t can save con-

siderable construction time.

As diagrammed in Figure 2.63,

up-down const ruct ion begins with

installation of a perimeter slurry wall.

In te rnal s te el col umns fo r the sub-

UP-DOWN

CONSTRUCTION

Somet imes speed of const ruct ion is

crucial, especially in expensive build-

ing projects, where the interest costs

on the money invested in the incom-

plete bui lding are high; ear ly occu-

S tee lco lumnsp lacedi n s l u rr y -f i ll e d s ha f ts

(a)FIGURE 2.63

Up-down construction.

Des igning Foundat ions / 67

s trUcture are lowered into dri ll ed,

slurry-filled holes, and concrete foot-

ingsare tremied beneath them. After

t il e ground f loor s lab is in place and

connected to the substructure

columns, erection of the superstruc-

ture may begin. Const ruct ion con-

t inues s imul taneously on t ile sub-

structure, largely by means of mining

machinery: Astory of soil isexcavated

from beneath the ground f loor s lab

and a level mud slab of CLSM is

poured. Worki ng on t he mud s lab,

workers r ei nfo rce and pour a con-

crete s tructural s lab for the f loor of

the topmost basement level and con-

nect thi s f loor to the columns. When

the slab issufficiently strong, another

story ofsoilis removed from beneath

it, along with the mud slab. The

process isrepeated until me substruc-

tu re is comple te , bywhi ch time t he

superst ructure has been bui lt many

stories into the air.

DESIGNING

FOUNDATIONS

It is a good idea t obeg in the de sign

o f t he foundat ions of a bu il ding at

the same time asarchitectural design

work commences. Subsurface condi-

tions beneath a site can strongly

influence several fundamental deci-

s ions about a bui lding- it s locat ion

on the site, its size and shape, its

Supers t ruc t u ree re ct io n u p




wei gh t, and t he requ ired degr ee o f

f lexibi li ty of i ts const ruct ion. On a

large building project, at least three

designers are involved in these deci-

sions: the architect, who has primary

responsibil ity for the locat ion and

form of the bui lding; the s tructural

engineer, who has primary responsi-

bilityfor its physical integrity; and the

foundation engineer, who must

decide, on the basis of site explo-

rat ion and laboratory repor ts , howbest to suppo rt it i n the ear th . Mor e

of ten t han not, it is possi bl e f or t he

foundation engineer to design foun-

dations for a building design dictated

entir el y by t he a rchi te ct . I n some

cases , however , the cost of the foun-

dat ions may consume a much larger

share of the construction budget

than the archi tect has ant ic ipated,

unless cer ta in compromises can be

reached on the form and locat ion of

the building. It issafer and more pro-

ductive for the architect towork with

the f ounda ti on eng ineer fr om the

outset, seeking alternative site loca-

t ions and bui lding configurations

that will result in the fewest founda-

tion problems and the lowest founda-

tion cost.

In designing a foundation, a

number of dif ferent des ign thresh-

olds need to be kept in mind. If the

designer crosses any of these thresh-

olds, foundation costs take a sudden

jump. Some of these thresholds are:

o B u il di ng b el ow t he w a te r t ab le . If the

subst ructu re and foundations o f a

building are above the water table, lit-

tle or no effort will be required to

keep the excavat ion dry dur ing con-

s truction. I f the water table i spene-tra ted, even by an i nch, expensive

steps willhave to be taken to dewater

the site, strengthen the slope support

system, waterproof the foundation,

and either s trengthen the basement

floor slab against hydrostatic uplift

p re ssur e o r p rovide f or adequa te

drainage to relieve this pressure. For

an extra inch or foot of depth, the

expense would probably not bejusti-

fied; for another story or two ofuseful

building space, itmight be.

o B u il d in g c lo s e t o a n e x is t in g s t ru c tu r e.

If the excavation can be kept well

away from adjacent s tructures, the

foundat ions of these s tructures can

r ema in undist urbed and no ef fo rt

and expense are requi red to protect

them. When digging close to an exist-

ing s tructure , and especia lly when

digging deeper than its foundations,

the s tructure wil l have to be braced

temporari ly against s tructural col -

l ap se and may r equire pe rmanen t

underpinning with new foundations.

Fur thermore, an excavat ion at a dis-

tance from an existing structure may

not require sheeting, while one

immed iate ly ad ja cent a lmost ce r-

tainlywill.

• Increa sing th e co lu mn o r w all loa d

f ro m a b ui ld in g b ey on d w ha t c an b e s up -

por ted b y a sha l low f oun d a ti o n . Shallow

foundat ions are far less expensive

than piles or caissons under most

conditions. If the building grows too

tall, however, a shallow foundation

may no longer be able to carry the

load, and a threshold must be crossed

into the realm ofdeep foundations. If

th is has happened for t he s ake o f an

extra story or two of height, the

designer should consider reducing

the height by broadening the bui ld-

ing. Ifindividual column loadings are

too high for shallow foundations, per-

haps they can be reduced byincreas-

ing the number of columns in the

building and decreasing their space-

ing.

For bui ldings at the scale ofone-

and two-family dwellings, foundationdesign is usual ly much simpler than

for large buildings because founda-

tion loadings are low.The uncertain-

ties in foundation design can be

reduced with reasonable economy by

adopting a large factor ofsafety in cal-

culat ing the bearing capacity of the

soi l. Unless the designer has reason

to suspect poor soi l condi t ions , the

footings are usual ly des igned using

rule-of-thumb allowable soil stresses

and s tandard iz ed f oo ti ng dimen-

s ions . The designer then examines

the actual soil when the excavations

have been made. If it is not of the

quali ty that was expected, the foot-

ings can be hastily redesigned using a

revised estimate ofsoil-bearing capac-

ity before construction continues. If

unexpected groundwater is encoun-

tered, better drainage provisions may

have to be provided around the foun-dat ion, or the depth of the basement

decreased.

FOUNDATION DESIGN

AND THE BUlI:LDING

CODES

Because of the public safety consider-

a tions that a re invol ved, buil di ng

codes contain numerous provisions

relat ing to the design and const ruc-

tion of excavations and foundations.

The International Building Code®

def ines which soi l types are consid-ered satisfactory for bearing the

weight of buildings and establishes a

set of requi rements for subsurface

exploration, soil testing, and submis-

sion of soil reports to the local build-

ing inspector. Itgoes on tospecify the

methods of engineering design that

may be used fo r t he founda ti ons. I t

sets forth maximum loadbearing val-

ues for soi ls that may be assumed in

the absence o f det ai led t es t p roce -

dures (Figure 2.5). It establishes min-

imum dimensions for footings,

caissons, piles, and foundation walls

and conta in s lengt hy d is cuss ions

relat ing to the ins ta llat ion of pilesand cais sons and the d ra inage and

waterproofing of substructures. The

In te rnat ional Bu il di ng Code al so

requires engineering design of

retaining walls.Through similar pro-

visions, each building code attempts

to ensure that every building willrest

upon secu re foundat ions and a dry

substructure.

SELECTED REFERENCES

I.Ambrose,james E.

S im p li f ie d D e s i !! " o f

Bu i ld ingFounda t ions (2nded.). NewYork,

john Wiley& Sons,Inc., 1988.

Afteran initial summary of soil proper·

ties, this small book covers simplified

foundation computation procedures for

bothshallowand deepfoundations.

2. Liu, Chen,andjack B.Evett.S o i ls a n d

Founda t ions (5th ed.). Englewood Cliffs,

l\!j, Prentice-Hall, Inc., 2000.

This isa fairlydetailed discussion of the

engineering properties of soils,subsur-

faceexploration techniques, soilmechan-

ics, and shallow and deep foundations,

but iswellsuited to the beginner.

3. Schroeder, W. L. S o il s i n C o n st r uc t io n

(4th ed.). NewYork,john Wiley& Sons,

Inc., 1995.

A well-illustrated,clearlywritten, moder-

ately detailed surveyof soils,soil testing,

Foundation Designand theBuilding Codes / 69

subsurface construction, and founda-

tions.

4. Henshel l, jus tin, and C. W. Griffin.

M a n u al o f B e lo w -G r a de W a t er p ro o fi n g S y s·

t em s . NewYork,john Wiley& Sons,1999.

A comprehensivepresentation of means

for keeping water out of building sub-

structures.



70 / Chapter 2 e Foundations

WEB SITES

Calweld, Inc. www.caIweld.com

Franki Foundations www.franki.co.gg

Griffin Dewatering Corporation www.griffindewatering.com

Soletanche www.soletanche-bachy.com

TERMS AND CONCEPTS

foundation test pit tieback caisson retaining wall

dead load test boring rock anchor belled caisson earth reinforcing

live load water table soil nailing socketed caisson geotextiles

wind load liquid l imit dewatering drilled pier drainage

thrust plastic l imit sump pile waterproof mem-

foundation settlement penetrometer well point timber p ile brane

uniform settlement excavation watertight barrier piledriver dampproofing

differential settlement ice lenses superstructure end bearing pile bentonite clay

rock sheeting substructure friction pile waterstop

soil soldier beam shallow foundation pile cap protection board

boulder lagging deep foundation grade beam blind-side water-

cobble sheet piling footing pile hammer proofing

gravel slurry wall column footing lead drainage mat

sand guide wall wall or strip f ooting H-pile backfill

coarse-grained soil clamshell bucket engineered fill pipe pile controlled 1011'-silt tremie slab on grade heaving strength material

clay p recas t s lu rr y wa ll c raw ls pace precast concrete pile (CLSM)

fabric soil mixing basement sitecast concrete pile mud slab

cohesive bracing tie beam mandrel up-down

cohesionless or crosslot bracing combined footing pressure-injected construction

frictional waler cantilever footing footing

strata raker mat or raft foundation base isolator

stability heel block floating foundation underpinning

REVIEW QUESTIONS

I. What is the nature of the most com-

mon type offoundation failure?

2. Explain in detail the differences

among fine sand, silt, and clay, especia lly

a s they re la te to the foundat ions ofbuild-

ings.

3. Listthree different waysof sheeting an

excavation. Under what circumstances

would sheeting not be required?

EXERCISES

1. Obtain the foundat ion drawings for a

n ear by bui ld ing. L ook f ir st at t he l og o f

test borings. What sorts ofsoils are found

benea th the s it e? How deep i s the water

t ab le? What t yp e o f founda tio n do you

t hi nk s houl d b e u se d i n t his s it uat io n

( keep ing i n mi nd t he rel at iv e we ight o f

the bui ld ing)? Now look a t the founda-

4. Under wha t condi tions would you use

a watertight barrier instead ofwell points

when digging below the water table?

5. Ifshallow foundations are substantially

less costly than deep foundations, why do

we use deep foundations?

6 . What s oi l cond it io ns f avor t he u se o f

belled caissons?

tion drawings. What type isactually used?

Why?

2. Wha t type of founda tion and subst ruc-

t ur e i s normal ly u sed f or hou se s i n your

area? Why?

3. Look a t s everal excavat ions for major

buildings under construction. Note care-

Foundat ion Des ign and the Bui ld ing Codes / 71

7. Which types of friction piles can carry

the heaviest load per pile?

8. List and explain some cost thresholds

f requently encounter ed in foundat ion

design.

f ul ly t he arr angemen ts made f or sl op e

support and dewater ing. How is the soi l

being loosened and carried away? What is

being done with the excavated soil? What

t yp e o f foundat io n i s b ei ng in st al led ?

What provisions are being made for keep-

ing the substructure permanently dry?

http://www.caiweld.com/

http://www.franki.co.gg/

http://www.griffindewatering.com/

http://www.soletanche-bachy.com/

http://www.soletanche-bachy.com/

http://www.griffindewatering.com/

http://www.franki.co.gg/

http://www.caiweld.com/



Strength of Wood

Structural Composite Lumber

Plywood Production

Specifying Structural Wood Panels

Other Wood Panel Products

C om p a ny Ph o to )

o

• Chemical Treatment

oWood Fasteners

Nails

Machine-Driven Nails

Wood Screws and Lag Screws

Bolts

Timber Connectors

Toothed Plates

Sheet Metal and Metal Pla te Framing

Devices

Adhesives

• Wood Manufactured Building

Components

Trusses

Wood I:Joists

Panel Components

• Types of Wood Construct ion

73



TREES

Because wood comes from trees , an

understanding of tree physiology is

es se nt ial t o know ing how to bu ild

with wood.

Tree Growth

The t runk o fa t re e i s co ve re d w ith a

protective layer of dead bark (Figure

3 .1 ). Ins ide the dead bark isa layer of

living bark composed of hollow longi-

tud inal cel ls tha t conduct nutrien ts

down the trunk from the leaves to the

roo ts . Ins ide this layer of l iv ing bark

l ies a ver y t hi n layer, the cambium ,

which c reates new bark cel ls toward

the outside of the trunk and new

wood c el ls t oward the ins id e. The

thick layer of living wood cells inside

7 4 1 t he c ambium is the sapwood. In thi s

zone of the tree, nutrien ts a re s to red

a nd sap i s p umped upward f rom the

roots to the leavesand distributed lat-

e ra lly in the trunk . At the inner edge

of this zone, sapwood dies progres -

s ively and becomes heartwood. In

many species of trees , hea rtwood is

easily distinguished from sapwood by

itsdarker color. Heartwood no longer

participa te s in the l ife p rocesses of

the tree but con tinues to con tr ibute

to i ts s truc tu ra l s trength. At the very

ce nt er o f the t ru nk , su rround ed by

hea rtwood , is the pith of the tree, a

smal l z on e o f weak wood c el ls t hat

were the first year's growth.

An ex amin at io n o f a smal l se c-

t ion o f wood und er a low-powered

microscope shows that it consists pri-

mar il y of tubu la r c el ls who se long

ax es are p aral lel t o the long ax is of

the trunk . The cel ls a re s truc tu red of

tough cellulose and are bound

togethe r by a softe r cementing sub-

stance called lignin. The direc tion of

the long axesof the cellsisreferred to

asthe grain of the wood. Grain direc-

t ion i s impor ta nt t o the d esigne r o f

wooden buildings because the prop-

er ti es of wood p aral lel t o grain and

perpend icular to gra in a re very d if-

ferent.

In tempera te c lima tes, the cam-

bium begins to manufacture new sap-

wood cells in the spring, when the a ir

is cool and groundwater is plentiful,

conditions tha t favor rap id growth.

G rowth i s s lower du ring the h eat o f

the s umme r when wa te r i s sc arce.

Springwood or earlywood cells are there-

fore l ar ger and less dense in sub-

stance than the summerwood or

latewood cel ls . Concentric bands of

springwood and summerwood make

up the annua l g rowth rings ina trunk

tha t can becoun ted tode te rmine the

a ge of a t re e. The rel at ive propor -

t io ns o f s pr in gwood and summer -

wood al so h av e a d ir ec t be ar ing on

the structural properties of the wood

a g iven tree wil l y ie ld because sum-

merwood isstronger and stiffer than

springwood. A tree grown under con-

t in uou sly moi st , c oo l cond it ion s

grows fas te r than ano ther tree of the

same species grown under warmer,

drier conditions, but itswood isnot as

dense or asstrong.

Softwoods and Hardwoods

Softwoods come from coniferous trees

and hardwoods from broadleafed trees.

The names can be deceptive because

many conif er ous trees produce

harder woods than many broadleafed

trees, but the distinction isneverthe-

lessa useful one. Softwood trees have

a relatively simple microstructure,

consisting mainly of large longitudi-

nal cel ls ( tracheids) t oge th er w ith a

small percentage of radial cells (rays) ,

who se fun ct ion i s the storage and

rad ia l t rans fe r o f nutrien ts (Figure

3 .2 ). Hardwood trees a re more com-

p lex in s truc tu re , with a much large r

percentage of rays and two different

typ es of longi tud in al ce ll s: smal l-

Cel l s truc ture of a softwood

Trees / 75

FIGURE 3.1

Summerwood r ings a re p rominent and

a f ew r ays a re f ai nt ly v is ib le i n tills cross

sec ti on o f an eve rg reen t ree, but t he

cambium , whi ch l ie s j u st benea tb t be

t hi ck l ayer o f bar k, i s t oo thin t o beseen ,

and heartwood cannot be distinguished

visually f rom sapwood in this species.

(Cour t es y ofFores t P roduc t s Laboratory ,

F or es t S e r v ic e , U SDA )

FIGURE 3 . : ; :Vertical cel ls ( tracheids, labeled TR)

dominate the structnre of a sof twood,

seen her e g reat ly enl ar ged, but r ays

(WR), which are cells tbat run radially

fr om t be c en te r o f t be t re e t o t be o ut-

side, are clear ly in evidence. An annual

r ing ( labe led AR) consi st s o f a l ayer o f

smaller summerwood cel ls (SM) and

layer of larger spr ingwood cel ls (8) .

S impl e p it s ( SP ) a ll ow sap to pass f rom

ray cel ls to longitudinal cells and vice

ver sa . Res in i ss to red inve rt ical and

hor izontal resin ducts (VRD and HRD),

with t be hor izon ta l duc ts cen te red i n

fusiform wood rays (FWR). Border pits

(BP) allow tbe transfer of sap between

longitudinal cells. The face of tbe sample

l abel ed RR r ep resent s a r ad ia l cnt

t lr rough tbe tree, and TG, a tangential

cnt. ( C ou rt e sy o f F o r e s t P rod u c ts L ab or a t or y ,

F o r es t S e r v ic e , U SDA )



76 / Chap ter 3 • Wood

diameter fibers and large-diameter ves-

sels or pores, which transport the sap of

the tree (Figure 3.3).

When cut into lumber, softwoods

generally have a coarse and relatively

uninteres ting gra in s truc tu re , while

many hardwoods show beautiful pat-

terns of raysand vessels (Figure 3.4).

Most of the lumber used t oday for

bu ilding f ram ing come s f rom sof t-

WR

Cell structure of a hardwood

FIGURE 3.3

R a y s (WR) const itute a large percentage of the mass ofa hardwood, a s s e en in th is

sample, and are largely responsible for the beautiful grain figures associa ted with

many species . The ver ti ca l c el l s truc ture i smore complex than tha t of a softwood, w ith

large pores (P) to t ransport the sap and sma ller wood f iber s (F) togive the t re e s truc -tural strength. Pore cells in some hardwood species end with crossbars (SC), while

those of other species a re ent ir ely open. P it s (K) pas s s ap f rom one cavity to another .

(Courtesy o fFore s tProducts Labora tory, Fore s tSe rv ice, USDA) .

woods, which are comparatively plen-

tiful and inexpensive. For fine furni-

ture and interior finish details,

h ardwoods are o ft en c ho se n. A few

sof twood and hardwood species

(a)

(b)

FIGURE 3.4

The grain figures of two softwood

species (left) and two hardwoods (right)

demonstra te the difference in cellular

structure between the classes of woods.

From left to right: (a) The cells in sugar

pine a re souniform tha t the gra in s truc -

ture isa lmost invisible except for scat-

tered resin ducts . (b) Vertical-grain

Douglas fir shows very pronounced dark

bands of summerwood,

, \~de ly u se d in Nor th Amer ic a a re

l ist ed in F ig ure 3 .5 , a lo ng w ith the

p rinc ip al u ses of ea ch ; however , i t

should be borne in mind that literally

thousands of species of wood are

(c) Red oak exhibits large open pores

amid its fibers. (d) This quartersliced

mahogany veneer has a pronounced

"ribbon" figure caused byvarying light

reflections off its fibers. ( P ho t o s b y t h e

author)

used in construction around the

world and tha t the ava ilab le species

vary conside rably with geographic

location. The major lumber-

p roducing fores ts in North America

Trees / 77

a re in the wes te rn and eas te rn moun-

t ains o f bo th the Uni te d S ta te s a nd

Canada. Other regions, chiefly in the

southeastern United States, also pro-

duce significant quantities.

SOFTWOODS HARDWOODS

Us e d f or F r am in g , S h e at h in g ,

Paneling

Alpine fir

Balsam fir

Douglas fir

Eastern hemlock

Eastern spruce

Eastern white pine

Englemann spruce

Idaho white pine

Larch

Loblolly pine

Lodgepole pine

Longleaf pine

Mountain hemlock

Ponderosa pine

Red spruce

Shortleaf pine

Sitka spruce

Southern yellow pine

Western hemlock

White spruce

Us e d fo r Mo l d in g s , W i n d ow

a n d D o or F r am e s

Ponderosa pine

Sngarpine

White pine

Us e d fo r F i n is h F l o or i ng

Douglas fir

Longleaf pine

De ca y- R e si s ta n t W o o d s , U s e d

f o r S h i ng l es , S i di n g , O u t d oo r

Structures

California redwood

Southern cypress

Western red cedar

White cedar

Us e d f or Mo l d in g s ,

Pane l ing , Furni ture

Ash

Beech

Birch

Black walnut

Butternut

Cherry

Lauan

Mahogany

Pecan

Red oak

Rosewood

Teak

Tupelo gum

White oak

Yellow poplar

Us e d fo r F i n is h F l o or i ng

Pecan

Red oak

Sugar maple

Walnut

White oak

FIGURE 3.5

Some species of woods commonly used in construction in North America , lis ted alpha-

betically in groups according to end use. All are domestic except Lauan (Asia),

Mahogany (Centra l America), Rosewood (South America and Africa), and Teal, (Asia).



78 / Chapter 3 • Wood

Sawing

The production of lumber , lengths of

squared wood for usein construction,

be gins w ith the fel li ng o f t re es a nd

the tr anspor tation of the logs t o a

sawmill (Figure 3.6). Sawmills range

in s ize from tiny family opera tions to

g iant , s emiautomated fac to ries , but

t he proc es s o f lumber p ro duc tion i s

much the s ame reg ardl es s of sc al e.

Each log is s tripped of i ts bark, then

p as se d repe at ed ly through a l arge

headsaw, which may be e ithe r a c ircu-

l ar s aw o r a b ands aw , to redu ce i t t o

untrimmed s labs of lumber (Figure

3.7). The sawyer (with the aid of a

compute r in the large r mil ls ) judges

how to obt ai n the maximum mar-

ketab le wood from each log and uses

hyd ra ul ic mach in ery to rot at e a nd

advan ce the log in o rd er to a ch ie ve

LUMBER

Wood: A Renewable Resource

c Wood is t he only major str uctural material t hat is

renewable.

o In the United Sta te sand Canada, tree growth each yea r

greatly exceeds the volume of harvested trees. However,

timberlands are owned by literally millions of different

ent it ie s, many ofwhom do not yet manage them in a sus -

tainable manner.

e On othe r c ont ine nt s, many count ri es long a go fel led

the las t o f the ir fores ts , and many fores ts in o ther coun-

tries a re being dep le ted by poor management pract ices

and slash-and-burn agriculture. Itiswise to look into thebackground of tropical hardwoods to determine whether

they are grown in a sustainable manner.

Forestry Practices

e Two basic forms offorest management are practiced in

North America: s u st a in a b le f o re s tr y and c l ea r cu t ti n g a n d

rep lant ing . The c learcu tt ing fores t manager a ttains sus -

t aina bl e p ro duc tion by cu tt ing a ll t he t re es in an a re a,

leaving the stumps, tops, and limbs to decay and become

compost, setting out new trees, and tending the new trees

until they a re ready for harvest . In sus ta inab le fores try,

t rees a re harvested selec tive ly from a fores t in such a way

as to maint ain a t a ll t imes the biod iv er si ty o f a n atural

forest.

e Environmental problems often associated with loggingofforests include lossof wildlife habitat, soil erosion, pol-

lut ion of waterways , and a ir pollu tion from machine ry

exhausts and burning of tree wastes. A recently clearcut

fores t is a shock ingly ugly tangle of s tumps, b ranches,

tops, and substandard logs left to decay. Itis crisscrossed

bydeep ly rut ted, muddy hau l roads . Within a few years ,

decay of the waste wood and new tree growth largely heal

the sca rs . Loss of fores t a rea may raise levels o f carbon

d ioxide , a greenhouse gas , in the a tmosphere, because

trees take up carbon d ioxide from the a ir , u ti lize the car-

bon for growth, and g iveback pure oxygen to the a tmos -

phere.

• The buyer of wood products can support sus ta inab le

forestry practices by specifying products from certified

fores ts . At the present t ime, however, only a small per-

centage of available lumber originates in certified forests .

Mill Practices

• Skilled sawyers working with good equipment can con-

vert a h igh percentage of each log into marke table wood

products. A measure ofsawmill performance isthe lumber

r e co v er y f a ct o r ( L R F) , whi ch i s the net volume of woodproducts produced from a cubic meter oflog.

• Manufac tu red wood products such as orien ted s trand

board, particleboard, finger-jointed lumber made byglu-

ing short p ieces end to end , wood Ijo is ts , and laminated

s trand lumber a re very e ffic ient in u ti lizing mos t o f the

wood fiber in a tree. Large solid t imbers a re was te fu l o f

wood because only one or two can becu t from each log .

e Kiln dry ing uses large amounts of fue l, but p roduces

more s table, unifo rm lumber than a ir d ry ing, which uses

no fue l o ther than sun ligh t and wind.

• Mil l was te s a re voluminous: Bark may be shredded to

sel l a sa landscape mulch , composted , burned, o r buried

in a l an df il l. Sawdu st , ch ip s, a nd wood sc ra ps may be

burned togenera te s team to power the mil l, used as l ive -

stock bedding, composted, burned, or buried in a landfill.

Transportation

e Because the major commerc ia l fores ts a re located in

c omers o f the Uni te d S ta te s a nd Canada , mos t lumber

must be shipped considerable distances. Fuel consump-

t ion i s minimi zed by plani ng and dr yi ng t he l umber

before i t is shipped , which reduces both weigh t and vol-

ume.

Lumber / 79

the required success ion of cuts. As

t he slabs f al l fr om t he log at each

p as s, a c onv eyor be lt ca rr ies them

< t w a y tosmaller sawsthat reduce them

to square-edged pieces of the desired

il'idths (Figure 3.8). The sawn pieces

at this stage of production have

rough-textured surfaces and mayvary

slightly in dimension from one end to

the other.

Mos t lumber intended for use in

the framing ofbuildings isp la insawed ,

a method ofd ividing the log tha t p ro -

duces the maximum yie ld and the re -

fore the grea te st e conomy (Figure

3 .9 ). In p la in sawed lumber , some

pieces have the annual rings running

virtually perpendicular to the faces of

the p iece , some have the rings onvar-

i ou s d ia gona ls , a nd some hav e the

rings running a lmos t paral le l to the

faces . These varying gra in orien ta -

t ion s ca us e the p iec es to d is to rt d if -

f er ent ly dur in g se asoning , t o ha ve

Content

• Wood has a s ignificantly lower embodied energy con-

tent than competing structural materials.

• An average 8-foo t-Iong 2 x 4 (2.4-m-Iong 38x 89mm)

has an embodied energy of about 30,000 BTU (120 000

Kcal).This includes the energy expended to fell the tree,

t ransport the log , saw and surface the lumber, d ry i t in a

kiln, and transport it to a building site.

o Woodcontains a large amount ofembodied energy that

doesnot appear in this accounting: Itwasobtained free of

ch arge in the form o fs un ligh t du ring the grow th of the

tree.• Wood construction involves large numbers of steel fas-

teners ofvarious kinds. Because steel isproduced byrela-

t ively ener gy-i nt ensi ve processes, fasteners add

conside ra bly to the tot al e ne rg y embod ie d in a wood

frame building.

Construction Process

o Asign ifican t fract ion of the lumber del ivered to a con-

st ruc ti on s it e i swast ed : I t i s c ut o ff when e ac h p ie ce i s

sawed to s ize a nd sh ap e a nd ends up on the sc ra p h ea p,

which isusually burned or taken to a landfill. On-site cut-

t ing oflumber a lso generates conside rable quantit ie s o f

sawdu st . Cons truc ti on si te was te ca n be reduc ed by

des igning build ings tha t u ti lize ful l s tandard lengths of

lumber and full sheets ofwood panel materials.

Indoor Air Quali ty (IAQ)

• Wood itself seldom causes IAQ problems. Only a very

fewpeople are sensitive to the odor ofwood. Preservative-

treated woods can give off small amounts of dust contain-

iug a rsen ic compounds , though such dus t has not been

shown to create health problems.

• Some o f the adhes ive s and b in de rs u se d in p lywood,

OSB,particleboard, and laminated timbers can cause seri-

o us IAQ p roblems by g iv in g o ff vo la ti le o rg an ic c om-

pounds such as formaldehyde. Some paints, varnishes ,

st ains , an d l acque rs for wood a lso emi t fumes tha t ar e

unpleasant and/ or unhealthful.

• In damp locat ions , molds and fungi may grow onwood

members, creating unpleasant odors and releasing spores

to which many people are allergic.

Building Life Cycle

• I f t he wood fr ame of a buil di ng i s k ept dr y and away

from fire, it will last indefinitely. However, if the building

is poorly maintained, wood components may decay andrequire replacement.

• Wood iscombus tible and g ives off tox ic gases when it

b urns . The se g as es a re the p rinc ip al ca us e of de ath in

build ing fires . I t is importan t to keep sources of ign it ion

away f rom wood and to prov id e smoke al arms and e asy

escape rou tes to ass is t build ing occupan ts in escap ing

from burning buildings.

• When a bui ld in g i s demol is he d, wood f raming mem-

bers can be recyc led d irec tly into the frame of ano ther

building, sawninto new boards or timbers, or shredded as

raw mater ia l for or ie nt ed -s tr an d mater ia ls . There i s a

g rowing indu st ry who se bu sine ss i s pu rc has in g a nd

demolishing o ld barns , mil ls , and fac to ries and sel ling

their timbers.

A rec en t Canad ia n study o f t il e en ti re l if e cy cl e o f asmall office building compares similar buildings framed

with wood, s teel , and concrete. The total ene rgy used for

the wood build ing isabout half o fthat for the s teel build -

ing and two-thirds of tha t for the concrete build ing. The

wood bui ld in g won handi ly in ev ery o th er ca te go ry as

well, including greenhouse gas emissions, air pollution,

solid waste generation, and ecological impact.




FIGURE 3.6

Loading southern pine logs onto a t ruck for the ir t rip to the sawmi ll . ( C ou rt e sy o f S ou t h -

ern Fores tP roduc t s A s soc iat ion )

FIGURE 3. 7In alarge, mechanized mill, the operator

controls the high-speed bandsaw from an

overhead booth. ( C o ur te sy o f W e st er n W o o d

Produc t s A s soc iation )

FIGURE 3.8

Sawn lumber issorted into stacks accord-

ing to its cross-sectional dimensions and

length. ( C ou r te s y o f W e s te r n Wood P r od uc t s

Association)

different surface appearances

one ano ther , and toe rode a td if-

r at es when us ed in appl ic a-

such as flooring and exterio r

For us es where a ny of thes e

, vartauv-c will cause a problem, espe-

for finish flooring, interior trim,

and furni tu re , hardwoods a re often

quartersawn toproduce p ieces oflum-

b er tha t ha ve the a nnu al r in gs run-

n in g more n ear ly p erpendicu la r to

face of the piece. Boards with thisgrain tend to remain flat

despite changes in moisture content

The v is ib le grain on the s ur fa ce o f a

quartersawn piece makes a t ighter

and more pleasing figure. The wear-

ing qualities of the piece are

improved because there are no broad

areas of soft springwood exposed in

PLAINSAWING

the face, as there are when the

annual rings run more nearly parallel

to the face.

Seasoning

Growing wood contains a quantity of

water that can varyfrom about 30per-

cen t to asmuch as300 percent o fthe

oven-dry weigh t o f the wood. After a

tree is cut , this water s lowlys tarts to

evapora te . F irst to leave the wood isthe f r ee w a t er that isheld in the cavities

of t he cells. When the f ree wat er i s

gone, the wood still contains about 26

to 32percent moisture, and the bound

water held within the cel lu lose of the

cell walls begins to evaporate. As the

first o f the bound water d isappears,

the wood start s to shr ink, and t he

QUARTERSAWING

Lumber / 81

s tr en gth a nd st if fn es s o f the wood

b egin to increa se . The sh rink ag e,

sti ff ness, and str ength increase

steadi ly as t he moist ure cont ent

dec reases . Wood can be dried to any

des ired moisture con tent , but fram-

i ng l umber i s considered to be sea-

soned when its moisture con tent is19

percent o r les s. For framing app lica -

t ion s that r equ ir e c lo se r cont ro l o f

wood shrinkage, lumber seasoned to

a 15 percent moisture content,labeled "Me 15, ' is ava ilab le . I t iso f

little use to season ordinary framing

lumber to a moisture con tent below

about 13 percent because wood is

hygroscop ic and wil l take on or g ive

off moisture, swelling or shrinking as

i t does so, in order to s tay in equ il ib -

r ium with the moisture in the a ir .

T YP IC A L S AW IN G O F A

L A RG E L O G

FIGURE 3.9

Pla insawing produce s boa rds with a broad gra in f igure, a s s een in the end and top views below the pla in-

s awed log. Qua rter sawing produce s a ver ti ca l gra in s truc ture , which i s s een on the fac e of the boa rd a s

tightly spaced paralle l summerwood lines. Alarge log of softwood istypically sawn to produce some large

timbers, some plainsawed dimension lumber, and, in the horizontal row of small pieces seen just below

the heavy timbers, some pieces of vertical-grain decking.

I llm ( (/ ( (( II( I



82 / Chap te r 3 • Wood

Mos t lumber is se asoned at the

sawmill , e ither by being air dried in

loose stacksfor a period ofmonths or,

more commonly, bybeing dried in a

kiln under closely controlled condi-

t ions of temperature and humidity

for a period of days (Figures 3.10 and

3.11) . Seasoned lumber i s s tronger

and stiffer than unseasoned (g r een )

lumber and more dimensional ly s ta-

ble. It isalso lighter in weight, which

makes i t mor e economical to sh ip .K i ln d ry in g i s general ly preferred to

a i r d r yi ng because it i s much fa st er

and produces lumber with fewer dis-

tortions and more uniform quality.

Wood does not shr ink and swell

uniformly with changes in moisture

content . Moisture shr inkage along

the length of the log ( l ong i tud inal

shr i nkage) is negligible for practical

pur poses . Shr inkage in t he r ad ia l

direction ( ra d ia l s h ri nk a ge ) i s very

large by comparison, and shrinkage

around the circumference of the log

( ta n ge n ti a l s h ri nk a ge ) is about half

again greater than radial shr inkage

(Figure 3.12) . I f an ent ire log issea-

Nor doI ever come to a

lumber yard with its cityHke,

graduated masses offresh

shingles, boards 'and timbers,

without taking a deep breath

ofits fragrance, seeing the

forest laid low in it by

processes that cut and shaped

it tothe architect's scale of

feet and inches . . .

Frank Lloyd Wright

soned before sawing, i t wil l shr ink

veryl i tt le a long i ts length, but i twil l

grow noticeably smaller in diameter,

and the dif ference between the tan-

gential and radial shrinkage will

cause i t to check, that is, to split open

all along its length (Figure 3.13).

These differences in shrinkage

rates are solarge that they cannot be

ignored in bui lding design. In con-

FIGURE 3.10

For prope r a ir dry ing, lumbe r i s supported wel lof f the ground. The s ti cker s, which

keep the boa rds separated for venti la tion , a re c arefnUy pla ced above one another to

avoid bending the lumbe r, and awater tight roof prote ct s e ach s ta ck f rom raln and

snow. ( C ou rt e sy o f F o r e s t P rod u c ts L ab or a to r y , F o r es t S e r v ic e , U SDA )

s tructing bui lding frames of pla in-

sawed lumber, a simple distinction is

made between pa ra ll el-t o- gra in

shr inkage, which is negligible, and

perpendicular-to-grain shrinkage,

which isconsiderable. The difference

between radial and tangential shrink-

age isnot considered because the ori-

entation of the annual rings in

p lai ns awed lumber is r andom and

unpredictable. As wewillsee in Chap-

ter 5,wood building frames are care-fully designed to equalize the amount

of wood loaded perpendicular to

grain f rom one side of the s tructure

to the other in order to avoid the

not iceable t il ting of f loors and tear-

ing ofwallfinish materials that would

otherwise occur.

The posit ion ina log from which

a piece of lumber issawn determines

i n la rge pa rt how it will d is to rt a s it

dries. Figure 3.14 shows how the dif-

fe rence s between tangent ia l and

r ad ia l sh ri nkage cause th is t o hap-

pen. These effects are pronounced

and are readily predicted and

observed in everyday practice.

FIGURE 3.11

Measuring moisture content in boards in

a drying kiln. ( C ou rt es y o f W e s t er n W o o d


Lumber is sur faced to make itsmooth

and more d imensi onall y p recis e.

Rough (unsurfaced) lumber is often

available commercia lly and is used

for many purposes, but surfaced lum-

ber iseasier towork with because it is

more square and uni form in dimen-

FIGURE 3.12

Shrinkage of a typical softwood

with decreasing moisture content.

Longitndinal shrinkage, not shown

on this graph, isso small bycompari-

son to tangentia l and radial shrinkage

tha t i t i s of no consequence inwood

buildings. ( C ou rt e sy o f F o r e st P r od u ct s

L ab o ra t o ry , F o r es t S e r v ic e , U SDA )

JlJ:GURE3.13

Because tangentia l shrinkage isso

much greater than radial shrinkage,

high internal stresses are created in

alog asit dries , resulting inevitably

in the formation of radia l cracks

ealled checks.

s ion and less damaging to the hands

of the carpenter. Surfacing isdone by

h igh-speed au toma tic machi nes

whose rotat ing blades plane the sur -

faces of the piece and round the

edges s lightly. Most lumber i s sur -

faced on all four sides (S4S), but

hardwoods are often surfaced on

only two sides (S2S), leaving the two

Lumber / 83

edges to befinished bythe craftsman.

Lumber is usually seasoned

before it issurfaced, which allowsthe

p lani ng proce ss t o remove some of

the distortions that occur during sea-

soning, but for some framing lumber

thi s order of operations i s reversed.

The designation S-DRYin a lumber

gradestamp indicates that the piece

8

1

~1---6

5 r - - - - -~1'-

1 ' - - - - ~ . > ; ,4

~~I'---I ' - - - - "' <

J

~~ r-.<,<

~ t-,I

r - - : : ; : : : : - - -0

FIGURE 3.14

The difference between

tangential and radial

shrinkage also produces

seasoning distortions inlumbe r. The nature of

the distortion depends

on the pos it ion the

piece of lumber occu-

pied in the t re e. The

distortions are the most

pronounced in plain-

sawed lumber (upper

right, extreme right,

lower right). ( C ou rt e sy o f

Fores tP roduc t s Labora-

t o ry , F o r e s t S e r vi c e, U SDA )

m m M $ m ro n ~ 5 _ M

MOISTURE CONTENT(PERCENT)




wassurfaced (planed) when in a sea-

soned (d ry) cond ition, and S-GRN

indicates that it was planed when

green.

and 3.16). Among the most common

growth character is ti cs are knots,

which a re p la ces whe re b ranche s

joined the t runk of the t ree; knotholes,

which a re hol es l ef t by loose knot s

dropping out of the wood; decay; and

i ns ec t d a ma g e. Knots and knotholes

r educe the s tr uct ur al s tr ength o f a

piece of lumber, make i t more dif fi -

cu lt to cut and shape, and a re o ft en

consi de red to be detriment al to i ts

appearance. Decay and insect dam-age that occu rred during the li fe o f

the tree mayor may not affect the

useful properties of the piece oflum-

ber, depending on whether the

organisms are s ti ll a live in the wood

and the extent of the damage they

have done.

Manufacturing characteristics

ari se largely f rom changes that take

place dur ing the seasoning process

because of the dif ferences in rates of

shrinkage with varying orientations

to the grain.Splits

andchecks

are usu-al ly caus ed by sh ri nkage s tr es se s.

Lumber Defects

Almost every p ie ce of l umbe r con-

tains one or more discont inui ti es in

i ts s tructure caused by g r ow t h c h ar a c-

teristics of the t ree f rom which i tcame

orm a nu f ac tu r in g c h ar a ct er is ti c s

thatwere created at the mill (Figures 3.15

C r oo k in g , b o w in g , t w is ti ng , and cupp ing

a ll occu r because of nonun if orm

shrinkage. Wane is an irregular

rounding of edges or faces that is

caused bysawing pieces too close to

the perimeter of the log. Experi-

enced carpenters judge the extent of

these defects and distortions in each

piece of lumber and decide accord-

inglywhere and how touse the piece

in the bui lding. Checks are of l it tl e

consequence in framing lumber, buta j oist or rafter with a crook in it is

usually placed with the convex edge

(the "crown") facing up, to allow the

f loor or roof loads to s traighten the

piece. Carpenters s traighten badly

bowed joist s or raf ters by sawing or

planing away the crown before they

apply subflooring or sheathing over

t hem. Bad ly twi st ed p ie ce s a re put

a si de to be cut up for bl ocking. The

e ff ec ts o f cupp ing in fl oor ing and

interior baseboards and trim are usu-

ally minimized by using quartersawnstock and byshaping the pieces soas

FIGURE 3.15

Surface features often observed in lumber include, in the left-

hand column f rom top to bot tom, aknot cut c rosswise , aknot

cut longitudinally , aud a bark pocket. To the right are a grade.

s tamp, wane on two edges of the same pie ce , aud a smal l check.

The gradestamp indicates that the piece was graded according

(0 the ruJes of the American Forest Products Associa tion, that it

i s #2grade Spruce-Pine -Fir , a nd tha t i twas sur fa ced a fter dry -

ing .The 27i s a code numbe r for the mil l tha t produced the

lumber. (Pho tos by t h e a u t ho r )

FIGURE 3.16

Four types of seasoning distortions in dimension lumber.

BO W TWIST

Lumber / 85

to reduce the likelihood ofdistortion

(Figure 3.17).

Lumber Grading

Each piece oflumber isgraded either

for appearance or for structural

strength and stiffness, depending on

i ts intended use, before i t l eaves the

mil l. Lumber i s sold by species and

grade; the higher the grade, tile

higher the price . Grading offers thearchitect and the engineer the oppor-

FIGURE 3.17

The effects of seasoning distor-

tions can often be minimized

through knowledgeable detailing

practices. Asan example , this

wood baseboard, seen in cross sec-

t ion, has been formed with a

relieved back, a broad, shallow

groove tha t a llows the pie ce to l i e

fla t against the wall even ifit cups

(broken lines). The sloping bottom

on the baseboa rd a ssures tha t i tcau be insta lled tightly against the

f loor despi te the cup. The gra in

orienta tion in tins piece is the

worst possible with respect to cup·

ping. If quartersawn lumber were

not available , the next best choice

would have been to mi ll the base-

boa rd with the c ente r of the t re e

toward the room rathe r than

toward the wall.

Re l ie v e d ba c k



86 / C hap te r 3 • W oo d

t un it y t o b u ild a s e c on om ic ally a s p os -

sible by using only as h igh a grade as

is required fo r a particular use . In a

spe cific b uilding , th e m ain be am s o r

co lumns may requ ire a very h igh

struc tura l gra de o f lum be r, w hile th e

re ma in de r o f th e fra mi ng m em bers

w ill pe rfo rm a de qua te ly in a n inte r-

m ed ia te, less ex pe nsive g rad e. F or

b lo ck in g, t he l ow e st g ra de i s p e rf ec tl y

a de qu ate . F or finish trim th at w ill b e

coa ted w ith a c lear finish , a h igh

appearance g rade is desirable ; fo r

pa in te d trim , a lo we r gra de w ill suf-

f ice .

S tr uc tu ra l g ra di ng o flu mb er m ay

be done e ither v isua lly o r by

m ach ine. In v is u a l g ra d in g , trained

inspecto rs exam ine each p iece fo r

lin g de nsity a nd fo r g ro wth a nd m an -

u fa ct ur in g c ha ra ct er is ti cs , t he n j ud ge

i t and stam p it w ith a grade in acco r-

d an ce w ith ind ustry-w id e g rad ing

ru le s (F ig ure 3 .1 8). In m a c h in e g r ad -

ing, a n a ut om at ic d ev ic e a ss es se s t he

s tr uc tu ra l p ro pe rt ie s o f t he w o od a nd

sta mp s a g ra de a uto ma tic ally o n th e

p ie ce . T hi s a ss es sm en t i s m a de e it he r

F I GU R E 3 . 18

A g r ad er m ar ks t he g ra de o n a p ie ce w it h

a l um h e r c ra y on , p re p ar at o ry t o a p pl yi ng

a g rades tamp . ( C ou rt e sy o f W e s te r n Wood


by flexing each p iece be tw een ro l-

le rs a nd m easu ri ng its re sistan ce to

bending o r by scann ing the w oo d

e lec tro nic ally to d ete rm ine it s de n-

s it y. A pp ea ra nc e g ra di ng , n at ur ally

e no ug h, i s d on e v is ua ll y. F ig ur e 3.19

o ut li ne s a t yp ic al g ra di ng sc he me f or

f ra mi ng lu mb er , a nd F ig ur e 3.20 ou t -

li ne s th e ap pe ara nc e g rad es fo r n on -

structu ral lum ber. L igh t fram ing

lum ber fo r ho uses and o th er sm all

b uild ing s is u sua lly o rde red a s "#2and bette r" (a m ixtu re o f #1 and #2

grades) fo r flo o r jo ists and roo f

rafte rs, an d a s "S tu d" gra de fo r w all

framing .

Structural Strength of Wood

The strength o f a p iece o f w ood

depends ch iefly o n its species, it s

g rad e, an d th e dire ctio n in w hich th e

lo ad a ct s w it h r es pe ct t o t he d ire ct io n

o f gra in o f t h e p iec e. W oo d is se ve ral

t im es stro nge r pa ralle l to g rain t ha n

p er pe nd ic ula r t o g ra in . W it h i ts u su al

a ss or tm en t o f d efe ct s, i t i s st ro ng er i n

c om pre ssio n th an i n te nsio n. A llo w-

a bl e s tr en gt hs ( st ru ct ur al s tr es se s t ha t

i nc lu de f ac to rs o f s af et y) v ar y t re m en -

d ou sly w it h s pe ci es a nd g ra de . A ll ow -

a ble c om pr es si ve s tr en gt h p ar alle l t o

g ra in , f or e xa m pl e, v ar ie s f ro m 32 5 to

1700 po unds per square inch

(2.24-11.71 M P a) f or c om m er ci al ly

a va ila ble g ra de s a nd s pe ci es o f f ra m-

i ng l um be r, a d if fe re nc e o f m o re t ha n

f iv e t im e s. F ig ur e 3.21 c om pa re s t he

s tr uc tu ra l p ro pe rt ie s o f a n " av er ag e"

fra min g lu mb er to th ose o f th e o th er

c om m o n s tr uc tu ra l m a te ri al s- br ic k

m aso nry , ste el, a nd co nc rete . O f th e

fo ur m ateri als, o nly w oo d a nd ste el

h av e u se fu l t en si le s tr en gt h. D ef ec t-

fre e w oo d is co mp ara ble to stee l o n a

st re ng th -p er -u ni t- we ig ht b as is , b ut

w ith th e o rd inary run o f defec ts, an

a ve ra ge p ie ce o f lu mb er i s s o me w ha t

i nf er io r t o s te el b y t hi s y ar ds ti ck .

W hen de si gni ng a w oo den struc -

t ur e, t he a rc hi te ct o r e ng in ee r d et er -

m ine s th e m axi mu m stre sse s th at a re

lik ely to o ccur in each o f th e struc-

t ura l m em be rs an d sele cts a n a pp ro -

p ria te sp ec ies a nd g rad e o f lum be r

F I GU R E 3 . 19

S t an d ar d s tr uc t ur a l g ra d es f o r

w e s te r n s o ft w o o d l um b e r. F o r

e ac h s pe ci es o f w o o d , t he a ll ow -

a b le s tr uc t ur al s tr e ss e s f o r e a ch

o f t h es e g r ad e s a r e t a bu la t ed in

t h e s tr uc t ur al e n gi ne e ri n g l it e ra -

ture. ( C ou rt e sy o f W e s te r n Wood


fo r ea ch . In a g ive n lo ca le, a lim ited

number o f species and grades are

usu ally a va ila ble in re tai l lum be r-

yards, and it is from these th at the

sele ctio n is m ad e. It i s c om mo n pra c-

t ic e t o u s e a s tr on ge r b ut m or e e xp en -

s iv e s pe ci es ( Do ug la s f ir o r S ou th er n

p in e, f or e xa m pl e) f or h ig hl y s tr es se d

m ajo r m em bers, an d to u se a w ea ke r,

le ss e xp en si ve s pe ci es (s uc h a s E as t-

e rn h em lock) o r spec ies g roup

( He mlo ck -F ir , S pr uc e- Pi ne -F ir ) f or

the rem ainder o f the structure.

W ith in each spec ies, the designer

se le cts gra de s b ase d o n pu blish ed

tables o f a llow able stresses. The

h igher the g rade, the h igh er th e

allo wable stress, but th e low er the

g ra de , t he l es s c os tl y t he l um b er .

Th ere are many fac to rs o ther

t ha n s pe ci es a nd g ra de t ha t i nf lu en ce

th e usefu l strength o f w ood : These

include th e leng th o f tim e th e w oo d

w ill be subjected to its m ax im um

lo ad , th e tem pe rat ure a nd m oistu re

c on di ti on s u nd er w h ic h i t s er ve s, a nd

th e size and sh ape o f th e p iece . C er-

ta in fire-re tard an t tre at me nts a lso

r ed uc e t he s tr en gt h o f w o od sli gh tly .

A ll these facto rs are taken into

a cc ou nt w h en e ng in ee ri ng a b ui ld in g

s tr uc tu re o f w o od .

Lumber Dimensions

L um be r size s in th e U nite d S ta te s a re

g iv en a s n om i n a l d i men si o n s i n i n ch e s.

A p iece nom inally 1 by 2 in ch es in

L um ber / 87

DIMENSION LUMBER GRADES Tab le 2.1

W W I 'A W e s te rn L u m be r

I P r o d u c t

G r a di ng R u le s

G r a d e s S e c t i o n R e fe r e n c e U s e s

Struc tu ra lL igh t SELECT STRUCTURAL (42.1 0) Structural applica tions

Framing (SLF) NO .1 (42 .11 ) wh e r e h i g h es t d e s ig n

2 " t o 4 " t h ic k NO.2 (42.12) va lues a re n eeded i n ligh t

2 " to 4 " w i d e NO .3 (42.1 3) framings izes .

L igh tFraming (LF) CONSTRUCTION (40.11) W here high -streng th values are n o t2 " t o 4 " t h ic k STANDARD (40.1 2) required, such as wall fram ing , pla tes,

2 " t o 4 "w i d e UTILITY (40 .1 3) sills, c ripp les, blo ck ing , e tc .

I StudSTUD (41 .1 3) An opt ional all-pu rpo se g rade

2 " t o 4 " t h ic k des igned p rimari lyfo rs tud uses ,

2 " a n d w i d e r inc lud ingbearing wal l s .

StructuralJoists SELECT STRUCTURAL (62.1 0) In tended to fi teng ineering

a n d P l a nk s ( S J&P ) NO.1 (62.1 1) applications fo r lumber 5" and wider,

NO.2 (62 .1 2) such as jo ists, ra fte rs, h eaders,

NO.3 (62 .13) b e ams, t r u ss e s, a n d g e n er a l f r am i n g .

1STRUCTURAL DECKING GRADES Table 2.2

W W P A W e s te rn L u m be r

G r a di ng R u le sP r o d u c t G r a d e s S e c t i o n l ie f e r e n c e U s e s

Struc tu ra lDeck ingSELECTED DECKING (55.11 ) U sed wh ere th e appearance o f th e

2 " t o 4 " t h i c kb e st f a c e i s o fp r ima ry imp o r t a nc e .

4" t o 1 2 " w i de COM M ERCIALDECK ING (55.12) Customarily used w hen appearance

i s n o t o fp r ima ry impo r t an c e .

TIMBER GRADES Table 2.3

W W P A W e st e rn L um b er

G r a di ng R u le s

S e c t io n R e f e r e n cer o d u c t G r a d e s E n d U s es

Beams an d S t r in g e rs

5 " a n d t h i ck e r ,

w i d t hmo r e t h a n

2 " g r e at e r t h a n

th icknes s

DENSESELECT

STRUCTURAL '

DENSENO . r -DENSENO . 2 '

SELECTSTRUCTURAL

NO.1

NO.2

(53 .00& 170 .00 )

(53 .00& 170 .00 )

(53 .00& 170 .00 )

(70 .10 )

(70 .11)

(70 .12)

Gra d es a r e d e s ig n e d fo rb e am

an d s t ri n g er t y p e u s e s wh en

s i z es l a rg e r t h a n 4" n om i na l

t h i ck n e ss a r e r e q ui r e d.

P o s t a n d l imbe r s

5 " x 5 " a n d l ar ge r ,

w i d t h n o tmo r e

t h a n 2 " g r e at e r

than th icknes s

Gra d es a r e d e s ig n e d fo rv e r ti c a ll y

l o a de d a p p li c a ti o n s wh e r e s i ze s

l a rg e r t h a n 4" n om i n al t h i c k ne s s

are requ ired.

DENSESELECT

STRUCTURAL '

DENSENO . r -

DENSENO . 2 '

SELECTSTRUCTURAL

NO.1

NO.2

(53.00 & 170 .00 )

(53 .00& 170 .00 )

(53 .00& 170 .00 )

(80 .10 )

(80 .11)

(80 .12)

' D o ug la s F i r o r D o u gl as Fir-larch on l y .



88 / C hapte r 3 • W oo d

Tab le2 .5

Finish

( u su a ll y a v a il a bl e o n ly i n

D o ug F ir a n d H e m -F ir )

SUPERIOR

PRIME

E

COLONIAL

STERLING

STANDARD

UTILITY

INDUSTRIAL

WWPAG r a d ing Ru l es

Se c t io n Numbe rr oduc l Grades1Eq u iva le n t G r a d es i n

I da ho W h it e P i ne

10.11

10.12

10.13

Selects ( a l l spec i e s) B &BTR SELECT

CSELECT

DSELECT

SUPREME

CHOICE

QUALITY

10.51

10.52

10.53

Spec ia l Wes tern

Red Ceda r

Pa t te rn Grades

CLEARHEART

AGRADE

BGRADE

20.11

20.12

20.13

Common Bo a rd s

(WWPARules )

( p r ima r i ly i n p i nes ,

s p ru c e s, a n d c e d ar s )

1 COMMON

2 C O M MO N

3 C O M MO N

4 COMMON

5 C O M MO N

30.11

30.12

30.13

30.14

30.15

Al terna te Board s

(WCLIBRules)

( p ri m a ri l y i n Do u g

F i r a n d H em - Fi r)

SELECT

MERCHANTABLE

CONSTRUCTION

STANDARD

UTILITY

ECONOMY

WCLIS'

118-a

118-b

118-c

118-d

118-e

Spec ial Wes tern

Red Ceda r

Pa t te rn ' Grades

SELECT KNOTTY

QUALITYKNOTTY

WCLIS'

111-e

111-f

' R ef er t o W W P P i s Vo l . 2, W e st er n W o od S p ec i es b o ok f o r f u l l- c ol o r p h o to g ra p hy a nd 1 0 W W P A' s N a tu r al W o od S i di n g f o r c o m p l et e i n fo r m at io n o n s id i ng g r ad e s, s p ec i fi c at io n , a n d

instal lation.

FIGURE3.20

N o ns tr uc tu ra l b o ar ds a re g ra de d a cc or di ng t o a pp ea ra nc e. ( C o ur t es y o fW es t cr n W o o d

ProductsAssocia t ion)

Nomina l

Dimens ion

Ac tu a l

D imens ion

I"

2"

3"

5"

6"

8"

10 "

12 "

over 12 "

v: (1 9 mm)

I V ," ( 3 8m m )

2 V ," ( 6 4m m )

3 V( ( 89 mm )

4 V," ( 11 4 m m )

5 Y , " ( l 4 0mm)

7 ' 1 / ' ( 18 4 m m )

9V t ( 23 5 m m )

II Vt ( 28 6 m m )

% " le ss ( 19 m m le ss )

c ro ss se cti on i s a 1 x 2 ( "o ne b y t wo ") ,

a p ie ce 2 by 10 inch es is a 2 x 10 , and

so on. At the tim e a p iece o flumber is

saw n, it m ay approach th ese dim en-

s io ns . S ub se qu en t t o s aw i ng , h ow e ve r,

se as on ing a nd su rf ac in g d im in ish i ts

FIGURE3.22

T he r ela ti on sh ip b et we en n om in al a nd

actual dimensions for the most common

s iz es o f k iln -d ri ed lu mb er i s g iv en i n t h is

s im p li fi ed c ha rt , w h ic h is e xt ra ct ed f ro m

t he c om ple te c ha rt i n F i gu re 3 .2 3.

L um ber / 89

s iz e su bst ant ia lly . B y th e t im e a ki ln-

dried 2 x 10 r ea ch e s t he l um b er ya rd ,

its a c tu a l d i m en s io n s are 1 ; ; 2 by 9;;1

inches (38 x 23 5 mm). T h e r el at io n-

ship betw een nom inal lumber di-

m ensions (w hich are alw ays w ritten

w ithout inch marks) and actual

dim ensio ns (w hich are w ritten w ith

inch marks) is given in simplified

fo rm in F igu re 3.22 a nd i n c om pl et e

form in Figure 3.23. Anyone w ho

d es ig ns o r c on st ru ct s w oo de n b ui ld-ings soon commits the simpler o f

these relationsh ips to memory.

STANDAIfiD SIZES-FIfiAMING LUMBEIfl

N o m in al & D re ss ed ( Ba se d o n W e st er n L u mb er G ra d in g R u le s)

Nom i nal S i ze

Tab le2 .4

Surfaced

Dr y

Thicknesses & Widths

(inches)

Surfaced

Unseasonedescr iption

S4 S

Th ic kness W id th

(inc hes) (inc hes)

2 2

3 3

4 4

5

6

8

10

12

o v er 1 2

1 ' 1 ,

2 ' 1 ,

3 ' 1 ,

4 ' 1 ,

5 ' 1 ,

7%

9%

11Y,

off 0 / . ,

Length

(Ieel)

6' and longer,

generally

s h ip p ed i n

multiples

of 2'

Allo w ab le A llo w ab le

M ate ri a l T ensile S t reng th C o m pressiv e S treng th Dens i ty

W oo d ( average) 7 00 ps i 1 ,100 p si 3 0 p ef

( 4. 8 3 M P a ) (7 .58 M Pa) (480 k g/n l')

B r ic k m a s on r y ( av e ra ge ) 0 250 p s i 1 20 pef

(1 .72 MPa) (1 ,920 kg/rrr')

S te el ( AS T M A 3 6) 22 ,000 p si 22 ,000 p si 4 90 p cf

(151 .69 M P a) (1 51 .69 M P a) (7 ,850 kg /rn")

C o n cr et e ( av er ag e ) 0 1 ,3 50 p si 1 45 pef

(9 .3 1 M Pa) (2 ,320 kg /rn")

M o du lu s o f

Elastic ity

1 , 20 0 ,0 0 0 p s i

( 8 ,2 7 5 M P a )

1 , 20 0 ,0 0 0 p s i

( 8, 27 5 M P a)

2 9 ,0 0 0, 00 0 p s i

( 20 0 ,0 0 0 M P a )

3 ,1 5 0, 00 0 p si

( 21 ,7 20 M P a)

R o ug h o r S 4 S

(shipped

unseasoned)

5 a n d l ar g er

2

Th ic kness Wid th

(unseasoned)

1/2o ff nominal (S4S) .

S e e 3 . 20 o fWWPAGrad in g

Ru les fo r Rough .

Th ic kness Wid th

(d ry) (d ry)

2" (Single T&G)

Th ickness W id th

8

10

1 2

1 ' 1 , 4

5

6 0 / . ,

8 %

3" a nd 4 "

(Doub le T&G)5 %

4

6 2 ' 1 ,

3 ' 1 ,

6' and longer ,

generally

s h ip p ed i n

multiples of 2'

6' and longer,

generally

s h ip p ed i n

multiples

of 2'

FIGURE3·U

C om pa ra ti ve p hy si ca l p ro pe rt ie s o f t he f ou r c om m on s tr uc tu ra l m at er ia ls : w o od , b ri ck

masonry, steel, concrete.

F OH C- Fr ee o f H e ar t C en te r

S4S-Surfaced four sides

T &G - T on gu ed a nd g ro o ve d R ou g h F ul l S a wn -U ns ur fa ce d l um b er c ut t o f u ll s pe ci fi ed s iz e

c ha rt o f n om in al a nd a ct ua l d im en si on s f or b ot h f ra mi ng lu mb er a nd

lumbe r . (Courtesy o f Wes te rnWood Products Associa tion)



90 / Chapter 3 • Wood

Because of chang ing moisture con-

tent and manufacturing tolerances,

however, i t is never wise to assume

tha t a p iece of lumber wil l conform

precisely to its intended dimension.

The experienced detailer of wooden

build ings knows not to treat wood as

if i t had the d imensional accuracy of

s teel . The des igne r working with an

exist ing wooden build ing wil l f ind a

great deal of variati on i n lumber

sizes.Wood members in hot, dry loca-

t io ns su ch a s a tt ics o ft en w il l h av e

shrunk to dimensions substantially

below tbeir original measurements.

Members in older buildings may have

been manufac tu red to ful l nominal

dimensions or to earlier standards of

actual dimensions such as I% inch or

1%inch (41 or 44mm) for a nominal

2-inch member.

Pieces of lumber less than 2

i nches in nomi nal t hi ckness ar e

called board s . Pieces ranging from 2

to 4 inches in nominal thickness a re

referred to collect ively as d imens ion

lumber: Pieces nominally 5 inches and

more in thickness are termed t imbers .

Dimension lumber isusually sup-

plied in 2-foot (610-mm) increments

ofleng th . The mos t commonly used

lengtbs a re 8 , 10, 12, 14, and 16fee t

(2.44 , 3 .05, 3 .66, 4 .27, and 4 .88 m),

but most retailers stock rafter mater-

ial in l engt hs t o 24 feet ( 7. 32 m).

Actual lengths are usually a fraction

of an inch longer than nominal

lengths.

Lumber in the Uni ted S ta te s i s

pr ice d by the bo a r d f o o t. Board foot

mea su rement i s ba se d on nominal

dimensions, not actual dimensions. A

bo ard foo t o f lumbe r i s d ef in ed a s a

sol id vol ume 12 square inches in

nominal c ross -sec tional a rea and 1

foot long. A 1 x 12 o r 2 x 6 10 f eet

l ong c on ta in s 1 0board feet . A 2 x 4

10 feet l ong c on ta ins [ (2 x 4 )/12 ] x

10" 6.67board feet, and soon. Prices

of d imension lumber and timbers in

the United States are usually quoted

on the bas is o f dolla rs per thousand

board feet. In other parts of the

wor ld, l umbe r i s so ld by the c ub ic

meter.

The architect and engineer spec-

ifylumber for a particular construc-

t ion use by des igna ting its species ,

grade, seasoning, surfacing, nominal

s ize, and chemica l treatment , i f any .

When ordering lumber, the contrac-

tor must additionally give tbe

requi red l en gths o f p ie ce s and the

requi re d numbe r o f p ie ce s of ea ch

length.

WOOD PRODUCTS

Much of the wood used in construc-

t io n h as be en proc es se d into l ami -

nated wood, wood panel products, or

"Your chessboard, si re , i s

inlaid with two woods: ebony

and maple. The square on

which your enl ightened gaze

is f ix ed was cut f rom the ring

of a trunk that gr ew in a year

ofdrought : you see how i ts

f ib er s are arr anged? Here a

bar ely h in ted knot can be

made out: a bud tried to bur-

geon on a premature spring

day, but the night' s f rost

forced it to desist .... Here is

a thicker pore: perhaps itwas

a larvum's nest .... " The

quantity ofthings that could

be read in a little piece of

smooth and empty wood over-whelmed Kublaij Polo was

already talking about ebony

fores ts, about raf ts l aden with

logs that come down the

r ivers, ofdocks , ofwomen a t

the windows . . .

Italo Calvino, in Invisible Cities,

NewYork,Harcourt Brace

Jovanovich, 1978

various composite products. These

products were originally designed to

overcome various of tbe shortcom-

ings of sol id wood s truc tu ra l mem-

bers. With diminishing forest quality

and a new consciousness of sustain-

ability, however, these products have

assumed a new importance. Empha-

s is in the fores t p roducts indus try is

steadily shifting awayfrom dimension

lumber and focus ing on maximum

uti liza tion of wood fiber from each

tree. Yearby year, a larger and larger

percentage of the wood fiber used in

build ings is in the form of manufac -

tured wood products.

Laminated Wood

Large s truc tu ra l members a re often

produced by joi ni ng many small

s tr ips of wood togethe r with g lue to

form g l ue -l a mi n at ed w o o d (called glu -

lam for short). There a re three major

reasons to laminate: size, shape, and

quality. Any desired size of structural

member can be laminated , up to the

capac it ie s o f the hoist ing and trans -

portation machinery needed to

deliver and erect it,without having to

se arch for a t re e o f su ff ici en t g ir th

and heigh t. Wood can be laminated

into shapes tha t canno t be obtained

in nature: curves, angles, and varying

cross sections (Figure 3.24). Quality

c an b e sp ec if ied a nd c lo se ly c on -

trolled in laminated members

because defec ts can becu t out o ftbe

wood before laminating. Seasoning is

carried out before the wood is lami-

nated (largely eliminating the checks

and distortions that characterize solid

timbers), and tbe strongest, highest-

quali ty wood can be placed i n the

parts o f the member tha t wil lbe sub-

jected to the highest structural

stresses. The fabrication of laminated

members obv ious ly adds to the cos t

per boa rd foo t, but this iso ften over-

come bythe smaller s izeof the lami-

nated member that can replace a

solid t imber of equal load-ca rrying

capacity. In many cases, solid timbers

a re s imply not ava ilab le a t any price

in tbe required size, shape, or quality.

FIGURE3.24

GluelaminatingaU-shaped timher for a ship.Severalsmaller membershave also

gluedaudclampedaud are dryingalongsidethe larger timber. ( C ou rt es y o f F o r es t

P ro d uc ts L a bo ra t or y, F o re s t Service. USDA)

J O IN T S I N L AM IN A T ION S

S C A RF J O IN T

F I NG E R J O IN T

FIGURE 3.25

withinalaminatiouof a glue-laminatedbeam,seen in theupper drawingin a

elevatiouview,mustbe scarfjointed or fmgerjointed to trausmit theten-

andcompressiveforcesfrom onepiece ofwood to the next.The individualpieces

areprepared forjointingby high-speedmachinesthat mill thescarf or fmgers

rotatingcuttersof theappropriate shape.

WoodProducts / 91

Ind iv id ual laminations a re usu-

ally IV2inches (38 mm) thick except

in curved members with small bend-

ing radii, where three-quarter-inch

( l9 -mm) stoc k i s us ed . End joint s

between individual pieces are either

f in g er j o in t ed or s c a r f j o i nt e d . These

types ofjoints allow the glue to trans-

mit tensi le and compressive forces

longitudina lly from one p iece to the

next within a l aminat ion ( Fi gure

3.25). Adhesives are chosen accord-ing to the moisture conditions under

which the member willserve. Anysize

member can be laminated , but s tan-

dard depths range from 3 to 75

inches (76- 1905 mm). Standar d

widths range from 21/8to 14V4inches

(54-362 mm).

The structural capacity of a glue-

l am in at ed b eam can be increas ed

considerably by gluing a thin strip of

fiber-reinforced plastic between the

first and second laminations nearest

t he edge of the beam (usual ly tbe

lower one) tha t acts in tension . The

f ibe rs u se d a re one s tha t ar e much

stif fer and st ronger than wood-

aramid, glass, carbon, or high-

performance polyethylene. These are

a ligned longitudina lly and embed-

ded in a p last ic matrix prior tobe ing

fabrica ted in to the beam. The result

is a sav ings of25 to40 percent in the

volume ofwood in the beam.

Structural Composite

Lumber

A number o f wood produc er s ha ve

developed s tr u ct u ra l c o m po s it e l u mb e r

p rodu ct s that ar e mad e up ei the r o f

ordinary plywood veneers or of long

s tr an ds o f wood f ibe r. Unl ik e p ly -

wood, however, in these products the

grains ofallthe veneers or strands are

orien ted in the longitudina l d irec -

tion ofthe piece oflumber to achieve

maximum bending st ren gth. One

type of product, l a mi n at ed v e ne er l u m-

b e r ( L VL ) , uses the veneers in sheets

and looks l ike a thick p lywood with

no crossbands ( Fi gure 3. 46). In

ano ther type, p ar al le l s tr an d l um b er

( P SL ) , the veneers are sliced into nar-



"Brayton F .W i l so n a n d Raben R. A r c he r , " T r eeD e s i g n : S ome B i o lo g i ca l

S o l ut i on s t o M e c ha n ic a l P r ob lem s ," Bioscience, V ol . 2 9 , N o .5 , p . 2 9 3.

Wood is the only major building material that isorganic

in origin. This accounts for much of i ts uniqueness asa

structural material. Because trees grow naturally, wood is

a renewable resource, and most of the work of "manufac-

tur ing" wood isdone for us bythe processes of l ife and

growth within the tree. The consequences ofthis are both

economic and practical. Wood ischeap to produce. Most

lumber need onl y be ha rvested, cut t o si ze , and d ri ed

before it isready for use. However, because wood is pro-

duced by t rees , we have l it tl e control over the product .

Unl ike other s tructural mater ia ls such as s teel or con-

crete, wecan do little to adjust or refine wood's properties

t o suit ou r needs . Rat he r, we must accep t i ts natu ra l

strengths and limitations.

Though weuse wood much aswef ind i t in the l iving

t ree, i t i s remarkably wel l sui ted to our bui lding needs .

This isbecause a tree isitselfa structure made from wood.

From a mechanical point of view, a tree isa tower erected

for the purpose of displaying leaves to the sun." Thus a

t ree i ssubjected tomany of the same forces as the bui ld-

ings we erect: It supports itself against the pull ofgravity

Itwithstands the forces ofwind, and the accumulation of

snow and iceon itslimbs.And itresists the natural stresses

of our environment, including temperature and moisture

extremes, attack by other organisms, and physical abuse.

Since wood isthe material that the tree usesto resist theseforces, i t i s not surpr is ing that we f ind wood a sui table

material for our structures aswell.

One ofwood's natural strengths isitshigh resistance

to loads or forces that act only for short per iods of t ime.

The greates t forces that t rees exper ience in nature are

f rom the wind, par ticularly in combination with ice or

heavy snow. Because ofthis, wood has evolved tobe more

rowstrands that are coated with adhe-

sive, oriented longitudinally, pressed

into a rectangular cross section, and

cured under heat and pressure (Fig-

ure 3.26) . L am in at ed s tr an d lu m be r

(LSL) i s s imilar to paral le l s trand

lumber, except that itis made oflong

shreds ofwood rather than str ips of

veneer. LVL and LSL are general ly

p roduced in sma ll er s iz es that a re

useful for headers and beams in light

92

capable ofwithstanding forces such as the wind that are

applied over shorter periods of time than forces applied

over much longer periods.

When webuild with wood, we can take advantage of

thi s unusual short -term strength. In the engineering

design ofanywooden structure, the maximum forces that

the s tructural members may car ry are increased as the

length of time these forces are expected to act decreases.

This adju stmen t is c all ed the d ura tio n o f lo ad [acun:

Increases from 15to 100 percent in the allowable stresses

are applied to wooden structures when considering the

effects ofwind, snow,and earthquake-the types offorces

alsoresisted best bythe tree.

The structural form ofthe tree isalsowellsuited toits

environment , Tree branches are supported at one end

only-an arrangement cal led a can ti l ever . The joint sup-

porting a cantilevered branch is a strong, stiff, efficient

connection that utilizes the tree's material resources to

the utmost. Despite the stiffness of thisjoint, however, the

b ranch it s uppor ts c an s ti ll def le ct re la ti ve ly gr ea t

amounts s ince i t i s fastened at one end only. This high

def lect ion can be advantageous for the t ree. A branch

t ha t dr oops under a heavy load o f snow can d rop t ha t

snow and relieve its load. Similarly, a tree that swaysand

bends in the wind can shed much ofthe force acting upon

i t. This capacity to def lect helps a t ree to survive forcesthat might otherwise break it.

When webuild with wood, wecannot directly exploit

the efficient structural form of the tree. The high deflec-

tions characteristic of cantilevered beams are unsuitable

for our buildings because such large movements would

cause discomfort to the occupants and undue distress to

other building components. Nor can wetake advantage of

the naturally strong joints that support these cantilevers,

though they could offer benef it s in their s ti ffness and

economy ofmaterials. In preparing wood for our uses, we

FIGURE3.26

This paral le l s tr and lumbe r, made up of

strands of wood veneer, is largely free of

defects , maldng itfar stronger and stiffer

than ordinary dimension lumber. (Cour -

t e sy o f T r u s J o is t MacM i ll a n)

sawthe tree into pieces ofconvenient sizeand shape. This

destroys the structural continuity between the tree's parts.

V\'benwe reassemble these pieces, we must devise new

waystojoin the wood. Despite the many methods devel-

oped for fabri ca ti ng wood connect ions , jo in ts wit h

strength and stiffness comparable to the tree's are impos-

sible tomake on the building site. This iswhymost of our

wood buildings relyon the much simpler type ofconnec-

tions that suffice when a beam issupported at both ends

simultaneously.

Wood in a tree performs many functions that wood in

a bui lding does not , because wood is fundamental ly

involved with all aspects of the tree's life processes and

growth. In order to meet numerous and specialized tasks,

wood has evolved a complex internal s tructure that i s

highly directional . The fact that wood is not a uni form

and " isot ropic" mater ia l ( the same in all directions)

placesbasic limitations on howit can be used. Virtually all

ofwood's physical properties vary greatly, depending on

whether they are measured parallel tothe direction ofthe

grain, or acrossit. This directional quality ofwood isoften

taken for granted, even though no other major building

material islike this. In fact, the direction of the grain in a

piece ofwood affects every aspect of how we can use i t,

including itsproduction, shaping, and fastening, itsstruc-

tural capacity,its durability, and its beauty.

Anexample that illustrates this isa comparison oftwo

earlyAmerican wood building types:the logcabin and theheavy timber frame. The log cabin was a building system

appropriate to the most primitive technology. The logs

werefound on or near the s iteand prepared for use with

a minimum of labor and tools . In their f inal form they

served many functions, including structural support, inte-

rior finish, exterior cladding, and insulation. But because

the logs were stacked with their grain running horizon-

tally,the wood was being used inefficiently from a struc-

tural point ofview Not onlydid thi s arrangement of the

timbers produce a relatively weak wall,it also maximized

frame construction, whereas PSL is

manufactured in a wide range of

s izes , including large dimensions

such as those of glue-laminated tim-

bers. Composi te lumber products

offer much the same benefits aslami-

na ted wood products , includi ng a

high degree of dimensional stability,

but they can be produced more eco-

nomically through a higher degree of

mechanization.

Wood Panel

Products

the effects of ver tical shr inkage and expansion in the

structure, resulting in a high degree ofdimensional insta-

bility.Where more sophisticated building techniques and

a wider range ofbuilding materials were available, the log

cabin wasnever used. Rather, buildings were framed with

heavy t imbers , uti li zing a s tructural sys tem that more

closely resembled the structure ofthe tree. This more nat-

ural configuration of timbers in which a wallwasframed

with ver tical posts and spanned with hor izontal beams

could be stronger, required lesswood, was more dimen-

sionally stable, and permitted the incorporation of other

materials better suited to the wall's nonstructural require-

ments. A simple difference in orientation of the timbers

produced a radically different building system that was

more efficient, durable, and comfortable.

Many of the modern developments in the wood

industry reflect a desire to overcome the natural limita-

tions ofwood. Plywood sheets are larger than could nor-

mal ly be obtained from trees , are dimensional ly more

s table t han na tu ra l wood , and min imize the adver se

effects of grain and natural defects in the wood. Newer

panel products such as par ticleboard, OSB, and hard-

board use scraps and waste that would otherwise be dis-

carded. Glue- laminated and veneer-based beams are

larger in sizeand of higher quality than can be obtained

from nature, and successfullyutilize techniques ofjoinery

that are asstrong as those of the original tree. Chemical

t reatments can increase wood's res is tance to f ire anddecay.Thus the trend istoward using wood lessin its nat-

ural s ta te , and more as a raw mater ia l in sophist icated

manufacturing processes . These new products share

more of the qualities of being highly refined and carefully

control led that are character is ti c of other man-made

building materials. However, despite such technological

changes the tree remains not onlya valuable resource but

also an inspi ra tion in i ts grace and strength, and in the

lessons itoffers in understanding our own building mate-

rials and methods.

in such a way that they numrrnze

many of the problems ofboards and

dimension lumber: Panels are more

nearly equal in s trength in their two

principal directions than solid wood;

shrinking, swelling, checking, and

splitting are greatly reduced. Addi-

tionally, panel products make more

efficient use of forest resources than

solid wood product s th rough l es s

wast ef ul ways o f reducing logs to

Wood in panel form isadvantageous

for many building applications (Fig-

ure 3.27). The panel dimensions are

usual ly 4 by 8 feet (1 .22 x 2 .44 m) .

Panels require less labor for installa-

tion than individual boards because

fewer pieces must be handled, and

wood panel products are fabricated

93

Wood P roduct s / 95



FIGURE 3.27

Plywood ismade of veneers selected to give the opthnum combination of economy and performance for each applica-

tion. This sheet of roof sheathing plywood is f ac ed witha Dveneer on the nnder side and a Cveneer on the top s ide.

These veneers , though nnattractive, perform well structurally and are much less costly than the higher grades incorpo-

rated into plywoods made for uses where appearance isimportant. ( C o ur t es y o f APA - T h e Eng i n ee r ed W o o d A s s o ci a ti o n)

bui lding products and through uti -

l iz at ion i n some types of pane ls of

ma te ri al t ha t would o therwi se be

thrown away-branches, undersized

trees,and mill wastes.

S tr u ct ur al w o od p a ne l products fall

into three general categories (Figure

3.28):

1. Plywood panels, which are made up

of thin wood veneers glued together.

The grain on the f ront and back face

veneers runs in the long direction of

the sheet, whereas the grain in one or

more inter ior crossbands runs per -

pendicular, in the short direction of

t he shee t. There is always an odd

number of layers in plywood, which

equali ze s the e ffe cts o f mo ist ure

movement, but an interior layer may

bemade up o f a si ng le venee r o r o f

twoveneers with their grains running

in the same direction.

2 . C o mp os it e p an el s, which have two

pa ra ll el fa ce veneer s bonded to a

core of reconstituted wood fibers.

3 . N o nu en ee re d p a ne ls , which are of

several differen t types:

a. O ri en te d s tr an d b oa rd ( OSB ) is

made of long shreds (st rands) of

wood compressed and glued into

three tofivelayers.The strands are

o riented i n the s ame manne r in

each layer as the grains of the

veneer layers in plywood. Because

ofthe length and controlled orien-

t at ion o f t he s trands , o ri en ted

strand board is generally stronger

and sti ffer than the other types of

nonveneered panels. Because i t

can be produced from small t rees

and even branches, oriented

st rand boar d i s genera lly mor eeconomical than plywood. It is

now the material most commonly

used for sheathing and subfloor-

ingoflight frame wood buildings.

b. WaJerboard iscomposed of large

waf erli ke f lake s o f wood com-

pressed and bonded into panels. It

has been largely replaced by ori -

ented strand board.

c. Particleboard ismanufactured in

several different density ranges, all

FIGURE 3.28

Five different wood panel products,

from top to bottom: plywood, composite

panel, waferboard, oriented strand

board, and particleboard. (Courte sy o f

APA - T h e Eng i n e er e d W o o d A s s o c ia t io n )

of whi ch ar e made up o f sma ll er

wood particles than oriented

strand board or waferboard, com-

pressed and bonded into panels. It

f inds use in bui ldings mainly as a

base mater ia l for veneered cabi-

nets and cabinets that are covered

with plastic laminate. Itisalsoused

commonly as an under layment

panel for resilient flooring.

d. Fiberboard is a very fine grained

boar d made of wood fi be rs andsynthetic resin binders. It is used

primarily in cabinets, furniture,

moldings, and other manufactured

products. The most common form

is me d ium · de n si t y f i be r bo a r d, known

a sMDR

APA - The Engineered Wood Asso-

ciation has established performance

standards that a llow considerable

interchangeability among these vari-

ous typesof panels for many construc-

tion uses. These standards are based

on the structural adequacy for aspeci-

f ied use, the dimensional s tabi li ty

under varying moisture conditions,and t he dur ab il ity of t he adhes ive

bond that holds the panel together.

Plywood Production

Venee rs fo r st ruct ura l panel s ar e

r o ta r y s l ic e d: Logs ar e soaked in ho t

water tosoften the wood, then each is

rotated in a large lathe against a s ta-

tionary knife that peels off a continu-

ous s tr ip ofveneer , much aspaper i s

unwound f rom a rol l (F igure s 3. 29

and 3.30). The strip of veneer is

clipped into sheets that pass through

a drying kiln where in a few minutes

their moisture content i s reduced to

rough ly 5 percen t. The shee ts a re

then assembled into larger sheets,

repai red as necessary with patches

glued into the sheet to fill open

defects, and graded and sorted

according to quality (Figure 3.31). A

machine spreads glue onto the

veneers as they are laid atop one

another in t he r equir ed sequence

and gr ai n or ient at ions . The g lued

96 / Chap ter 3 • Wood Wood P roduct s / 97



(a)

(c)

FIGURE3.29

Plywood manufacture. (a, b) A250-horsepower lathe spins a

sof twood log a sa kni fe pee ls off a continuous sheet ofveneer

for plywood manufacture. (c) An automatic clipper removes

unusable a re as ofveneer and t rims the res t into sheet s of the

prope r s iz e for plywood panel s. (d) The c lipped sheet s a re fed

into a continuous forced-air dryer, a long whose 150-foot (45-m)path they will lose about half their weight in moisture. (e) Leav-

ing the dry er , the sheet s have amois ture content of about 5

percent . They a re graded and sor ted a t th is point in the

process. (fI The highe r grade s ofveneer a re patched on thi s

machine that punches out defects and replaces them with

tightly fitted wood plugs. (g) In the layup line, automatic

machine ry appli es glue to one s ide of e ach sheet ofveneer , a nd

alternates the grain direction of the sheets to produce loose

plywood panels. (h)After layup, the loose panels are pre-

pre ss ed with a force of 300 tons per panel toconsol idate them

for easier handling. (X I Following prepressing, panels are

squeezed individually between platens heated to 300degrees

Fahrenheit (150°C) to cure the glue. (j) After trimming, sand-

ing, or grooving asspecified for each hatch, the finished ply-

wood panels are sorted into hins bygrade, ready for shipment.

(Pho tos band i our/elY oJGeorgia -Paci f ic; o the rs cour tesy o jA PA-The

1ngineered Wood Associa tion)

(f)

(h)

(j)

98 / Chapt er 3 • WoodWood P roduct s / 99



v ene er s are t ran sformed into p ly -

wood in presses tha t app ly e leva ted

temperatures and pressures to create

d ens e, f la t p ane ls . The pa ne ls a re

trimmed to size, sanded as required,

and graded and gradestamped

b efore sh ip ping. Grad e B ven eer s

and higher are always sanded

smooth, but panels intended f or

sheathing are always left unsanded

because sanding slightly reduces the

thickness, which diminishes the struc-tur al st ren gth of the p ane l. Pa nel s

i nt ended for subfl oors and fl oor

underlayment are lightly t o uch s a n de d

to produce a more fla t and unifo rm

surface without seriously affecting

their structural performance.

Veneers for hardwood plywoods

intended for interio r paneling and

cabinetwork are usually sliced from

square blocks ofwood called flitches in

a machine that moves the flitch verti-

callyagainst a stationary knife (Figure

3 . 3 0 ). F l it c h -s l ic e d veneers are analo-

g ou s to quar te rs awn lumbe r: Th ey

exhibit a much t ight er and more

interesting grain figure than rotary-sl iced veneers. They can al so be

arranged on the plywood face in such

a wayasto produce symmetrical grain

patterns.

Standard plywood panels are 4 by

8 fee t (1220 x 2440 mrn) in surface

area and range in thickness from one-

quart er to I i nch (6.4- 25.4 mm ),

Longer panels are manufactured for

siding and industrial use. Actual sur-

f ac e d imen sion s o f the st ru ctural

g ra des of p lywood are sl igh tly l ess

than nominal . This permits the pan-

e ls to be ins ta lled with small spaces

between them to a llow for moisture

expansion . Composite panels andnonveneered panels a re manufac -

tured by ana logous processes to the

same set of standard sizes asplywood

panels and to some l ar ger si zes as

well.

FIGURE 3.31

Veneer grades for softwood plywood.

( c ou r te s y o j APA - T he Eng in e er e d W o o d A s s o·

ciation)

TABLE 1

VE NE E R GRAD E S

AS m oo th , p oi nt ab le . N ot m or e t ho n 1 8 n ea tl y m od e r ep ai rs , b oa t, s le d, o r r o ut er f yp e, a nd

p a ra l le l t o g r o i n, p e rm i tt ed . W o o d o r s y n t he ti c r e p ai rs p e rm it te d . M a y b e u s ed f o r n a tu r a l

f i ni sh i n l e ss d e ma nd i ng a p pl i ca ti on s .

BS o li d s u r f ac e . S h i ms , s l e d o r r o u te r r e pa i rs , a n d t ig h t k n o ts t o 1 i n ch a c ro s s g r o i n p e rm i tt ed .

W o o d o r s y n t he t ic r e p a ir s p e rm it te d . S om e m i n o r s p l it s p e rm i tt e d .

CI mp ro ve d C v en ee r w it h s pl it s l im it ed t o 1 /B ·i nc h w id th a nd k n ot ho le s o r o t h er o pe n d ef ec ts

l im i te d t o 1 / 4 x 1 /2 i nc h , W o o d o r s y n th e ti c r e pa ir s p e rm i tt ed . A d mi ts s o m e b r o k en g r oi n .

Plugged

R O TA R Y S L IC I NG

CT ig h t k n o ts t o 1 -1 /2 i nc h. K n ot ho le s t o 1 i n c h a c r o ss g r oi n a n d s o m e t o 1·1/2 i n c h i f t o t a l

w i dt h o f k n o t s a n d k n o th o l es i s w i th i n s p e ci f ie d l im i ts . S y n th e ti c o r w o o d r e p a ir s . D i s co l -

o r a ti o n a n d s a n d in g d e fe c ts t h a t d o n o t i m p ai r s t r en g t h p e rm it te d . l im i te d s p l it s a l l o w ed .

S t i tc h i ng p e rm i t te d .

DK n o t : a n d . k~oth~le~ t o 2 - 1 _ l2 - i n c h wid~hocro~sg~oin a n d . 1 /2 in~h l a rg e r w i t h in

s p ec i fi e d l im it s . L im i te d sp i r t s a re permitted. Sl t tchrng permitted. L Im i te d t o

E x p osu re 1 o r I n te r io r p a n e ls .

be used asroof sheathing over rafters

spaced 32 inches (813 mm) apart o r

as subflooring over joists spaced 16

i nches (406 mm) apart . The long

dimension of the sheet must be

placed perpendicular to the length of

the suppor ti ng membe rs . A 32/16

panel may be plywood, composite, or

OSB, may be composed of any

accep ted wood species , and may be

any of several thicknesses, so long as

i t passes t he st ruct ur al t ests for a

32/16 rating.

The d es ig ne r must al so s el ec t

from three e x po s u re d u r ab i li t y c l a ss i fi c a -

tions for structural wood panels: Ex te -

r io r ,Exposure 1,and Exposure 2. Panels

marke d "Exter io r" are s ui ta bl e for

use assiding or in other permanently

exposed applications. "Exposure I"

s p an r a ti ng . The span rat ing isdeter-

mined bylaboratory load testing and

i s g iven on the grades t amp on the

backof the panel, asshown inFigures

3.32 and 3. 33. The pur pose of the

span-ra ting sys tem is to permit the

use of many dif ferent species of

woods and types of panels to achieve

the same s truc tu ra l o bj ect iv es . A

panel with aspan rating of32/16 may

FIGURE 3.3~

Typical gradestamps for structural wood

panel s. Grade stamps a re found on the

back of each panel. (Courte sy o jAPA -The

Engineered Wood Associa tion)

T I l E E N G I N E E R E DWOOD ASSOC lAr lOH

T H E E N G I N E E R EDWOOD A S SOC IA T l aN

TH E E N G l H f f R E DWOOD A S SOC IA T l ON

1 - R A TE D S T UR O ·I -F L OO R

2 -24 D C 2 3 f. l2 IN C H- 6

3 _ T : ~ ' h F O ~ ~ ~ ~ ~ f l l4 - - E X P O S U R E 1

_OOO~75 - PSH5~~LA~8

1 R A T E D S H E A TH I N G

2 - 48/24 2 31 J 21 N C H 6

4SI.UDF O R 8 P AC IN G

- - E X P O S U R E 1 7

5-""j:S'2-sz°~~mlllGa-Pf tp·1CSHUD4lM. .wt : -11

C O N S TR U C IH I HS H E A ll II H G- 1 2

13 - 2R48/2F24

14--JllJmQ U A RT E R S L IC I NG

FIGURE 3.30

Veneers for structural plywood are rotary sliced, which isthe most economical

method. For bet te r control of g ra i n figure in face veneers ofhardwood ply_

wood, flitches are pla insliced or quartersliced. The g ra i n figure produced by

rotary slicing, asseen in the detail to the right, isextremely broad and uneven.

The finest figures are produced byquarterslic ing, which results in a very close

g ra i n pattern with prominent rays.

15 1S T R E H G n l A X I S

- nilS D lREtnDU

9 S id in g f a ce g ra de

1 0 S pe de s g ro u p n u mb er

1 1 H U D /F HA r e c o gn i t i o n

1 2 P an el grode, C a na d ia n s ta n da r d

1 3 P an el m a rk - R at in g a n d e nd -u sed es i gn a ti on , C a na d ia n s t an d ar d .

1 4 C a n ad i an p e rf a rma n ce - ro l edp a n e l s t a n da r d

1 5 P a ne l f a ce o r ie n ta ti o n i nd ic a to r

1 P o ne l g ra de

2 S pa n R at in g

3 T o n g u e- a n d -g r o o v e

4 E x p os u r e d u r a bi l it y c l as s i fi c at i on

5 P r o du c t S t a n da r d

6 Th i ck n es s

7 M i ll n u mb er

8 A PI \ s P er fo rm a nc e R at ed P an el

Standard

Specifying Structural Wood

Panels

For structural uses such as subfloor-

ing and sheathing , wood panels may

be specified either by thickness or by

100 / Chap ter 3 • Wood Chemica l Tre atment / 101



TABLE2

G UI DE T O A PA P ER FO RM AN CE R AT ED P AN EL S( o) (b )

F OR A PP LIC AT IO N R EC OM ME ND AT IO NS , S Ee F OL LO WIN G P AG ES .

APA

APARATED SHEATH ING

Typi cal T r ademarkmEmoHERD

WOODASSOClAOOU

R A T E O S H E A T I l m G

40/20 1 9 / 3 2 I N C H

S I U D f D R S P A C I l I 6

E X P D S U A E 1_ _ 0 0 0 __

PSHSlATfflNN 1 fUD-1 IlH«

APATHfNaHUUD

WOOD ASSOC/AllOH

R A T E D S H E A T l I l N G

2 4 / 1 6 m 6 I N C H

S l U D f O n l P A C l X G

E X PO S UR E 1_ _ 0 0 0 __

f ' I W o l N I t U D - U ~

S pe ci al ly d es ig ne d f or s ub fl oo ri ng a nd w o ll o nd r oo f s he at hi ng .

A ls o g oo d f or a b ro od r an ge o f o t he r c an st ru ct io n a nd i nd us tr ia l

a pp li ca ti on s. C an b e m an uf oc tu re d a s p ly wo od , a s a c om po si te ,

o r o s O S B. E X P OS U RE D U RAB I LI T Y CLA S SI F ICA T IO N S: E x te r io r ,

E xp os ur e 1 , E x po su re 2 . C O M MO N T HI CK NE SS ES : 5 /1 6, 3 /8 , 7 /1 6,

15/32,1/2,19/32,5/8,23/32,3/4.

APAA P A S T RU CT U RAL I

RATED SHEATHINGi,1

Typi col T r o demarkmEEHO/UI fRED

WOODASSOCIAJIOH

R A T E D S H E A T H I H GsmUCTURAll

3 2 / 1 6 " " " H C HS l Z I D f O l l $ P A C l l f n

E X P O S U R E 1_ _ 0 0 0 __

I S HS C ·D F f P · "

APAr E NG I NUR E O

WOODASSOCUlTlOH

f i A T E D S H E A T H l H G

3 2 / 1 6 1 5 1 3 " H C H

SlZt1 l1DRIPAtma

E X PO S UR E 1_ 0 0 0 __

S T RU C TU /W .I R A T EDPWIflU&MMHWIWAU.$

rAIIRllWllOOfSI ' f U ' . .U l a tMH /M - 4 «:

U ns an de d g ra de f or u se w he re s he er a nd c ro ss -p an el s tr en gt h

p ro p er ti es o r e o f m a xi m um i m po r ta n ce , s u ch a s p an el iz ed r o of s

a nd d ia ph ra gm s. C on b e m an uf ac tu re d a s p ly wo od , a s a c om po si te ,

o r a s O S B. E X P OS U RE D U RAB I LI T Y CLA S SI F ICA T IO N S: E x te ri o r,

Exposure 1. C OM M ON T HI CK NE SS ES : 5 /1 6, 3 /8 , 7 /1 6, 1 5/ 32 , 1 /2 ,

19/32,5/8,23/32,3/4.

APA RATEDSTURD -I-FLOOR =::===Typi co l T r ademark APA

tlrrHGNlfHOWOODASSOaA OU

R A T E D S T U R D · I · F l O D l \

24 O C " " ' , " C HS i l : E D 1 G f U P A t m G

T A i l an w w r n . t H / Z

E X P D S U R E 1_ 0 0 0 __

I 'U·tlilMGI.HlOOflm·1NW).~

APAntEfNllNRfD

WOODA UOC I A T lO H

R A T E D S T U R D - J . F l O O R

20 DC",,"HeHrnlOll)flSPMlliG

T&GKnWIOTll4H/Z

E X PO S U RE 1_ 0 0 0 __

r n 1 N H M J M«

S pe ci al ly d es ig ne d a s c om b in ot io n s ub fl oo r- un de rl oy m en t. P ro vi de s

s mo ot h s ur fa ce f or o pp li ca ti an o f c a rp et a nd p ad o nd p os se ss es h ig h

c on ce nt ra te d a nd i mp ac t lo ad r es is ta nc e. C on b e m an uf ac tu re d a s

p ly wo od , o s a c om po si te , o r a s O SB . A va ila ble s qu ar e e dg e o r t o ng ue -

o n d- g ro o v e. E X P OS U RE D U RAB I LI T Y CLA S SI F ICA T IO N S: E x te ri o r,

E xp o su re 1 , E x po su re 2 . C O MM O N T H IC K NE SS ES : 1 9/ 32 ,5 /8 ,

23/32,3/4,1,1-1/8.

APAAPA RATEDS ID ING

Typi cal T r ademarkm E l N GI H E lR E D

WOODASSOC IAT l OH

R A T E O S I D I H G

2 4 0 c " n 2 I H C HSIl10RlflSPAClHa

E X P O S U R E 1_ 0 0 0 __

fftP·lt1HIIO-II1d43C

APAf E O Hm o

WOODA $ S O C I A T JOH

R A T E O S I D l H G

3'J.1~SNl

1 6 D C = ~ N , C 1 lSlzro l~s rAtlml

m a l l O R_ 0 0 0 __

P l l- 9 S F 1 IP · lt 1f I l A . . . . . .

F o r e x t er io r s id i ng , f en ci ng , e tc . C a n b e m a nu fa ct u re d a s p ly w o od ,

a s a c om po si te o r a s a n o ve rl ai d O SB . B at h p on el o nd lo p s id in g

a va il ab le . S p ec io l s ur fo c e t re at me nt s uc h a s V -g ro o ve , c ha nn el g ro o ve ,

d ee p g ro ov e ( su ch o s A PA T ex tu re 1 -1 1) , b ru sh ed , r ou gh s aw n a nd

o v er la id ( M DO ) w i th s mo o th - o r t ex tu re -e m bo ss ed f oc e. S pa n R o ti ng

( st u d s p ac i ng f o r s i di n g q u a li f ie d f o r A P A S t u rd - I-W a ll a p pl ic a ti o ns )

a nd f ac e g ra de c la ss if ic at io n ( fo r v en ee r- fo ce d s id in g) i nd ic at ed i n

t ra d em a rk . E X P OS U RE D U RAB I LI T Y CLA S SI F ICA T IO N : E x te r io r .

COMMON THICKNESSES : 11 / 32, 3 / 8, 7 /16 , 15/ 32 ,1 / 2 ,19 / 32 , 5/ 8 .

( a ) S p ec i fi c g r od e s, t h ic k ne s se s a n d e x po s ur e d u ro b il it y c lo s si fi c at i on s m o y b e

i n l i m it ed s up pl y i n s o m e a re as . C h ec k w i th y ou r s up pl ie r b ef or e s pe ci fy in g.

( b) S pe ci fy P er fo rm a nc e R o te d P an el s b y t h ic kn es s o nd S po n R o ti ng .

S pa n R at in gs ' :I re b o s ed o n p an el s tr en gt h o n d s ti ff ne ss . S in ce t h es e p ro pe rt ie s

o re a f un di on o f p o n el c om p os it io n o n d c on fi gu ra ti on o s w e l l a s t hi ck ne ss ,

t he s om e S pa n R at in g m a y o pp eo r o n p cn el s o f d i ff er en t t hi ck ne ss .

C o nv er se ly , p on el s o f t h e s om e t hi ck ne ss m a y b e m o rk ed w i thd i ff er en t S p an R o ti n gs .

( c) A I ! p li es i n S t ru ct ur al I p l yw o od p o ne ls o re s pe ci e! i mp ro ve d g ro de s o nd

p on el s m ark ed P S l ore li mi te d t o G r ou p 1 s p ec ie s. O th er p cn el s m ar ke d

S t ru c tu r ol l R o t ed q u al if y t h ro u gh s pe c ic l p e rf o rm a nc e t e st i ng . S t ru c tu r a/ II

p ly w oo d p on el s a re a ls o p ro v id ed f or , b u t r ar el y m o nu fo d ur ed . A pp li ca ti on

r ec om m en da ti on s f or S tr ud ur al ll p ly w oo d a re i de nt ic al t o t ho se f or

A P A R A TE D S H E AT H IN G plywood.

FIGURE 3.33

Aguide to specifying structural panels. Plywood panels for use in wall paneling, furni-

ture, and other applications where appearance isimportant are graded by the visual

quality of their face veneers. ( C ou r te s y o f APA - T h e Eng i n ee r ed .W o o d A s s o c ia t io n )

panels have fullywaterproof glue but

do not haveveneers ofashigh a qual-

ity as those of "Exterior" panels; they

are suitable for structural sheathing

and subflooring, which must often

endure repeated wetting during con-

struction. "Exposure 2"is suitable for

panels that wil l be ful ly protected

from weather and willbe subjected to

a minimum of we tti ng dur ing con-

struction. About 95 percent of struc-

tural panel products are class if ied

"Exposure 1."

For panels intended asfinish sur-

faces,the quality of the face veneers is

o f obv ious concer n and should be

specified by the designer, For some

t)1)esofwork, fine flitch-sliced hard-

wood face veneers may be selected

rather than rotary-sliced softwood

veneers, and the matching pattern of

the veneers specified.

Oilier Wood Panel Products

Several types of nonstructural panels

ofwood f iber are often used in con-

struction. Hardboard i sa thin, dense

panel made of highly compressed

wood fibers. It isavailable in several

thicknesses and surface finishes, and

in some formulations i t i s durable

again st wea ther exposur e. Ha rd-

board is produced in configurations

for residential siding and roofing as

well as in general -purpose and peg-

board panels of standard dimension.

C a ne f ib e r b o ar d is a thick, low-density

panel with some thermal insulating

value;it isused in wood construction

chieflyas a nonstructural wall sheath-

ing. Panels made of recycled newspa-

per are low in cost, help to conservefores t resources , and are useful for

wall sheathing, tackboards, carpet

underlayment, and incorporation in

ce rta in p ropri et ar y t ypes of roof

decking and insulating assemblies.

CHEMICAL TREATMENT

Various chemical t reatments have

been deve loped t o coun te ra ct two

major weaknesses of wood: its com-

bustibi li ty and i ts suscept ibil ity to

at ta ck by decay and i ns ect s. Fi re -

retardant treatment is accomplishedby p lac ing lumber in a ves sel and

impregnating it under pressure with

cer ta in chemical sal ts that great ly

reduce itscombustibility. The cost of

fire-retardant-treated wood is such

that i t i s l it tl e used in s ingle- fami ly

residential construction. Its major

Usesare roof sheathing in attached

houses, and framing for nonst ruc-

tural par ti tions and other inter ior

componen ts in build ings o f f ire -

resistant construction.

I shall always remember

how as a child I played on the

wooden floor. The wide

board s were warm and

fri endly, and in the ir texture I

d is covered a r ich and

enchant ing wor ld ofveins and

knots. I also remember th e

comfort and security expert-

enced when fal ling asleep

next to th e round logs of an

old t imber wal l; a wall which

was not just a pla in surface

but had a plast ic presence

like everything alive. Thus

sight, touch, and even smell

were satisfied, which is as it

shou ld be when a ch ild meets

the world.

Christian Norherq-Schulz

Decay and insec t re si st ance is

veryimportant inwood that isused in

ornear the ground and inwoodused

for exposed outdoor structures such

as marine docks, fences, decks, and

porches. Decay-resistant treatment is

accomplished by pre ssure impregna tion

with any of several types of preserva-

tives. Creosote i s an oily der ivat ive of

coal that is widely used in engineer-

ingstructures, but because ofitsodor,

toxic ity , and unpa in tabil ity , i t is

unsuitable for most purposes in

building construction. Pentachlorophe-

nol is also impregnated as an oil

solut ion, and aswi th other oilypre-

servatives, wood treated with it can-

not be painted.

The most widely used preserva-

t ives in bui ld ing cons tr uct ion ar e

wa t er bo rn e s a lt s . For several decades,

the most common of these has been

c h ro m at ed c o pp er a r se na te ( C G A ) , which

imparts a greenish color to the wood

but permi ts subsequent paint ing or

s ta ining. But CCA preservat ives ,

despi te scanty evidence, have long

been under fire from environmental

organizat ions as a possible cause of

arsenic poisoning, especially in chil-

dren. Wood preservers have recently

made an agreement with the U.S.

government to phase out CCA in

favor of salts that do not contain

arsenic. One of these, a lk al in e c o pp er

q ua t ( AC Q ), i s bas ed on coppe r and

quaternary ammonia. Itcauses wood

to weather t o a wa rm brown colo r.

Another, c o pp er b o ro n a zo l e ( C BA ) , is a

boron compound. Preservatives can

be b rushed o r sp rayed on to wood,

but long-lasting protection (3 0 years

and more) can only be accomplished

by p re ssur e impr egnat ion. Wood

treated with waterborne salts isoften

sold without drying, which is appro-

p ri at e fo r u se i n the g round, but fo r

interior use, it must be kiln dried

after treatment. Because ofthe poiso-

nous nature of wood preservatives,

their use istightly controlled byenvi-

ronmental regulations.

The heartwood of some species

ofwood isnaturally resistant to decayand insects and can be used ins tead

of preservat ive- treated wood. The

most commonly used decay-resistant

species are Redwood, Bald cypress,

and Red and vVhitecedars. The sap-

wood of these species is no more

r es is tant to at ta ck than t ha t of any

other tree, so "All-Heart" grade

should be specified.

Woo d -- p o l) ,m er com p os i te p l a nk s are

intended for use inoutdoor construc-

tion. Atypical product ofthis typeisa

wood decking board made from

wood f iber and recycled polyethyl -

ene. I ts advantages are decay res is -

tance and easy workability. Most suchproducts are not asstiff assawn lum-

ber and require more closely spaced

supporting joists to avoid sagging.

Most wood-attacking organisms

need both a ir and moi st ure to l ive.

Accordingly, most can be kept out of

wood by constructing and maintain-

ing a bui lding so that i ts wood com-

ponents are kept dry at all t imes. This

includes keeping all wood well clear

o f the soil, ventil at ing a tt ic s and

crawlspaces to remove moisture,

102 / Chapter 3 • Wood WoodFasteners / 103



using good construction detailing to

keep wood dry , and repa ir ing roof

and plumbing leaks as soon as they

occur. Wood that is fully submerged

in f re shwa te r is immune to decay

because the water does not allow suf-

f ic ient oxygen to come i n cont ac t

wi th the wood for decay -caus ing

organisms to survive. Submersion in

salt water does not prevent deteriora-

t ion because of mar ine borers that

attack wood under these conditions.

WOOD

FASTENERS

Fasteners have alwaysbeen the weak

link in wood construction. The inter-

locking t imber connect ions of the

past, laboriously mortised and

pegged, were weak because much of

the wood in a joint had to be

removed tomake the connect ion. In

today's wood connections, which are

generally based on metal connecting

devices , i t i s usual ly impossible toinsert enough nails, screws, or bolts

i n a connec tion to deve lop t he f ul l

strength of the members being

joined. Adhesives and toothed plates

are often capable of achieving this

s trength, but are largely l imited to

factory installation. Fortunately, most

connections in wood structures

depend primarily on direct bearing

ofone member on another for their

strength, and a variety of simple fas-

.reners are adequate for the majority

of purposes.

Nails

Nails are sharpened metal pins that

are driven into wood with a hammer

or a mechanical nai l gun. Common

nails and finish nails are the two

types most frequently used. C ommon

na i ls havef lat heads and are used for

most structural connections in light

frame construction. F in is h n ai ls are

virtually headless and are used to fas-

ten finish woodwork, where they are

le ss obtru sive than common nail s

(Figure 3.34) .

~1 1 1 1 1 1 1 1 ~

CO MMON NA IL

~llllill 8>-

BOXNA I L

[ : 1 1 1 1 1 1 1 . 8>-CAS INGNA I L

01111111 S >-F INlSHNAlL

BRAD

D 1 1 1 1 1 1 I O D D l i l O O U l J l l l l l l D 1 l l l D I 1 0 n n O n n D n l l D ! l [ ! ! l E : l >DE FO R ME DS H A N K NA IL

;;§~

HARDENED CONCRETE

NAIL

CUTNA I L

ROOF ING NA I L

FIGURE 3.34

AIl nailed framingconnectionsaremade

wi th c om m on n ai ls o r t he ir m a ch in e- -

drivenequivalent.Box nails,whichare

madeof Iighter-gaugewire,do not have

asmuchholdingpoweras commonnails;

theyareused in constructionfor attach-

ingwood shingles.Casingnails,finish

nails,and bradsare usedfor attaching

finishcomponentsof a building.Their

headsare setbelowthe surface ofthe

woodwitha steel punch, and theholes

arefilledbeforepainting.Deformed

shanknails,whichare veryresistant to

withdrawalfrom thewood,are usedfor

such applicationsas attachinggypsum

wallboardand floor underlayment, mate-

rials thatcaunotbe allowedtoworkloose

in service. The most common deforma-

tionpattern isthe ring-shankpatternshownhere. Concretenailscanbe driven

short distancesinto masonryor concrete

for attachingfurringstripsand sleepers.

Cutnails,once usedfor framingconnec-

tions,nowservemostlyfor attaching fin-

ishflooringbecausetheirsquaretips

punch throughthe woodrather than

wedgethrough, minimizingsplittingof

brittlewoods.The large head on roofmg

nailsis needed to applysufficienthold-

ingpower to thesoft materialof which

asphaltshinglesare made.

FIGURE 3.35

Standardsizesof commonnails,

reproduced fullsize.The abbrevia-

t io n " d" s ta nd s f or " pe nn y. " T he

lengthof eachnailis givenbelowits

sizedesignation.Thethree sizesof

nailusedin lightframe construc-

tion, 16d,IOd,and8d, areshaded.

In the Uni ted States, the s izeof a

nail is measured in pennie s . This

strange unit probably originated long

ago asthe price ofa hundred nai ls of

a given size and persists in usedespite

the effects ofinflation on nail prices.

Figure 3.35 shows the dimensions of

the var ious s izes of common nai ls .

F in ish nai ls a re the s ame l ength a s

common nails of the corresponding

penny designation.

Nai ls are ordinar ily furnished

brigh t , meaning that theyare made of

plain, uncoated steel. Nails that will

be exposed to the weather should be

of a corrosion-resistant type, such as

h o t- d ip g a lv a ni z ed , a l um i nu m , or s ta in-le sss te e l . (The zinc coating on electro-

galvan iz ed nails is very t hin and is

often damaged during driving.) Cor-

ro si on re sis tance is pa rt icul ar ly

important for nails in exterior siding,

trim, and decks, which would be

stained by rust leaching from bright

nails.

The three methods of fas tening

with nai ls are shown in Figure 3.36.

Each of these methods has i ts uses in

building construction, as illustrated

FIGURE 3 - 36

Facenailing is thestrongest of the

three methodsof nailing.Endnailing

is relativelyweakand isuseful primar-

ilyforholdingframingmembersin

alignmentuntilgravityforces and

appliedsheathingmakea stronger

connection.Toe nailingis usedin situ-

ationswhereaccessfor endnailingis

notavailable.Toenails aresurpris-

inglystrong-load testsshowthem to

carry about five-sixthsasmuchload

asface nailsof thesamesize.

-

12d IO d 9d 8d 7d 6d 5d 4d 3d 2d

3 114 " 3" 2*" 2V 2 " 20" 2" IW llhfl l 1 f 4 " l"

83mm 76mm 70mm 64mm 57mm 51mm 44mm 38mm 32mm 25mm

I

ENDNAl£ TOENAI£

fundamentals of building construction-2

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