Trusses Print E-mail
Trusses are an effective way of using smaller dimension timber rather than larger dimension timber to carry a particular load. Trusses can often span greater distances than commercially available large dimension timber.

Roof trusses use less timber than a conventional pitched roof and are able to span the full width of the building where as a conventional pitched roof usually requires support from internal load bearing walls.

Parallel chord trusses are often used as floor joists. They use less timber than solid floor joists and can span much greater distances. Their structure allows services to run "through" the trusses.

Heavy engineering type trusses use large dimension timbers but by their design they allow those timbers to be used more effectively. Examples of "heavy engineering type trusses" can be seen in wooden truss bridges spanning waterways in the road network.

A truss is a structure comprising one or more triangular units. Each triangle is constructed with straight and mostly slender members whose ends are connected at joints. External forces and reactions to those forces are considered to act only at the joints and result in forces in the members which are either tensile or compressive forces. A truss gets its strength from this triangulation, which bands elements together to act in structurally advantageous ways. For instance, each member can be assigned to work in a network of tension and compression members, and by doing this, greater structural efficiency is possible. For the types of trusses described above, compression members often dictate the size of the elements, and for this reason, designs that have short compression members, or restraint against lateral buckling, are generally more efficient than trusses with long compression members.

In a building, timber roof trusses can be either hidden from the occupant's view or exposed to the occupant's view.

Those roof trusses that are to be hidden from the occupant's view are described as nail plated trusses and use nail plates as connectors.

Those roof trusses that are to be exposed to the occupant's view are described as architectural trusses to denote attractively detailed timber truss work exposed to view.

A nail plated truss is usually loaded (and restrained) on the bottom chord by the weight of the ceiling while Architectural Trusses usually carry the ceiling and roofing on the top chord.

Architectural trusses are often used as part of a "cathedral ceiling" system and this part of the structure creates an eye-catching appearance and they may be light or heavy to suit the architectural theme. They could be left natural or they could be oiled, stained or even highly decorated. Connections may be very much in evidence or concealed.
Figure 1: Truss with raised bottom chords
Figure 1: Truss with raised bottom chords

Architectural Trusses are designed to create visual impact. For this reason, it is common to space them in the order of 3 to 6 metres apart, so that each truss is seen almost in isolation and as an individual feature. To make such wide spacings between the trusses possible, purlins are used to span between trusses for the purpose of supporting in-fill rafters (i.e. between the trusses). As a result, the truss chords (top) are subjected to extra bending from the purlin loads. This can require larger top chords than desired, and if there is a need to reduce this size, the problem can be solved by incorporating more webs into the truss (i.e. more top chord support), thus allowing a smaller chord to be used. Even wider spacings are more suited to glued laminated, trussed or ply-webbed purlin systems.
Figure 2: Scissors truss
Figure 2: Scissors truss

Some truss designs involve situations where the bottom chord is raised to give a greater feeling of space - as shown in Figure1. The main disadvantage of this type of truss is that larger and stronger members are required to deal with flexure in the top chord, resulting from the acquired loads brought about by the raised bottom chord. As a result, care must be taken to ensure that the desired appearance and budget can still be attained. If appropriate, a more attenuated option is the scissor truss - as shown in Figure 2. It uses the inner members to create ties that are always in tension - even under wind reversal loads. These inner members create a notional ceiling line and may be made from timber, cable or steel rod.


 
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