. . Since our first attempts to enclose space for shelter, the most challenging process in building has been the carrying of loads over horizontal distances without touching the ground, ie. spanning between supports. Because of scale effects, it can be quite challenging to span the distances enc
1. Introduction to Timber Roof Trusses
10. Can we get rid of the double joint failure to save spaceCan we get rid of the double joint failure to save space
12. Truss Terminology Given the previous discussion, a truss can be described as a pre-fabricated, engineered building component which functions as a structural support member.
There are different types of trusses but the same basic terms apply:
Members are either top chords, bottom chords or webs
Each will be in tension or compression according to the type of truss involved
21. Truss Types - Standard Trusses King Post – has only one central vertical web. Used for small spans e.g. spans up to 5.0m.
23. Truss Types for Hip Roofs
24. Truncated Girder Truss - is the main truss in a hip end. It occurs below the standard truncated trusses and takes the load of the outer hip trusses including the hip, jack and creeper trusses. It is made stronger than the standard truncated trusses to take these loads.
25. Jack Truss – runs into the hip truss. It is also similar to a half truss but with an extended top chord extending over the truncated girder and meeting the hip truss.
26. Other Truss Types Scissor Truss - are modified standard trusses to suit a sloping ceiling. Most scissors have an equal pitch ceiling each side of the apex. Other ceiling lines are also possible
27. Variations to Truss Types
29. Can we get rid of the double joint failure to save spaceCan we get rid of the double joint failure to save space
31. Trusses are engineered to help in the following ways:
Much lighter timber members can be used because the predominant actions are tension and compression, not bending
The lighter timber can be predominantly sourced from plantations and easily kiln dried so there are no surprises as they dry out
Deflections are much smaller, particularly in the long term
The roof frame can be planned and prefabricated off-site, making it more possible to take advantage of an engineered design
32. Putting the Camber into Trusses During fabrication, trusses further improve on traditional rafter design by forcing an upward bend into the chords of trusses referred to as a “Camber”.
Camber helps to resist loads e.g. the amount of bend is calculated to help resist the load of tiles and ceiling lining. The calculations are designed to ensure the truss eventually flattens out to provide straight chords, once fully loaded.
33. Clear Spanning Internal Walls A benefit of trusses is that they can span long distances in one go. External walls are usually used to provide support but internal walls are not needed.
Internal walls cause problems if used for support because they change the way the truss works. To prevent this:
External load bearing walls are made slightly higher than internal walls, leaving a gap between the bottom chord and the internal wall
Special brackets fix the bottom chord to the internal wall – the brackets allow the bottom chord to move up and down in the gap (but not sideways)
34. Codes used in Truss installation Once trusses have been designed and manufactured according to previous engineering principles, the emphasis is on site installation practices
Australian Standard AS4440-2004 Installation of Nailplated Timber Trusses is the standard applied
It relates to residential construction (including BCA building classification 1,2,3 and 10) and light commercial structures.
It covers a broad variety of issues including:
Terms and definitions
Eaves and gables
Lifting, storage and temporary bracing practices
35. Limits to the application of AS4440 include:
Roofs with a maximum roof pitch of 45O
Roofs that are essentially rectangular layouts or a combination of rectangular elements
Roofs with a maximum truss span of 16m
Truss spacings at a maximum of 900mm for tiled roofs or 1200mm for metal sheet roofs
Nail plated trusses only
Maximum wind speeds - refer to either AS1170.2 or AS4055
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