How to Choose Truss Type?

Trusses can be divided into many types, truss type is used in a design is usually influenced by architectural aspects and customer requirements as well as geometric and economic factors.

In Pratt truss, under normal vertical loads, the diagonal web members bear tension, while the relatively short vertical web members bear compression. To a certain extent, this force characteristic compensates for the fact that the compression chords in the mid-span bear greater forces than the tension chords under conventional vertical loads. It is worth noting that wind loads on a gently sloping roof pratt truss may cause load reversal, resulting in stresses in the longer web members.

Converting the web members of a pratt truss from inward to outward oblique becomes a Howe truss (or English Truss). There are certain advantages to using Howe trusses for roofs with very small loads. Due to the influence of wind loads, it will produce reverse vertical loads. In addition, under the action of vertical load, the mid-span tension chord has to bear greater internal force than the compression chord.

When the Fink truss as shown in the figure is used on a roof with a long span and a steep slope, each member is subdivided into shorter members, so it is more economical in terms of steel consumption.

Mansard truss: It is a variation of the Fink truss and has the advantage of reducing unusable roof space and therefore reducing the building's operating costs. However, the main disadvantage of this kind of frame is that the internal force of the upper and lower chords will increase due to the small span-to-height ratio.

Warren truss: Since the lengths of the oblique web members of the trusses are equal, the production cost is reduced. Unlike the Pratt truss, the diagonal web members in the middle sections of the Warren truss are compressed under gravity loads.

When the span is large, an improved warren truss can be used. This truss adds vertical web members, and the spacing between the chord members becomes smaller. This reduces the effective buckling length of the compression chord and reduces secondary stresses caused by local bending.

The zigzag truss, or butterfly truss, is just one of many examples of trusses used in multi-spans buildings.

In terms of material usage, trusses can provide very efficient structural solutions. However, it is worth noting that the method of saving steel consumption by using a large number of components with relatively small cross-sections usually increases production costs and long-term maintenance costs.

Generally speaking, a simple design with roughly the same component specifications is the best choice. Designers should also consider construction feasibility, such as transportation of elements and machinery for on-site erection, as these factors may also control the truss design.