Steel Girder Bridge

What It Means

A steel girder bridge uses steel beams, typically rolled wide-flange sections for short spans or welded plate girders for longer spans, as the primary load-carrying superstructure elements. Steel girders are the most common superstructure type in the U.S. NBI, representing roughly 30% of all bridges and a higher share of medium-span highway bridges (100-300 feet). Girders are typically spaced 6 to 15 feet apart under a concrete deck, with intermediate cross-frames or diaphragms providing lateral stability and load distribution between girders. Steel offers high strength-to-weight ratio, predictable material properties, ductility under overload, and relatively fast construction, girders can be fabricated in shops to precise dimensions and erected quickly with cranes. The trade-offs are corrosion vulnerability (requiring paint maintenance every 25-30 years or weathering steel), fatigue susceptibility at welded details, and potential for fracture critical configurations in two-girder layouts. Historically, many pre-1980 steel girder bridges were built with Category E or E' fatigue details (cover plate ends, transverse stiffener-to-flange welds) that have required retrofit or replacement as truck volumes increased. The 2007 I-35W Minneapolis collapse occurred on a steel through-truss superstructure with fracture critical gusset plate details, though the proximate cause was inadequate gusset plate thickness combined with dead load additions. Modern steel girder design under AASHTO LRFD uses high-performance steels (HPS 50W, HPS 70W, HPS 100W) with improved toughness and weatherability, redundant multi-girder configurations, and Category C or better fatigue details. The federal Buy America provisions require domestic steel for federally funded bridges with limited exceptions.

Frequently Asked Questions