Truss Bridge

Triangulation. A triangle is the only polygon that can't deform without changing the length of a side. By assembling triangles, designers create a rigid structure where every member is loaded purely axially, and steel is far more efficient axially than in bending. The result: long spans with much less material than a solid beam of equivalent depth.

Pratt has vertical members and diagonals sloping down toward the center; the diagonals carry tension while the verticals carry compression. Howe is the inverse: diagonals slope outward and carry compression, verticals carry tension. Pratt is preferred for steel (efficient in tension); Howe was common in early timber bridges (timber handles compression well; iron rods handled tension verticals).

Alternating equilateral triangles with no verticals (or with optional verticals for deep spans). Members alternate between tension and compression as load passes over. Warren trusses are simple, symmetrical, and easy to fabricate, making them a common choice for highway overpasses. The classic 1840s patent by James Warren and Willoughby Monzani is still in use.

Each panel point (joint) sees forces from connected members. By assuming pin connections (no moments at joints), engineers solve for axial forces using the method of joints (force balance at each node) or method of sections (cutting through the truss). For statically determinate trusses, equations of equilibrium yield exact solutions; indeterminate trusses need stiffness methods.

Deck truss: deck on top of the truss, traffic above. Through truss: deck inside, traffic between the trusses with overhead bracing—needed for tall trusses. Pony truss: deck inside, but trusses too short for top bracing, requiring stiffer top chords. Choice depends on span depth, vertical clearance, and aesthetics.

Compression members buckle (sudden lateral instability when load exceeds Euler's critical load). Tension members yield or fracture at connections, often at bolt holes or welds. Connection failure is the most common cause: corroded gusset plates on the I-35W bridge in Minneapolis collapsed in 2007, killing 13. Modern inspections focus heavily on connections.

An arched top chord with a horizontal bottom chord; a hybrid of arch and truss. The top chord acts in compression, the bottom in tension, and verticals carry deck loads. Bowstring trusses are particularly efficient and were common for early railroad bridges. Modern variants are still used for medium spans where the arched profile is desirable.