A Technical look into Pratt Trusses and Pratt Truss Design
Firstly, what is a Pratt Truss?
A Pratt Truss is a style of truss which is styled as internal (web) truss members travelling in a diagonal top-down direction. This formation is quite simple and extremely effective in long spans such as bridges. It differs to other types of trusses, in its simplicity and is 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).
This is a highly effective use of materials, as the vertical members absorb a lot of the force in compression which is an effective use of material. The diagonal web members act as a form of bracing in tension. Simply put: The vertical members are in compression, whilst the diagonal members are in tension. Overall, the system behaves quite consistently, making it very easy to design:
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) is transferred in the form of bending moment force. We’ll discuss this further under the Pratt Truss Design part below..
Advantages of a Pratt Truss
The following are some advantages of a Pratt Truss:
- Simple Design – Pratt Trusses are a very simple design. With simple geometry these are easy to model, design and build
- Well-Understood Behavior – The vertical and top chord members undergo compression, whilst the diagonal and bottom chord members are in tension. This means the sections and members can be designed quite easily
- Efficient – the benefit of Pratt Trusses (and most Trusses in general) is that their members are in either compression or tension. Also, since there are really only 4 types of members (top chord, bottom chord, diagonal web, vertical web) these can be efficiently designed to serve a specific purpose, reducing the amount of material required to withstand the load.
- Proven -Pratt Trusses have been used and trusted for centuries (The Pratt truss was invented by Thomas and Caleb Pratt in 1844)
Pratt Truss Design
Pratt Truss Design is generally based on the type of material, section, application (for instance Pratt truss bridges and Pratt roof trusses may be designed differently) and regional design standards used. For proper design, specific design standards should referred to and a structural engineer should alway be consulted. However, for the purpose of education, there are a few important things to look for when designing trusses, so let’s look at some of the basics. First, the analysis model:
Model Setup – Member End Fixities
It’s important to consider that in most truss designs, the member end connectivity is extremely important and can drastically change your results and the behaviour of the internal members. When designing a truss, the member fixities should be pinned. Using the SkyCiv Structural 3D analysis software as reference, the following two analysis models show the differences when members are set as Pinned vs Fixed (not recommended):
Fixed connections will drastically alter your designs, particularly at connections, as they will require a moment connection to support this load. Note: To learn more about this topic, visit member end fixities or how to model a pinned connection in structural analysis software.
Deflection and Span Ratios
Deflection results are a really important aspect of roof truss design or bridge truss design. A common rule of thumb, or calculation are span ratios. These are calculated as span length / deflection and represented as L / (this number). For instance, the peak deflection in the member is 19.243mm (not shown below) and the span is 25.6m. The denominator would be calculated as 25600/19.243 = 1,330, or simply L / 1330.
The lower the number, the worse the deflection. The deflection span limit varies on different design codes. For instance for roof trusses, the deflection due to live load cannot exceed a limit of 240 (L/240) for live load and L/180 for total load. Whereas floor trusses, don’t need to be as ‘stiff’ so may required deflection spans of L/240 and L/360 for minimum code deflection criteria.
A number of design standards are used to design Pratt and other Trusses. These are dependent mainly on the material used (for instance the US may require NDS for timber or AISC for steel members) and the type of application (roof design is significantly different to bridge design standards).
These are also generally accompanied with Load Combination requirements to test a range of different loading scenarios. These can include various live and dead loads, as well as wind, snow and even dynamic loading depending on your local standard requirements.