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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