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BUILDING CONSTRUCTION

An intimate knowledge of building construction is essential for the safety and

efficiency of Firefighters, Captains and Chief Officers. Possessing the knowledge of

construction types and the effect the fire will have on various construction methods are an

integral part of successful firefighting efforts, and substantially increasing the safety of

Firefighters working on scene.

It is not necessary for a firefighter to know how to build a wall or building from

scratch, but it is essential that they understand the strengths and weaknesses of a structure

and the probable effects fire will have on that structure.

The components in any structure are designed with one purpose, to counter the

effects of gravity. All buildings are designed to transfer the weight of the structure and

its contents, thru the structure to the ground. All structures will eventually collapse. Fire

has the effect of speeding up that collapse. Understanding how the components will fail

can tell Firefighters the approximate time a building can be expected to collapse under

various fire conditions.

Buildings can be basically divided into four styles, masonry, frame, metal, and

concrete.

MASONRY

Masonry buildings can be classified into two types. Unreinforced and reinforced.

Up until the late1940’s, most masonry buildings were constructed of brick. These

structures usually had no reinforcement in the brick. Mortar between the bricks was

usually substandard, containing a high amount of lime and little if any Portland cement.

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Unreinforced masonry can usually be identified by a “king’s row”, usually every six

courses, used to hold the bricks together structurally.

As unreinforced masonry increases in height the thickness must also increase to support

the weight of the bricks above. Because of this, usually unreinforced masonry is limited

to a few stories. To span openings in unreinforced masonry, pored concrete or arched

lintels are used to span door or window openings. Steel and wood lintels were also used.

Roof or floor members are usually set into beam pockets in the wall or nailed to rim

joists bolted thru the wall. Beam pockets are usually unrestrained and very often fire cut

to help prevent wall collapse in the event of fire or structural collapse.

Joist

Fire Cut

Furring Strip

Fin. floor

Metal Strap

Plaster

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Hazards of unreinforced masonry are that walls can collapse easily if roof or floor

sections fail and push or pull on the wall sections. Parapets are common in these

structures and pose substantial collapse potential from hose streams and the weight from

cantilevered overhangs or facades, as they pull on an unreinforced masonry wall having

little lateral strength.

REINFORCED MASONRY

Reinforced masonry is usually associated with block construction. In the late

1940’s concrete masonry units (CMU) or cinder block begin to replace brick

construction. Lime mortar is usually no longer present with block construction. After the

mid 70’s, block construction has the advantage of having steel reinforcement throughout

the structure in the form of steel lintels, grout cells and bond beams, greatly reducing the

potential for catastrophic collapse if properly constructed.

LintelBlock

ReinforcedSteel

PouredConcrete

WallOpening

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FRAME CONSTRUCTION

Frame construction involves building wall, roof, and floor structures from lumber.

Wall members are 2x4 or 2x6 dimensioned lumber 16” or 24” on center. Headers

composed of solid wood beams, box beams, or lightweight truss assemblies span window

and door openings. The exterior can be wood siding, stucco, brick or rock veneer.

Roof structures can be classified in two styles, conventional and lightweight

construction.

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CONVENTIONAL FRAMING

Conventional framing was the primary method of framing roof structures prior to

1960. Conventional framing consists of 2x4 or 2x6 roof rafters and ridge beams. King

posts help to support the roof structure. Collar ties run horizontally connecting the roof

rafters, helping to prevent the roof from spreading laterally.

In conventional framing, interior walls running in the direction of the ridge are

usually bearing walls. The ceiling joists are not part of the roof structure. Roof sheathing

prior to 1960 is 1x solid sheathing, 30 style buildings usually contain skip sheathing.

Planks

Beams

Post

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LIGHTWEIGHT CONSTRUCTION

In 1959 gusset plates were patented. This revolutionized the construction

industry by allowing lightweight trusses to become commercially available. Lightweight

trusses are an engineered product and are designed for a specific structure. Lightweight

trusses are designed as a series of triangles, each component of the lightweight truss is

dependent on the other for its support and strength.

2x4

2x6

Optional Overhang4

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3/8" W.P. Plywood Gussets

Members are held together by gusset plates or gang nails. In lightweight trusses

the top cord is under compression, the bottom cord is under tension.

12x

1/2 Span

Lightweight trusses, unless specifically designed, are designed to be supported by

the ends only. The bottom cord is unsupported. At this writing, lightweight trusses were

being designed for up to 128’ of clear unsupported span.

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Lightweight trusses come in different styles. Some of these are peaked truss,

scissor truss, bow, and parallel cord.

In the late 50’s plywood became commercially available, replacing 1x material

for sheathing, decreasing cost and labor.

In the early 80’s OSB (oriented strand board) becomes readily available. With

that came TJI’s or truss joist I beam. TJI is a wood I beam used in floor and roof

systems.

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TJI’s are composed of laminate top and bottom cords usually 2x2 with the center

webbing of OSB. The wood I beam concept has taken the mass out of older solid beam

components, removing the mass equates to lower building costs.

Structural Element

Non Structural

Solid Beam TJI

In tests, an equal size TJI is superior in strength to an equal size solid timber.

Unfortunately for firefighters it is true only until the material burns.

Framing multiple stories is accomplished two ways. The older method was called

balloon framing. This involved straight runs on sidewalls usually with no fire stops from

first to second floor. Interior floors were generally attached on the outside wall with a

rim joist. In older cities, pancaking of floors occurs when floors are impacted or burned

and the rim joist is sheered of the sidewalls.

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Rafter

Stud

1"x4'Let-InBrace

Let-InRibbon

Sub-Floor

Sill

Foundation

Ledger

Girder

Joist

Single Plate

PartitionStud

Double Plate

Joist

In platform framing each floor level is built before the next floor is built, virtually

eliminating fire travel, because of the fire stopping, and lessening the potential for a

pancake type collapse.

ROOF STYLES

There are numerous roof styles constructed using either conventional or

lightweight construction. In the Phoenix area the most common seen are.

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Gabel, common in ranch style. These can be constructed using conventional or

lightweight truss.

Hip, common in ranch style, can be conventional or lightweight truss.

Dutch Gabel, common in ranch style, can be conventional or lightweight

truss

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Saw tooth roofs are generally seen in industrial applications. Saw tooth roofs are

generally well constructed using larger lumber or steel trusses. The hazard is these

buildings have been neglected and are reaching advanced age.

True mansard, not common in Phoenix area but can pose a similar problem as hip

or gable roofs.

Roofing Material

Wood Sheathing

Rafters

Glass

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False mansard, attached to the outside of buildings, generally not fire stopped.

Basically a horizontal chimney. There can be breeches thru the buildings walls or

parapets into the roof structure. Any fire has the potential for rapid extension in a

mansard building.

Bow strung, is the older method of spanning large areas without interior

columns. Generally, larger dimensional construction plated and bolted together. Attic

spaces in Bow strung buildings can be 10’-20’ high. Most Bow strung buildings are

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approaching 50 – 80 years old. Originally, extremely strong and solidly built, fatigue and

neglect have taken a tole on many of the Bow strung buildings in the Phoenix area. Many

have been retrofitted with tie rods along the lower cord to increase their strength. Bow

strung trusses are usually placed 15’-25’ on center with 2x10 or greater perlins

connecting each truss, covered by 1x sheathing. In addition, most Bow strung buildings

are constructed using unreinforced or under-reinforced masonry walls. In a fire situation

the large attic space can conceal a large amount of fire for and extended amount of time

before detection. The failure of one truss has the potential of failing an extremely large

amount of roof area, along with collapsing other trusses in a domino effect. Collapse of

the front or rear of the building is probable because of the attachment of the perlins to the

front and rear facades. This is compounded by the placement of windows and doors in

the front and rear of the structures further weakening those areas.

Wide span, are gable style truss roofs presenting all the hazards of the Bow

strung.

Flat roof, commonly seen in residential and commercial occupancies, can be

constructed in several techniques. Conventional framed, constructed of larger

dimensional solid lumber, generally spanning small to medium buildings.

TJI, lightweight constructed TJI’s can span unsupported nearly 50’ and greater.

TJI’s when exposed to fire can fail rapidly. As one TJI fails the weight is transferred to

other members that cause a domino effect, failing large areas fairly rapidly. TJI’s are

easy to fire block across the trusses, but the space between the trusses can transmit fire

rapidly in that enclosed space. TJI’s can be used for both roof and floor joists.

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Parallel cord truss, is a lightweight truss where the top and bottom cord are

parallel and connected with gusset plates. All features of a lightweight truss apply.

Generally supported on the ends and intended to be unsupported in the center.

Parallel cord bar truss, constructed with either steel or wood top and bottom cord.

Top Chord(Compression)

Plywood RoofingMaterial

Metal Open Webbing

Bottom Chord(Tension)

Webbing can be relatively lightweight steel, capable of spanning large areas. These

trusses are supported by the top cord only. Commonly found in “Big Box” style

buildings.

Roofing Material

Concrete

Wire MeshCorrugated Metal

Many commercial bar trussed buildings may be covered with a lightweight

concrete or Q-decking material.

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A corrugated Q-decking material is placed over the trusses. Reinforcing wire is

added and lightweight concrete is pored over the metal decking. A waterproof material is

then placed over the concrete.

Top Chord(Compression)

Plywood orQ-Decking

Roofing Material(If Applicable)

Bottom Chord(Tension)

Unsupported

Open Webbing

Open Web

Steel Bar

Weld

Angle Irons

Another example is drywall or plywood sheets placed over the Q-deck. Then

foam panels or spray foam is applied, followed by the application of a waterproof

covering. These roofs can be confusing as fire will travel thru the voids and the roofs

will often appear compromised while probing.

Panelized roofs are commonly found in tilt slab buildings and allow large spans

with relatively few columns. The panelized roof generally is divided with large glue

Corrugated Metal

DrywallBuild -up

Roofing Material

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laminated beams. Perlins will be of a smaller dimension spanning a smaller area; each

additional segment is decreased in dimension and span until the smallest lumber may be a

2x4. Areas of the main girder may be very spongy feeling due to the small dimension

lumber in the final panel. Sheathing on panelized roofs is commonly only ½” plywood or

OSB. Under fire conditions, these roofs can be expected to fail rapidly because of their

lightweight makeup.

Roofing Material

4"x8" Plywood

Joist

Beam

Purlin

Metal Hanger Detail

13' to 40'

24"

8'

Metal buildings or metal clad buildings are normally wood or metal framed in

construction, covered with steel, aluminum or fiberglass panels. They are usually

commercial industrial buildings. They are easily recognized by the corrugated

appearance to the exterior. Skylights are commonly incorporated into the roof structure

and may be difficult to detect from the exterior, especially at night. The lightweight

construction and early failure under fire conditions, makes roof and interior operations in

a well-involved structure extremely hazardous.

Light Panel

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Tilt slab is a method of forming large buildings. The walls of these buildings are

constructed of onsite pored, reinforced concrete that are lifted into place and welded

together with steel plates that are imbedded into the panels. Roof and floor structures are

usually lightweight construction; panelized roofs often compose the roof. These

buildings are normally commercial and can be quite large. Occasionally seen in these

buildings are roof assemblies composed of pre-stressed concrete I-beams. Vertical

ventilations are ruled out if this structure is present.

Post and beam construction are usually associated with buildings requiring a great

deal of strength and stability. These buildings can be constructed of steel girders or

poured reinforced concrete. A series of columns tied together by beams form the

supporting framework. Most of these buildings are large industrial buildings. Roof

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structures in these buildings can be of various types running from saw tooth to panelized

depending on the date of construction.

A hazard to older post and beam construction are that wall panels between the

posts are typically unreinforced masonry. Although the structural components are

extremely strong the unreinforced masonry panels can fail independently of the main

structure when subjected to fire and or lateral movement.