Roof Trusses

Definition

LoadTransferred

LoadTransferred

Load notTransferred

Truss Types

• Trusses are categorised into 3 groups depending on the shape of the top chord

1. Triangular Roof Trusses2. Crescent roof Trusses3. Other Types

Triangular Roof Trusses

• Simple Triangular geometric shape• Web Bracing• Straight Top Chord

Triangular Roof Trusses

Crescent Roof Trusses

•Top Chord is manufactured with a curved top chord•The Harbour bridge is a good example

Other Types

• Top Chords may be parallel – such as floor joist trusses

• Or they may be nearly parallel – such as bridges

Terminolgy

Roof Truss Members

Roof Truss Panel Points

Roof Truss Stress Types

Parallel Chord TrussesTop Chord & Bottom Chord are parallel

Used as RaftersAdvantages•Lighter•Larger Spans •Allow for easy access for services

Disadvantages•Cannot be site modified

Parallel Chord TrussesTop Chord & Bottom Chord are parallel

Used as RaftersAdvantages•Lighter•Larger Spans •Allow for easy access for services

Disadvantages•Cannot be site modified•Generally Deeper

Parallel Chord Trusses

Truss Manufacture

• Designed by Structural Engineer• No Site Modification or repair without

engineer supervision• Trusses manufactured in controlled factory

conditions to ensure design is strictly followed

Truss Manufacture

• Nailing Plates– Claw Type, only suitable

for use with a press

– Knuckle type, can be nailed with a hammer or pressed

Truss Manufacture

• Member Sizes must be specified by engineer• Trusses Manufactured in factory in controlled

environment• Members are assembled and cut in jigs and

presses

Camber

Trusses are manufactured with camber in the bottom chord1.To allow for calculated deflection while dead loads such as Roof Covering & Ceiling Linings 2.Bottom chord should not be supported between supports, unless specifically designed

Transfer of Loads

Internal WallMin 12 Clear

Click to show load flow on correctly installed trusses

Compression Load

Tensile Load toCounteract Compressive Load

No load in this Area

Transfer of Loads

Bottom Chord bearing on Internal Wall

Click to show load flow on incorrectly installed trusses

Load transmits Horizontally to wall.

Bottom Chord is not designed to take horizontal load and will fail

Support to Trusses

• Top Plates Based on AS 1684 Span Tables• As no internal support walls, spans are large• Loads imposed on top plates are greater than

conventional roofs

Top Plates• Option1• Using Nominal Thickness Top

Plates (i.e. 90 x 35) • Place Studs directly under trusses

Top Plates

• If Trusses are not placed directly over studs

• Top Plates may be overloaded and deflect and/or fail

Top Plates• Option 2• Increase Top Plate

as per 1684

Top Plates & Girder Truss

• It is always good practice to have studs as per concentrated loads beneath a girder truss

Lintels

• Similarly Lintels should be sized according to AS 1684

• As the spans are larger than a conventional roofs , large members may be required

• An options may be to use C & Z metal lintels

Z Lintels

Lifting Roof Trusses

• Never lift by the Apex• This will damage the roof trusses

Lifting of Trusses

Note That lifting is done at Panel Points

Bundle Lifting

Ensure that trusses are tied or banded together at base

Storage of Trusses

• Trusses should be inspected on delivery• No site repairs with out design engineers

supervision• Stored flat on timber dunnage

Carrying of Trusses

Installation of Trusses

Erection of Trusses

Erection of Trusses• Note – All Trusses need to Temporarily Braced

during Installation

The purpose of temporary bracing is to hold the trusses plumb & true until permanent bracing is installed

Erection Tolerances

Erection Tolerances

Complex Roofs• Trusses can be made to suit just about any

roof shape

Complex Roofs• Trusses can be made to suit just about any

roof shape

Truss Layout

Trussed Hip Roofs

• Jack End Truss

Hip Truss

Truncated Girder Truss• Placed at position determined by Engineer• Will take load of Jack Truss, Hip Truss &

Creeper Truss• Designed to take more load than other trusses

Truncated Truss•Placed between girder truss and gathering point•Important to be placed at specified spacing at they will increase in height to match the hip end•Made to similar specification to standard truss as they take no extra load

Truss Layout – Scotch Valley

Scotch Valley

Saddle or Valley Truss• Used to form diminishing Minor Roof• Edges form valley

End Truss Support