What is a truss bridge?
A truss bridge is a fixed bridge whose main girders are trusses: frameworks of members arranged in rigid triangles. A triangle cannot change shape unless one of its members gets longer or shorter. So a truss is strong and stiff with relatively little material. Nearly all modern truss bridges are steel; timber and wrought iron were common in the past. In Dutch it is called ‘vakwerkbrug’.
How it works
A truss consists of a top chord, a bottom chord and web members between them: diagonals and often verticals. On a simply supported bridge, the top chord is in compression and the bottom chord in tension. The web members carry the shear and, depending on their direction, act in tension or compression. Designers often model the joints as pins, so the members carry mainly axial force. In reality modern connections are welded or bolted and therefore more or less rigid.
Truss types
- Warren: diagonals only, alternately sloping up and down, with no or few verticals.
- Pratt: verticals with diagonals sloping down towards the middle of the span. Under self-weight and traffic the diagonals are in tension and the verticals in compression.
- Howe: the mirror image of the Pratt, with diagonals sloping the other way; diagonals in compression, verticals in tension.
- K-truss: the diagonals meet at mid-height of a vertical, forming a K. The compression members become shorter, which limits buckling; suited to deep trusses.
By contrast, a Vierendeel girder has no diagonals, only rectangular panels with rigid joints. Its members work in bending and are therefore heavier.
Application
Truss bridges are common as railway bridges and for crossing wide rivers and canals. A special case is the Bailey bridge: a portable, prefabricated truss bridge built from standard panels, developed during the Second World War and still used for temporary and emergency crossings, sometimes on pontoons.
Tips and points of attention
- Compression members govern: they must be checked for buckling, while tension members can be slimmer.
- Joints are critical: fatigue and corrosion often start at connections and gusset plates. Inspect there first.
- Maintenance: many members mean a large surface to protect against corrosion.
Related terms
- Truss
- Fink truss
- Lattice girder
- Beam bridge
- Cantilever bridge
- Pontoon bridge
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