tech brochures truss facts aus
TRANSCRIPT
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Truss acts book
An introduction to the history design and mechanics
o preabricated timber roo trusses.
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Table o contentsWhat is a truss? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
The evolution o trusses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
History... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Today . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6The universal truss plate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Engineered design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Proven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
How it works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Truss terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Truss numbering system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Truss shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Truss systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Gable end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Hip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Dutch hip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Girder and saddle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Special truss systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Cantilever . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Truss design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Truss analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Truss loading combination and load duration . . . . . . . . . . . . . . . . . . . . . . 20
Load duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Design o truss members. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Webs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Chords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Modication actors used in design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Standard and complex design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Basic truss mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Truss action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Defection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Design loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Live loads (rom AS1170 Part 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Top chord live loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Wind load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Terrain categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Seismic loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Truss handling and erection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Table o contents
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What is a truss?
What is a truss?
A truss is ormed when structural members are joined together
in triangular congurations.
The truss is one o the basic types o structural rames ormed
rom structural members. A truss consists o a group o ties and
struts designed and connected to orm a structure that acts as
a large span beam.
The members usually orm one or more triangles in a single
plane and are arranged so the external loads are applied at the
joints and thereore theoretically cause only axial tension or axialcompression in the members. The members are assumed to
be connected at their joints with rictionless hinges or pins that
allow the ends o the members to rotate slightly.
Because the members in a truss are assumed to be connected
to rictionless pins, the triangle ormed is the only stable shape.
Studies conducted on the truss show it is impossible or the
triangle to change shape under load, unless one or more o the
sides is either bent or broken.
Shapes o our or more sides are not stable and may collapse
under load, as shown in the ollowing images:
These structures may be deormed without a change in length
o any o their members.
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The evolution o trusses
The evolution o trusses
In only a ew decades, timber trusses have almost completely
replaced traditional roo construction methods.
Their advantage in allowing greater reedom o design and in speeding up construction, while
reducing the impact o external infuences including weather and building site thet, are major
actors contributing to their success.
Since 1979, Multinail has pioneered development o the engineering technology that made
these changes possible and ensures Multinail Fabricators continue to provide the highest quality
products at competitive price.
Multinail also produce all the specialised hardware or manuacturing building components
including roo trusses, wall rames and foor trusses, and also oers a large range o
high production machines and equipment used during abrication.
History...
Beore the 1940s, trusses were primarily constructed or large
buildings and bridges and manuactured rom heavy steel, with
wood members limited to timbers with bolted connections.
World War II, saw the demand or speedy military housing
construction that required less labour-intensive practices and
reduced job site time or raming roos. Timber members were
used to meet these new requirements, and connected together
using glued and nailed plywood gussets, or simply nailed to
joints, to orm wood trusses.
This method continued ater the war, with the boom o single
amily housing. To urther reduce the labour-intensive practice o
cutting plywood gussets and then gluing or nailing the gussets
to the timber, a light gauge metal plate was created with pre-drilled holes that allowed nails to be hammered through.
As the pre-drilled metal plates were inadequate and still labour
intensive, Arthur Carol Sanord developed an alternative - the
truss plate. His truss plate was the rst to use stamping to
create triangular teeth embedded at the truss panel points to
transer the structural loads across the joints.
In 1979, one o Americas leading building industry magazines
Automation in Housing and Systems Building News honoured
Arthur Carol Sanord or his singular invention o the toothed
metal connector plate in 1952.
Sanord made other contributions to the growing truss industry,
including contributing to the development o the rolling press,
a method where abricators use extremely high pressures to
embed nailplates into timber during truss manuacture. The
strength o the joints constructed using this method generates
trusses with predictable engineering properties.
Pre-abricated timber trusses have extended rom a simple
collection o individual timber members to become complete
building components or building entire roos or foors.
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TodayOver the next decade truss plates naturally progressed rom the
early truss plates that required hand-applied nails to modern
truss plates that require no nails.
As part o the nailplate progression, the Multinail Truss Plate was
developed.
Multinail nailplates are manuactured rom G300 steel with a
galvanised coating o 275 grams per square metre. Stainless
steel nailplates are also available, and nailplates can also
be powder-coated or have additional galvanising applied i
required.
Multinail nailplates incorporate many renements, especially
in the tooth shape that is designed to grip the timber more
securely. The bending and twisting o teeth during manuacture
was careully designed to aid the transer o orces across the
nished joint. This also increase the joints resistance to damage
during handling when orces may be applied rom virtually any
direction.
The reliability o Multinail trusses results rom the ollowing
actors:
Truss plate - made rom high grade steel to exacting
tolerances that maintains the reliability and perormance
essential to sae truss construction.
Quality o engineering design - based on Multinails years o
eort, talent and experience.
Methods or cutting and assembling timber members -
using automated saws and computer aided controls helps
ensure members and joints t accurately.
Care taken by Multinail abricators to ensure trusses are
manuactured in strict accordance with designs and
handled appropriately.
The evolution o trusses
Over time, the wood truss has become a highly-engineered,
preabricated structural product using two very reliable
resources, Wood and Steel.
The predicted lie o nailplates depends on the protection o the
nailplates rom the weather, wind and other corrosive elements
including salt spray and chemicals.
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The universal truss plate
The universal truss plate
FeaturesLong teeth;
Low plate cost per truss;
Penetrates high density hardwoods;
Eliminates tooth bending and wood splitting;
High orce transer per unit area;
High holding power in hardwoods and sotwoods and
Prime quality galvanised steel.
Engineered design
Manuacturing an engineered truss requires accurate cutting,jigging and pressing. Fabricators do not want timber splitting or
truss plate teeth bending during pressing as this can result in
production delays, site calls and increased costs o repairs and
rebuilding.
Timber splitting, teeth bending and associated problems can be
eliminated by using Multinails Universal Truss Plates.
The Universal Truss Plate is the ideal choice or abricators as
they are designed to provide excellent holding power (with eight
teeth per square inch) at a low cost per truss.
Multinail was also the rst company to introduce South East
Asia to the concept o eight teeth per square inch in nailplates.
At Multinail, we didnt just stop there. I you look careully at the
Universal Truss Plate, you can notice a uniquely-designed tooth
shape that gives the nailplate ull penetration and holding power
- other companies have tried to copy this method but have been
unable to reproduce.
ProvenExtensive tests in Australia and Asia have proved the versatility
o the Universal Truss Plate.
The Universal Truss Plate is suitable or use with high densitywoods (e.g. Ironbark and Karri rom Australia, Kapur and
Selangur Batu rom Malaysia) as well as other high and low
density hardwood timbers and low density woods (e.g. Radiata
Pine and Oregon).
How it worksThe Universal Truss Plate provides high density tooth
concentration that ensures high strength transer. When
combined with the universal tooth shape, this virtually eliminates
teeth bending or wood splitting.
The Universal Truss Plate produces tight tting joints that helpresist rough truss handling during delivery or on site.
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Truss terms
Truss terms
TrussA preabricated, engineered building component which unctions
as a structural support member.
MemberAny element (chord or web) o a truss.
ApexThe point on a truss at which the top chords meet.
Axial orceA orce (either compression or tension) that acts along the length
o a truss member. Measured in newtons (N) or kilo newtons
(kN)
Axial stressA measure o the intensity o an axial orce at a point along a
member, calculated by dividing the axial orce at that point by
the cross-sectional area o the member. Measured in mega
pascals (MPa).
BattensStructural members which are xed perpendicular to the top
chords o a truss to support roong material or to the bottom
chords to support ceiling material and to restrain truss rom
buckling.
Bending momentA measure o the intensity o the combined orces acting on a
member; ie, the reaction o a member to orces applied perpendicularto it (including the perpendicular components o applied orces).
The maximum bending moment is generally towards the centre o
a simple beam member.
Bending stressA measure o the intensity o the combined bending orces acting
on a member, calculated by dividing the bending moment by
the section modulus o the member. Measured in mega pascals
(MPa).
Bottom chordThe member which denes the bottom edge o the truss.
Usually horizontal, this member carries a combined tension and
bending stress under normal gravity loads.
Butt jointA joint perpendicular to the length o two members joined at
their ends.
CamberAn vertical displacement which is built into a truss to compensate
or the anticipated defection due to applied loads. All trusses
spanning relatively large distances are cambered.
CantileverWhere the support point o the truss is moved to an internal
position along the bottom chord o the truss.
Combined stressThe combined axial and bending stresses which act on a member
simultaneously; ie, the combination o compression and bending
stresses in a top chord or tension and bending stresses in a
bottom chord which typically occur under normal gravity loads.
Concentrated or point loadA load applied at a specic point; ie, a load arising rom a man
standing on the truss.
Pitching Point
Cantilever
Web
Ceiling
Top ChordNailplate
Roong
Bottom ChordBottom Chord
TieCeiling
Batten
CantileverTruss
Overhang
Truss Span
Truss
Overhang
Fascia
Pitching
Point
Web Tie
(Web Bracing)
Battens
Pitch
Web
Panel
Point
Overall
Height
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Truss terms
Cut-oThe term used to describe a truss which is based on a standard
shape but cut short o the ull span.
Dead load
The weight o all the permanent loads applied to member oa truss; ie, the weight o the member itsel, purlins, roong
ceilings, tiles, etc.
DeectionThe linear movement o a point on a member as a result o the
application o a load or combination o loads. A measure o the
deormation o a beam under load.
Eaves overhangThe extension o the top chord beyond the end o the truss to
orm the eaves o a roong structure.
HeelA point on a truss where the top and bottom chords join.
Hip jointThe joint between the sloping and horizontal top chords o a
truncated truss.
Interpanel spliceA splice in a member (at a specied distance rom a panel
point).
Laminated beam or trussTwo or more members or trusses mechanically astened to
act as a composite unit. Lamination allows the achievement
o increased strength without the use o solid, larger section
timber.
Lateral or longitudinal tieA member connected at right angles to a chord or web member
o a truss to restrain the member.
Live loadTemporary loads applied to the truss during maintenance by
workers and during constructions.
Load duration coefcientThe percentage increase in the stress allowed in a memberbased upon the length o time that the load causing the stress is
on the member. (The shorter the duration o the load, the higher
the Load Duration Coecient). (K1)
Manuacturing detailsDrawings which contain the data or truss abrication and
approval by local building authorities. (Produced automatically
by the sotware used by Multinail Fabricators.)
Mitre cutA cut in one or more members made at an angle to a plane o
the truss. I.e. the top or bottom chords o a creeper truss aremitred at 45 degrees at the end o the truss where they meet
the hip truss.
OverhangThe clear extension o a chord beyond the main structure o a
truss.
Panel
The chord segment o a truss, usually top or bottom chord,between two panel points.
Panel pointThe connection point between a chord and web.
Panel point spliceA splice joint in a chord which coincides with a panel point.
PitchThe angular slope o a roo or ceiling. Also the angular slope o
the top or bottom chords o a truss which orm and/or ollow the
line o a roo or ceiling.
Plumb cutA vertical cut. A plumb cut is perpendicular to a horizontal
member. All splices are plumb cut.
PurlinA structural member xed perpendicular to the top chord o a
truss to support roong.
SpanThe distance between the outer edges o the load-bearing walls
supporting the trusses.
Splice jointThe point at which top or bottom chords are joined (at or
between panel points) to orm a single truss member.
Support reactionsThose orces (usually resolved into horizontal and vertical
components) which are provided by the truss supports and are
equal and opposite to the sum o the applied orces.
Top chordsThe generally sloping members o a truss which dene its top
edge. Under normal gravity loads, these members usually carry
a combined compression and bending stress.
Truncated girder stationThe position o a truncated girder. Dened in terms o its distance
rom the end wall.
WebsMembers which join the top and bottom chords, and together
with them, orm a truss by which structural loads are transerred
to the truss support.
Wind loadsWinds loads are the orces applied to roo trusses by virtue
o wind blowing on the structure, typically (but not always)upwards; ie, opposite to dead loads.
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Truss numbering systemMultinail uses a simple, fexible and very versatile system o member and joint numbers to identiy all members and connectors.
LH Let heel
RH Right heel
TO Always allocated to the apex
BSO Always allocated to the joint immediately below the
apex joint
T1R Joint immediately to the right o TO joint
T2R Joint immediately to the right o T1R joint
T1L Joint immediately to the let o TO joint
T2L Joint immediately to the let o T1L joint
TCR Top chord on the right hand side o TO joint
TCL Top chord on the let hand side o TO joint
A variation to this Numbering System occurs when the Top
Chord contains a splice. The Top Chord is then allocated two
denotations:
TC1-R The upper Top Chord on the right hand side o the TO
joint
TC2-R The lower Top Chord on the right hand side o the TO
joint and on the right hand side o the splice
I the Top Chord contains three members, than the next Top
Chord would be marked TC3-R, etc.
Bottom ChordsFor the bottom chord, the numbers and markings are similar. I
the truss does not have a BO joint, then the joints are marked asB1R, etc. and B1L rom an imaginary line rom the TO connector.
Hence, the more joints, the more numbers to each side o this
imaginary line.
With standard trusses, there is normally only one splice joint per
bottom chord and the size and stress grade o each member is
the same, thus the bottom chords are numbered BC1 and BC2
as it is not critical to which side o the truss it is positioned.
For trusses with multiple bottom chords such as Cathedral
trusses in which there may be up to ve bottom chords (and
each may be a dierent size), the members are numbered rom
the let hand side o the truss and are marked as BC1, BC2BC5. This numbering refects the manner in which the truss
drawing is developed.
SplicesWhen a chord is spliced between panel points, it is marked as
LTS1 (being the rst splice in the top chord on the let hand sideo the TO position). Similarly to RTS1.
When the chord is spliced at a panel point, the joint is marked as
LTS2 or LBS3 relevant to the joint number in the top or bottom
chord.
WebsWebs (the internal truss members) are marked according to
their position in relation to the apex joint TO and the vertical web
under this joint, or the imaginary line rom the TO position.
Thus, webs to the let o TO are marked W1L, W2L, etc. and
webs to the right o this line are marked W1R, W2R, etc.
Note that it is possible that there may be more webs on one side
than the other o the TO position.
Truss numbering system
TO
BSO
HL HR
T1L T1R
TCL TCR
BC1 BC2
WOW1L
W1R
TO
HL HR
T1L T1R
TC1LTC1R
BC1 BC2
W1L
W1R
T2RT2L
TC2L
TCR2
BS@1R
(splice)
B2RB1LB2L
TS (splice)TS (splice)
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Truss shapes
Kingpost trussFor spans approximately 4m. Used primarily in house and
garage construction.
Queenpost trussFor spans approximately 6m. Used mainly or house
construction.
A-trussFor spans approximately 9m. This is the most commonly used
truss or both domestic and commercial applications.
B-trussFor spans approximately 13m. Used primarily in residential and
smaller commercial buildings, this truss is generally preerred
to the A-truss or larger spans since it oers greater strength
(additional web members) at lower cost due to the reduction in
size o top and bottom chord timber.
C-trussFor spans approximately 16m. Used principally or commercial
and industrial buildings. Can be constructed with lower
strength timbers.
These diagrams indicate the approximate shapes o the trusses in most common use. The choice o truss shape or a particular
application depends upon the loading and span requirements (general spans mentioned are or 90mm top and bottom chords).
Hal A-trussFor spans up to 6m. Used or residential construction where
the trusses may orm a decorative eature.
Hal B-trussFor spans up to 9m. Uses are similar to those or the A-truss.
Hal C-trussFor spans up to 11m. Uses are similar to those or the A-truss
but tends to be preerred to the hal A-truss or reasons givenunder C-truss.
Truncated trussSpans depend on depth. There are two types o Truncated truss
- the truncated girder truss and the standard truncated truss.
Together they acilitate hip roo construction.
Hip trussA hal truss with an extended top chord which is used to orm
the hip ridge o a hip roo.
Truss shapes
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Jack trussSimilar to the hal A-truss but with an extended mitred top
chord which overlies a truncated truss to meet the extended
top chord o a Hip truss.
Creeper trussSimilar to a jack truss but mitre cut to intersect the Hip truss
between the outer wall and the truncated girder.
Girder trussSpecial truss o any standard truss shape used to support other
trusses which meet it at right angles. (The standard shape is
maintained but girder truss members are generally larger in
both size and stress grade.) Girder can be any shape and carry
trusses at any angles.
Scissor trussNot a standard truss design but oten used to achieve special
vaulted ceiling eects, sometimes with relatively wide spans.
Hal scissor trussAs its name implies, one-hal o the standard scissor truss
used or industrial, commercial and residential buildings.
Pitched warrenGenerally used or industrial or commercial buildings to achieve
low pitch with high strength over large spans.
Dual pitch trussNon-standard truss used to achieve special architectural
eects. The let top chord is a dierent pitch to the right top
chord.
Cut-o trussAny truss, the shape o which orms part o a standard truss. As
the name implies, the cut-o truss has a shorter span than that
o the standard truss on which it is based; it is terminated by a
vertical member along the line o the cut-o.
Bowstring trussUsed or large span industrial and commercial buildings
including aircrat hangars, where the roo prole is curved.
Bowstrings can be used everywhere domestic included.
Howe trussFor spans up to 12m. Used mainly or applications which
involve high loading o the bottom chord (in preerence to the
A-truss).
Truss shapes
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Pratt trussFor spans up to 12m. May be used in preerence to Howe
truss or circumstances o high bottom chord loading.
Fan fnk trussFor spans up to 9m. Used mainly or applications which involve
high loading o the top chord (eg, where the truss is exposed
and the ceiling load is carried on the top chord).
Double howe trussFor spans up to 20m. Used or the same reasons as the Howe
truss (in preerence to the B-truss).
Parallel chordedAs its name implies, the top and bottom chord are parallel.
Used or both foor and roo applications.
Attic truss
Special purpose truss to simpliy attic construction.
Portal rameA standard commercial and industrial design or wide spans.
Inverted cantileverUsed to achieve special architectural eects in churches,
restaurants, motels, etc.
Cathedral trussA non-standard truss used mainly in residential buildings to
achieve a vaulted ceiling eect.
Bell trussA standard truss used to achieve a bell-shaped roofine. E.g. 4
top chord at dierent pitches.
Truss shapes
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Truss systems
Gable endThe Gable is one o the simplest and most common roo types.
Gable ends may be either fush or may overhang the end wall o
the building with overhangs being either open or boxed.
Gable ends can be ormed with a cutdown truss - a truss o
reduced height sometimes supported along its length by the
end wall. This truss can in turn support either verge raters (or a
fush gable) or an outrigger superstructure where an open gable
overhangs the end wall.
There are several possible orms o superstructure shown in the
diagrams. For example, the outrigger purlins may be supported
by the top chords o the cutdown truss extending inwards
to intersect the top chords o the next truss and extending
outwards to end in verge raters.
Alternatively under-purlins can be used, attached to the lower
edges o the top chords o the last two or three trusses.
A boxed gable can be constructed using a standard truss as
the end truss xed to cantilevered beams supported by the
buildings side walls. The cantilevered beams should extend at
least 2.5 times the cantilever distance and must be specically
designed or the expected load.
The position o the gable truss on the end wall is determined by
structural demands as well as by requirements or conveniently
xing purlins and roong material, ceiling material, gable end
battens and cladding.
Standard truss
Gable blocks
Outriggers
Gable verge rater
Batten
Cutdown truss
Outrigger purlins
Standardtruss
Batten
Prop
Under purlinsStandard
truss
Cantilevered beam
Standard truss
Cutdown truss
Under
purlins
Truss systems
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Hip
The hip roo truss system is built around a truncated girder that
transers the weight o the hipped roo sections to the side walls.
The system design species a girder station - the distance where
the Truncated Girder is located relative to the end wall.
The trusses that orm the hips o the roo (called the hip trusses)
are supported at their outer ends by the corners o the end or
side walls and at their inner ends by the truncated girder. Their
top chords which orm the hips are extended to meet the ridge
line.
On the end wall side o the truncated girder, the roo structure is
ormed by jack trusses (between the end wall and the truncated
girder) and creeper trusses (between the side walls and each o
the hip trusses).
On the internal side o the truncated girder, the roo structure is
ormed by truncated trusses o increasing height towards the
end o the ridge. The top chords o the jack trusses extend over
the truncated girder and truncated standard trusses.
The hip roo truss system allows the construction o traditional
roo design without needing to locate load-bearing walls to
support the hipped roo sections.
A number o variations are possible and are usually derived by
intersecting the rst hip system with another or dierent type o
truss system.
Truncated
girder
Truncated
standard
Standard
truss
Truncated
standard
Truncated
girder
Jack truss
Standard
trusses
Creeper
truss
Hip truss
Jack truss
Truss systems
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Dutch hip
The dutch hip truss system is built around a special girder truss
with a waling plate xed to one side.
As with the other girder trusses, it is placed at the specied
girder station. The roo structure is similar to the hip truss
system; however there is no need or truncated trusses - rather
than continuing over the girder truss, the top chords o the jack
trusses sit on the waling plate.
The result is a roo structure that combines some eatures o the
hip roo with eatures o the gable.
Girder truss
Girder truss Standard
truss
Waling
plateJack truss
Truss systems
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Girder and saddle
The girder and saddle truss eliminates the need or a load-bearing
wall at the intersection o two gables in the roo structure.
In this system, a girder truss placed parallel to the trusses in thesecond roo is used at the intersection to support the ends o
the trusses orming the rst roo. Truss boots are usually xed to
the girder truss to transer the load rom the trusses to the girder
and through the girder to the side walls.
The secondary roo line is continued past the girder truss by
saddle trusses that diminish in size.
Laminated
girder truss
Saddle
trusses
Laminatedgirder truss
Standard truss
Truss systems
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Special truss systems
ChimneysThe diagrams show typical roo structure treatment around
chimneys with standard trusses used at either side o the
chimney. The intervening cuto trusses are supported by beams
xed to the side walls o the chimney.
CutosCuto trusses are created when the truss must be stopped
short o its normal span (e.g. to allow or a chimney). This is
supported at one end by the heel in the standard manner and
at the other by the bottom chord immediately below the end
member. Double cuto trusses can be ormed by modiying
standard truss designs at both ends.
Hot water systemsSpecial provision must be made in the design and abrication
o trusses that carry additional loads such as those imposed byHot Water Systems.
Solar hot water systemsSpecial provision should be made in the design and abrication
o a roo truss system to carry the additional load imposed by a
solar hot water system.
Where this load will be carried by an existing roo not specically
designed or the purpose, a solar hot water heater with a
capacity o up to 300 litres may generally be installed on the roo
system provided the trusses within a roo length o approximately
3600mm are suitably modied. Details o these modications
(which take the orm o strengthening both the chords and jointso each truss) can be obtained rom Multinail Fabricators.
Chimney
Chimney
Chimney
Chimney
Parallel chorded girder trussesBeam
Cuto trusses supported by beam
Cuto
trusses
Beam or parallel chorded girder trusses
Cuto
trusses
Solar hot water
system
Truss systems
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CantileverA cantilever exists where a truss is supported inside its span
rather than at the end o its bottom chord (i.e. at the heel). A
truss may be cantilevered on one side only, or on both.
The diagrams show the three main types o cantilever:
A. Where the bearing o the truss alls wholly within the solid
length o the heel joint, the standard truss requires no
alteration.
B. Where the bearing o the truss is relatively close to the
heel but outside o the solid length o the heel joint, a
supplementary top chord is required to convert a standard
truss into a cantilever truss.
C. Where the distance between the heel joint and the bearing
o the truss is relatively large, additional members must be
used.
Cantilever
Cantilever
Cantilever
Truss systems
The maximum length o cantilever that can be handled using
standard design inormation is limited (e.g. trusses with cantilever
distances up to 1/5 nominal span or Type A Trusses, 1/6
nominal span or Type B Trusses and a combined distance o1/4 the nominal span or any truss). Larger cantilevers demand
special design.
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Introduction
This section provides a brie introduction to the techniques ortruss design; it is not intended as a comprehensive guide.
The design o the truss members can begin immediately ater
determining the anticipated loadings (i.e. Dead Load, Live Load
and Wind Load) .
Truss analysisFor truss shapes, where members and joints orm a ully
triangulated system (i.e. statically determinant trusses), truss
analysis makes the ollowing assumptions;
i) Chords are continuous members or bending moment,
shear and defection calculations. Negative moments at
joint (nodes) areas evaluated using Clapyrons Theorem o
Three Moments and these moments are used to calculate
the shear and defection values at any point along the chord,
or distributed and concentrated loads.
ii) Member orces can be calculated using a pure truss (i.e.
all members pin-jointed) and calculated using either Maxwell
Diagram or equilibrium o orces at joints.
iii) Total truss defection used to evaluate truss camber and to
limit overall defections can be calculated using the system
o virtual work. Again, the members are considered as pin-
ended and a dummy unit load is placed at the required point
o defection.
Truss loading combinationand load duration
The ollowing load combinations are used when designing all
trusses:
a) SW + DL: permanent duration
b) SW + DL + SLL: short term live load combination
c) SW + DL + MLL: medium term live load combination
d) SW + DL + WL: extremely short duration
Where, SW = Sel Weight (timber trusses)
DL = Dead Loads (tiles, plaster)
SLL, MLL = Live Loads (people, snow)
WL = Wind Loads
Each member in the truss is checked or strength under all
three combinations o loadings. Dead loads plus live loads and
dead loads plus wind loads may constitute several separate
combinations in order to have checked the worst possible
combination.
Load duration
The limit state stress in a timber member is depends on the loadduration actor (K1). For a combination o loads, the selected
load duration actor is the actor corresponding to the shortest
duration load in the combination.
For trusses designed according to AS1720.1-1997, the duration
o a load considered to act on a truss is o major importance or
dead loads only; the load is considered permanent and thus
actor K1 at 0.57 is used. For dead and wind load combinations,
the wind load duration is considered as gusts o extremely short
duration and K1 o 1.15 (or timber) is used. For dead and live
load combinations several load cases may have to be checked
due to diering load durations. In general, live loads are taken as
applicable or up to 5 hours with a K1 o 0.94 (or timber). Liveloads on overhangs are applicable or up to 5 hours with a K1 o
0.97. Either may be critical.
Design o truss membersTruss webs are designed or axial orces and chords, or axial
orces plus bending moments and checked or shear and
defection between web junctions.
WebsTension webs are checked or slenderness and the nett cross-
sectional area is used to evaluate the tension stress. The cross-
sectional area is taken as the product o the actual member
depth and the thickness - less 6mm to allow or timber bre
damage by the Multinail Connector Plate.
Compression webs are also checked or slenderness. Eective
length is used or buckling o the web in the plane o the truss
and out o the truss plane.
Truss design
Truss shapes
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ChordsTension chords are designed or strength and stiness and must
withstand combined tension and bending. The slenderness o
the member is checked as tension webs and also as a beam.
The shear o the member is also checked but is usually criticalonly on heavily loaded members (e.g. girder trusses).
The stiness criteria is to limit the defection o a chord between
the panel points. The long term defection is calculated or dead
loads only, as the instantaneous defection under this load
multiplied by duration actor (rom AS1720) is determined by the
moisture condition o the timber. The limit on this defection is
(panel length) 300.
Live load and wind load defections are calculated separately
without consideration o the dead load defection. Defection is
limited or live and wind loads to overcome damage to cladding
materials and to reduce unsightly bows in the roo or ceiling.
Compression chords are also designed or strength and
stiness. The strength o compression chords depends largely
on the lateral restraint conditions o the chords. The combined
compression and bending stresses in the members are
checked using the index equation in AS1720. Shear stress is
also checked as or tension chords.
Defections are checked as or tension chords and designed or
similar allowable values.
Modifcation actors used in design
The ollowing is a typical calculation or bending strength. The
capacity in bending (M) o unnotched beams, or strength limit
state, shall satisy -
(M) M*
where:
(M) = k[bZ]
and,
M* = design action eect in bending
= capacity actor
k = k1 x k4 . x k12 and is the cumulative eective
o the appropriate modication actors
b = characteristic strength in bending
Z = section modulus o beam about the axis o bending
(bd2/6)
Standard and complex designBoth standard truss designs and complex truss designs can be
generated by Multinail Fabricators or Multinail Engineers.
When a complex design is generated by the Fabricator or a
quotation job, it is standard practice or a copy o the input and
output to be checked by an engineer - either an independent
consultant or a Multinail Engineer - beore manuacturing the
truss.
For large projects (e.g. hospitals, schools, oces, etc.) the
entire project is initially analysed and an overall truss and
bracing layout completed. Each truss is then individually
analysed, designed, drawn to scale, costed and presented with
ull cutting and jig layout dimensions to ensure accurate and
uniorm manuacture.
Trusses are usually analysed and designed or dead, live and
wind loads; however the analysis and design may be extended
to include concentrated point loads as required. Trusses can
also be analysed and designed or snow load, impact loads,
moving loads, seismic loads, etc.
I the drawing species the purpose o the structure and the
anticipated loads, all the loads will be considered during truss
analysis and design and clearly itemised on the drawing.
Computations can also be supplied i required.
Truss shapes
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Basic truss mechanicsBending
Beams are subject to bending stress (e.g. scaold plank, divingboard, etc.). The actual bending stress b = Bending/Section
Modulus.
Introduction
The Australian Standard AS1720.1 Timber Structures Codeoutlines the design properties o timbers or bending, tension,
shear and compression.
Multinail sotware checks that truss member stresses do not
exceed the allowable values and, i required, larger members or
higher strength timber are considered to help ensure stresses
do not exceed allowable values.
Stress levels in nailplates are checked against the allowable tooth
pickup and steel strength values that Multinail has determined
over the years with dierent timbers.
TensionA member in tension is subject to tensile stress
(e.g. tow rope or chain).
Tension Stress (in a member)
= P/A in MPa
Where: P = Load in Newtons
A = Area in Square mm
Compression
A member in compression is tending to buckle or crush. Long
compression members buckle and are weaker than short ones
which crush. The allowable compression stress or a particular
timber depends on the slenderness ratio which is the greater
o length/width or length/depth.
By doubling the length (L) or load (P), you double the bending
moment.
Pull Pull
Push
D B
L
Push
Foc
Allowablestress
10 20 30 40 50
Slenderness ratio L or L
D B
P
P
2L
2
L
2
P
2
The maximum bending moment = P x L = PL2 2 4
Basic truss mechanics
The Section Modulus (Z) is the resistance o a beam section
to bending stress. This property depends upon size and cross
sectional shape and or a uniormly rectangular shaped beam:
Z = bd2
6
Where: b = width in mm
d = depth in mm
NOTE:Z depends on the square o d, so doubling the depth increases
the strength o the beam our times.
For example:
Increasing 125 x 38 to 150 x 38 gives a sectional modulus
o 1425003 which is stronger than a 125 x 50 which has a
section modulus o 130208 mm3.
The depth is simply more important than the width. Deep beams
require more careul lateral restraint.
The Bending Moment depends on the load and the length o
the beam.
For example:
Consider a simply supported beam carrying a Point Load P at
midspan.
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Truss actionA truss is like a large beam with each member in tension or
compression, the chords acting as beams between the panel
points as well as carrying axial load.
At any joint, the sum o the orces acting must be zero
(otherwise motion would occur) - this enables the orces to be
determined.
For example:
By measuring (or scaling) or by using simple trig. The orces are
ound to be 37.3 kN tension in the bottom chord and 38.6 kNcompression in the top chord.
The ollowing diagrams show typical Tension (T) Compression
(C) orces in the modulus o a truss under uniormly distributed
gravity loads.
DeectionDuring the analysis process when designing a truss, a number
o defection calculations are made to determine:
A) Chord inter-panel deectionThe actual defection o the timber chord between panel pointsis calculated and compared to the allowable defection by the
Australian Standard or stricter limits that may be applied.
B) Joint deectionEach joint within the truss is checked or vertical and horizontal
defection. In a fat (i.e. horizontal) bottom chord truss, there is
no horizontal defection in the bottom chord panel points and
only some small horizontal defection in that top chord panels.
The defection calculated in the bottom chord panel points is used
to calculate camber built into the truss during manuacture.
NOTE:For trusses without horizontal bottom chords, the horizontal
defection is very important as it may cause the supportingstructure to defect outwards.
Care must be taken in applying the truss loads, xing the truss
to the bearing point and maybe even design o the supporting
structure, to resist these loads.
P1-topchord
P2 - bottom chord
Reaction 10.0 KN
15
P1=38.6
kNcompr
ession
P2 = 37.3kN tension
10kN
O C C O
T T T
O
C T C C T
O
C
C C C
O T O
C T C O C T C
C
C
C
C
T T
C
T T
C
T
T T
C
C
C
C
C C
T
T T T
T
Joint Inter-panel defection
Basic truss mechanics
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Design loadsThe ollowing details contain the basic dead, live and wind
loads used or all truss design. The loads are used in as many
combinations as required to achieve the most adverse loads on
a particular truss.
Dead loads are those loads considered to be applied to a truss
system or the duration o the lie o the structure. They include
the weight o roo sheeting and purlins, ceiling material and
battens, wind bracing, insulation, sel-weight o the truss, hot
water tanks, walls, etc.
Loads are considered in two major combinations:
Maximum dead load value - used or calculations involvinga.
all dead and live load combinations and or wind load (acting
down on the truss) which is an additional gravity load.
Minimum dead load value - used in combination with windb.load causing maximum possible uplit on the structure, thus
achieving the largest stress reversal in the truss members.
The ollowing tables show examples o loads used or truss
details.
NOTE:This inormation is subject to changes according to Code
requirements.
Tiles = Approximately 55kg/m2
Sheet Roo = Approximately 12kg/m2
Plaster = Approximately 10kg/m2
Live loads (rom AS1170 Part 1)
Top chord live loads
For non-tracable roos (rom AS1170 Part 1 Section 4.8).
Where the area supported by the truss exceeds 14m2, a valueo 0.25KPa live load is applied over the plan area o the roo.
I the supported area is less than 14m2, the value o live load is
taken as:
= 1.8 + 0.12KPa(Supported Area)
The supported area is usually the product o the truss span and
spacing.
Bottom chord live loads(From AS1170 Part 1 Section 3.7.3)
The load is assumed as that o a man standing in the centre o a
particular panel o the truss bottom chord. The value is taken as
1.4kN where the internal height o the truss exceeds 1200mm
and 0.9kN or height less than 1200mm.
For exposed trusses (section 4.8.3.2), the bottom chord load
is taken as 1.3kN applied at each end panel point in turn and
centrally in a panel where the internal height exceeds 1200mm.
For exposed Industrial and Commercial Buildings (Section
4.8.3.1) the bottom chord load is taken as 4.5kN applied at
each bottom chord panel point, taken one at a time.
Basic truss mechanics
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Wind load
1. Basic wind velocity VpBasic wind velocity Vp is determined rom Table 3.2.3 or capital
cities in AS1170 Part 2. For other areas, the ollowing basic
wind velocities can be used in timber structures:
Region A - 41m/sec;
Region B - 49m/sec;
Region C - 57m/sec;
Region D - 69m/sec
2. Design wind velocity VzVz = V x M(z, cat) x Ms x Mt x Mi
Where: M(Z, cat) = Terrain-Height Multiplier
(Table 3.2.5.1 and Table 3.2.5.2)
Ms = Shielding Multiplier
(Table 3.2.7)
Mt = Topographic Multiplier
(Table 3.2.8)
Mi = Structure Importance Multiplier
(Table 3.2.9 in AS1170 Part 2)
3. Wind pressure (P)Wind Pressure (P) = 0.0006 x Vz2 (KPa)
4. ExamplesAssuming a house with an eaves height o less than 5 metres
and in a Category 2 Region C area.
The Wind Load is calculated as ollows:
Basic Wind Velocity = 57.0 (m/sec)
M(z, cat) = 0.91
Ms = 1.0
Mt = 1.0
Design Wind Velocity = 57 x 0.91 x 1 x 1 = 52(m/sec)
Wind Pressure (P) = 522 x 0.0006
P = 1.6224 (KPa)
The results are summarised in the ollowing
Other design criteriaRoo Span - 10 metres
Roong - Sheeting
Ceiling - Plasterboard
Roo Pitch - 15
Truss Spacing - 900mm
Timber - Green Hardwood
Web Conguration - A Type
The Wind Uplit orce o each truss at support is calculated as
ollows:
Wind Uplit = (P-DL) x Spacing x (Int. + Ext.) x Span/2
Where:
Dead Load = DL, including roong and ceiling material and sel
weight o truss.
NOTE:This inormation is provided as a guide only.
Wind load calculation changes occur continuously and you
must careully consult the relevant Codes and other sources
beore undertaking this task.
Multinails TrusSource (Truss Design Sotware) perorms these
calculations automatically, based on the latest Code renementsand best practice design criteria.
Case Int. + Ext. TC Size Web1 Size Uplit (KN)
1 0.8 100*38-F11 75*38-F11 3.662
2 1.2 125*38-F11 100*38-F11 6.340
3 1.3 100*38-F11 125*38-F11 6.972
4 1.7 125*38-F11 175*38-F11 9.631
Basic truss mechanics
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Terrain categoriesThe surrounding terrain aects the wind orces acting on a
structure. The design wind velocity depends upon whether the
structure is exposed, is on an open or hilly terrain with or without
scattered obstructions o varying heights, or is in a well woodedor heavily built up area such as suburbs, industrial areas, cities,
etc. These and many other actors aect the wind orces on the
structure.
To assist in the selection o the terrain category, the ollowing
sketch and notes have been produced. Also reer to AS1170
Part 2 or more detailed explanations.
NOTE: For structures located in areas with gradual terrain
changes (e.g. rom a low category number to a high category
number, or conversely a high category number to a lower
category number), the structure is subject to either a reduction
or increase in the design wind velocity. This relationship is calledetch/height. For details reer to AS1170 Part 2.
Category 1Exposed open terrain with ew or no obstructions. Average
height o obstructions surrounding structure less than 1.5m.
Includes open seacoast and fat treeless plains.
Category 2Open terrain with well scattered obstructions having heights
generally 1.5m to 10m. Includes airelds, open parklands and
undeveloped, sparsely built-up outskirts o towns and suburbs.
Category 3Terrain with numerous obstructions the size o domestic houses.
Includes well wooded areas, suburbs, towns and industrial areas
ully or partially developed.
Seismic loadsThe Australian Standard or Earthquakes AS1170.4 considers
houses as ductile structures. In order to determine whether
seismic loading aects the structure, a number o actors must
be known including the acceleration coecient that depends onthe geographic location o the structure; and the site actor that
depends on the soil prole.
When considering seismic loading, the connection o the wall
supporting members to the roo trusses must be capable o
resisting horizontal orces generated by the seismic activity.
According to Australian Loading Code, AS1170.4-1993, only
ductile structures in the highest earthquake design category -
H3 - require the design o the connection to resist a horizontal
orce at the top plate equal to approximately 5% o the dead load
reaction. All other earthquake design categories or domestic
structures require no specic earthquake design requirements.
In general, i a structure needs to withstand seismic loads, you
should consult a Multinail engineer.
Category 3
Category 2
Category 1
Category 2
Category 1
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B.0
1.Au