Pratt Truss Calculator
A Pratt truss arranges its diagonal web members to carry tension and its vertical members to carry compression — the reverse of a Howe truss — a pattern well suited to steel construction, where slender tension members are efficient. This calculator sizes a Pratt-style truss from span and pitch.
Size Your Pratt Truss
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How the Pratt Truss Carries Load
In a Pratt truss, the diagonal web members slope away from the center of the span and carry tension, while the vertical members carry compression. Because steel performs efficiently as a slender tension member, the Pratt configuration became the standard for steel truss bridges and steel roof trusses after the material shift from timber to iron and steel.
A method-of-joints force analysis of a Pratt truss shows the diagonal tension members pulling the joints together under load, while the shorter vertical compression members act more like simple struts — the opposite force pattern from a Howe truss.
Pratt-style webbing is common in wide-span steel roof trusses for commercial and agricultural buildings, where its tension-efficient geometry allows lighter overall truss weight compared to a Howe layout at the same span and load.
Frequently Asked Questions
What is a Pratt truss best used for?
The Pratt truss is best suited to steel construction and wide-span steel roof trusses, since its diagonal tension members can be made from slender, efficient steel sections while the compression verticals stay short.
Is a Pratt truss stronger than a Howe truss?
Neither is inherently stronger — they distribute the same total load differently. The best choice depends on the construction material: Pratt suits steel (tension-efficient), Howe historically suited timber (compression-efficient).