1. Roof Trusses

2. Definition
Load
Transferred
Load not
Transferred
Load
Transferred

3. Truss Types
Trusses are categorised into 3 groups depending on the shape of the top chord
Triangular Roof Trusses
Crescent roof Trusses
Other Types

4. Triangular Roof Trusses
Simple Triangular geometric shape
Web Bracing
Straight Top Chord

5. Triangular Roof Trusses

6. Crescent Roof Trusses
Top Chord is manufactured with a curved top chord
The Harbour bridge is a good example

7. Other Types Top Chords may be parallel – such as floor joist trusses
Or they may be nearly parallel – such as bridges

9. Terminolgy

11. Roof Truss Members

12. Roof Truss Panel Points

13. Roof Truss Stress Types

14. Parallel Chord Trusses
Top Chord & Bottom Chord are parallel
Used as Rafters
Advantages
Lighter
Larger Spans
Allow for easy access for services
Disadvantages
Cannot be site modified

15. Parallel Chord Trusses
Top Chord & Bottom Chord are parallel
Used as Rafters
Advantages
Lighter
Larger Spans
Allow for easy access for services
Disadvantages
Cannot be site modified
Generally Deeper

16. Parallel Chord Trusses

17. 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

18. Truss Manufacture Nailing Plates
Claw Type, only suitable for use with a press
Knuckle type, can be nailed with a hammer or pressed

19. 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

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

22. Transfer of Loads
Click to show load flow on correctly installed trusses
Tensile Load to
Counteract Compressive Load
Compression Load
Internal Wall
Min 12 Clear
No load in this Area

23. Transfer of Loads
Click to show load flow on incorrectly installed trusses
Bottom Chord is not designed to take horizontal load and will fail
Bottom Chord bearing on Internal Wall
Load transmits Horizontally to wall.

24. 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

25. Top Plates Option1 Using Nominal Thickness Top Plates (i.e. 90 x 35)
Place Studs directly under trusses

26. Top Plates If Trusses are not placed directly over studs
Top Plates may be overloaded and deflect and/or fail

27. Top Plates
Option 2
Increase Top Plate as per 1684

28. Top Plates & Girder Truss
It is always good practice to have studs as per concentrated loads beneath a girder truss

29. 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

30. Z Lintels

31. Lifting Roof Trusses Never lift by the Apex
This will damage the roof trusses

32. Lifting of Trusses
Note That lifting is done at Panel Points

33. Bundle Lifting
Ensure that trusses are tied or banded together at base

34. Storage of Trusses Trusses should be inspected on delivery
No site repairs with out design engineers supervision
Stored flat on timber dunnage

35. Carrying of Trusses

36. Installation of Trusses

37. Erection of Trusses

59. 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

60. Erection Tolerances

61. Erection Tolerances

62. Complex Roofs
Trusses can be made to suit just about any roof shape

63. Complex Roofs
Trusses can be made to suit just about any roof shape

64. Truss Layout

65. Trussed Hip Roofs
Jack End Truss

66. Hip Truss

67. 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

68. 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

69. Truss Layout – Scotch Valley

70. Scotch Valley

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

73. End Truss Support