Architectural Trusses

Figure 5: Typical truss designs

Architectural trusses are often used as part of a ‘cathedral ceiling’ system. Aesthetically, this part of the structure creates an eye-catching appearance, and typical options are shown in Figure 5. Many of these designs, were patented during the 19th century, and still bear the name of the original designers e.g., Pratt, Howe etc.

Trusses get their strength via 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 trying to maximise the visual impact of trusses, it is common to space them in the order of 3 to 6 metres apart, to distinguish them as individual features. To make this 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) take 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.

Some truss designs involve situations where the bottom chord is raised to give a greater feeling of space – as shown in Figure 6. 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 is attained. If appropriate, a more attenuated option is the scissor truss – as shown in Figure 7. It uses the inner members to create ties that are always in tension - even under wind reversal loads – these create a notional ceiling line and may be made from timber, cable or steel rod.
 

Figure 7: Scissors truss configuration

Figure 6: Truss with raised bottom chords