What Is a Bridge Strut? The Vertical/Diagonal Members That Stiffen Trusses

A bridge strut is a compression member in a truss bridge that connects the upper chord (top horizontal) to the lower chord (bottom horizontal). Struts are typically vertical or diagonal and work with ties (tension members) to form rigid triangles—the geometric basis of truss stability.

Function

• Transfer loads: Move compressive forces from deck → upper chord → strut → lower chord → piers/abutments.
• Prevent buckling: Subdivide chord lengths, reducing unsupported span and Euler buckling risk.
• Maintain geometry: Keep chords parallel, resist shear deformation.

Common Truss Types & Strut Patterns

• Warren truss: Equilateral triangles — alternating diagonal struts/ties (no verticals).
• Pratt truss: Vertical struts (compression), diagonal ties (tension) — efficient for long spans.
• Howe truss: Vertical ties, diagonal struts — historic for timber/iron.
• K-truss: Two diagonals per panel forming K — shorter struts, less buckling.
• Baltimore / Pennsylvania (Petit): Subdivided panels with multiple struts/ties for very long spans.

Materials & Design

• Steel: Wide-flange (W), hollow structural section (HSS), built-up box sections.
• Concrete: Precast or cast-in-place compression struts in segmental bridges.
• Timber: Historic covered bridges; glulam in modern pedestrian spans.
• Connections: Gusset plates (bolted/welded), pin joints (historic), node plates.

Beyond Trusses

"Strut" also appears in:

• Arch bridges: Spandrel columns (vertical struts) from arch rib to deck.
• Cable-stayed bridges: Pylon struts (cross-bracing between pylon legs).
• Suspension bridges: Tower bracing struts.

In all cases, a strut is a slender compression element — the unsung hero keeping the bridge from folding.