Modeling a Pratt Truss Bridge

Fanny Littmarck | December 28, 2012
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The Pratt truss is one of several different types of truss bridge designs out there. Said to be the oldest type of modern bridge, the truss is simple and lends itself well to analysis. This type of bridge is typically constructed for the use of pedestrians, cars, trucks, trains, and pipelines.

Pratt truss bridge geometry
Geometry of a Pratt truss bridge.

Brief History of the Pratt Truss Bridge

The Pratt truss bridge was invented by Caleb and Thomas Pratt in 1844. Its initial use was mainly to carry trains, replacing the older railway bridges made of stone or timber. The biggest advantage of this bridge was its low construction costs; the iron parts needed could be transported to the building site easily, and the assembly was quick, requiring only semi-skilled labor.

Structural Mechanics of a Pratt Truss Bridge

As seen in the image above, the design of Pratt truss bridges combines diagonal and vertical beams into a web, leading to a fairly light yet very strong structure. The diagonal and vertical beams are subject to tension and compressive forces, respectively. It is the tension force that eliminates the risk for buckling, in turn allowing for thinner diagonal beams, resulting in reduced material costs.

Using COMSOL Multiphysics 4.3a and the Structural Mechanics Module, two different analyses can be performed on the Pratt truss bridge. First, the stress and deflection fields can be evaluated as the bridge is exposed to a pure gravity load, as well as the passing of trucks across the bridge. Second, an eigenfrequency analysis would show the eigenfrequencies and eigenmodes of the bridge. Eigenfrequencies are important for taking wind loads and earthquakes into account.

Truss bridge under self weight Axial forces in a truss bridge

Left: Deformation of the truss bridge under self weight. Maximum deflection is 3 centimeters on the roadway. Right: axial forces in the beams, where red is tension and blue is compression.

If you want to see the full analysis and get instructions on how to simulate this in COMSOL Multiphysics, check out the Pratt Truss Bridge Model Documentation.

You can also download the model files for the Pratt truss bridge model from the Model Gallery. Note that you need to sign into your COMSOL Access account to do so. (If you don’t already have an account, it’s free and easy to sign up for COMSOL Access).

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