A truss is an important structure type in structural engineering. So what are trusses? A truss is a triangulated system of members that are structured and connected in a way such that they only incur axial force. These members are considered two-force members as the forces are only applied at either end of the member, resulting in either a compression or tension force. They are commonly used as bridge designs, given their ability to efficiently span long distances. A typical truss might look something like this: The joints are typically pinned connections, such that no shear or moment forces are transferred from member to member. This is a major, yet commonly misunderstood, the difference between truss and frame structures. A frame member will typically take a combination of shear, axial, and bending forces; whereas a truss member will only take axial force.Benefits of a TrussAs we have explained the definition of a truss, let’s explore its benefits. When designed correctly, trusses are an efficient way to span long distances whilst minimizing the amount of material used. This is because the internal loads of the members are induced axially (in the direction of the member) in the form of compression or tension. This means less material can be used, and the system as a whole is more efficient, as the force is distributed among a number of members. Source: Roof Truss Calculator Types of TrussesTo maximize the efficiency of the structure (which is often measured in the material used or labor), an appropriate truss type should be selected for the design and construction. Today we will explore what types of truss bridges are out there and how they can benefit your design. Pratt TrussA Pratt Truss has been used over the past two centuries as an effective truss method. The vertical members are in compression, whilst the diagonal members are in tension. This simplifies and produces a more efficient design since the steel in the diagonal members (in tension) can be reduced. This has a few effects – it reduces the cost of the structure due to more efficient members, reduces the self-weight, and eases the constructability of the structure. This type of truss is most appropriate for horizontal spans, where the force is predominantly in the vertical direction. Pratt Trusses are most closely related to the Howe Truss, where the diagonal members are in the opposite direction (causing an inverse compression/tension behavior in the members). It’s also important to note that in Pratt Truss Design, and most Truss design for that matter, is that the members have their end fixities set to pinned. This ensures that the force is all transferred in either compression or tension, with little (theoretically, none) transferred in the form of bending moment force. Below is an example of a Pratt Truss, constructed and analyzed using our SkyCiv Truss Calculator. Compressive members are shown as green and tension as red.
|