Geometric patterns that optimize triangulation
A truss is a framework of triangles, the most stable rigid geometric form. Pratt trusses (developed 1844) use vertical members in compression and diagonal members in tension, with diagonals angling inward toward midspan. This pattern puts the longer diagonals in tension (where steel is efficient) and shorter verticals in compression. Howe trusses reverse the angles, putting longer diagonals in compression (better for wood in older bridges). Warren trusses use equal-length diagonals alternating between tension and compression, creating a uniform aesthetic and distributing load evenly. K-trusses add vertical members for shorter unbraced lengths, improving buckling resistance. Each pattern balances material efficiency, fabrication simplicity, and visual appearance.
Selection by span, material, and context
Short spans under 50 meters often use simple Warren trusses due to material efficiency and fabrication ease. Long spans of 100-500 meters require Pratt or K-trusses to manage buckling in compression members. The material dominates: steel truss bridges prefer Pratt (efficient use of steel in tension); wooden trusses historically used Howe (compression on shorter wood members is safer than tension on joinery). Historic bridges (19th-century through mid-20th century) are predominantly Pratt in America, reflecting the period's steel availability and engineering knowledge. Modern pedestrian and rail bridges increasingly use contemporary designs (cable-stayed, suspension) rather than simple trusses for longer spans, but truss bridges remain common for short-span road crossings and industrial structures due to low cost and established design practices.