Girder Bridge: I-Girders, Box Girders and How They Work

A girder bridge is a beam bridge built with large, efficient beams called girders. Rather than a solid rectangle, the girder is shaped so that most of its material sits at the top and bottom, where bending stresses are highest, joined by a thinner web in between. Shaped as an I or a closed box, in steel or prestressed concrete, girders carry most of the world’s road and rail bridges of short and medium span.

This guide explains how girders work, the main types, how they combine with the deck, notable lessons from history, and the failure modes that engineers inspect for.

How a girder bridge carries load

Like any beam, a girder resists load by bending between its supports. Bending creates compression on one face and tension on the other, and the further the material sits from the neutral axis, the more effectively it resists. This property is captured by the second moment of area: doubling the depth of a section increases its bending stiffness roughly eightfold. Our article on moment of inertia explains why depth is so valuable.

The girder exploits this. Its flanges, the wide plates at top and bottom, carry most of the bending as a pair of forces, one compressive and one tensile, separated by the depth of the section. The web joins them, holds them apart and carries the shear. Remove material from the web, add it to the flanges and make the section deeper, and you get much more stiffness per tonne of steel or concrete.

Load travels from the deck slab to the girders, along the girders by bending to the bearings, and down through piers and abutments to the foundations. Cross-frames or diaphragms between girders share load between them and stop them twisting.

I-girders and box girders

I-girders

The I-girder is the workhorse. Small spans use rolled steel sections; larger ones use girders welded up from plates, known as plate girders, with webs perhaps several metres deep. In concrete, factory-made prestressed I-beams and T-beams are lifted onto piers and topped with a cast deck. The plate girder bridge has its own guide in this series, and our comparison of H-beams and I-beams covers the rolled sections.

I-girders are open sections, so they are relatively weak in torsion and rely on bracing between them. They are simple to fabricate, inspect and maintain, which keeps them popular.

Box girders

A box girder closes the section with a bottom flange and two or more webs, forming a hollow tube. Closed sections are very stiff in torsion, so box girders suit curved bridges, wide decks on single piers, and long spans where twisting under eccentric load would trouble an open section. Most balanced cantilever concrete viaducts, and many elevated rail lines, are box girders. The cost is a more complex section that has to be entered to be inspected.

Steel, concrete and composite girders

Steel girders are light, quick to erect and good for longer spans or where weight matters, but they need protection against corrosion and attention to fatigue at welded details. Concrete girders, usually prestressed, are heavier but durable and need less routine maintenance. Prestress puts the concrete into compression so that it stays largely uncracked under service load.

Most modern girder bridges are composite. A reinforced concrete deck slab is connected to steel girders by shear studs welded to the top flange, so the slab and girder bend together as one section. The concrete takes compression at the top, the steel takes tension at the bottom, and the combined section is far stiffer than either alone. The same principle is used in composite floor construction in buildings. Our comparison of steel and concrete structures explains the trade-offs in more depth.

Girders can be simply supported or continuous over several piers. Continuity reduces the peak moments and the number of joints but introduces hogging over the supports, where the bottom flange is in compression and must be checked for buckling.

Lessons from the 1970 box girder collapses

Thin steel plates are efficient but they can buckle locally, and box girder bridges taught that lesson painfully. In June 1970 a span of the Cleddau Bridge at Milford Haven in Wales collapsed during construction, killing four workers. In October that year, a 112-metre span of Melbourne’s West Gate Bridge collapsed during construction, killing 35 people. A Royal Commission in Victoria found serious design and construction failings.

Both failures involved large, thin steel plates in compression, during construction, when the structure was incomplete and the loads unusual. The response included major reviews of steel box girder design, notably the Merrison Committee in the United Kingdom, whose interim rules shaped later bridge codes. The lasting lessons are that plate buckling and construction stages must be checked as rigorously as the finished bridge, and that design must be independently checked.

Common failure modes in a girder bridge

Local and lateral-torsional buckling. Compression flanges and thin webs can buckle if unbraced or under-stiffened, especially during erection before the deck provides restraint.
Fatigue cracking. Repeated traffic loading can start cracks at welded attachments, the ends of cover plates and stiffeners, and around cut-outs. Cracks can then grow into the flange or web.
Corrosion. Steel girders corrode where water collects, typically beneath leaking deck joints and at bearings. Concrete girders suffer reinforcement and strand corrosion from chlorides and carbonation.
Bearing problems. Seized or deteriorated bearings restrain thermal movement and transfer unintended forces into girders and piers. See our guide to elastomeric bearings.
Impact. Over-height vehicles striking the lowest girder can bend steel flanges or spall concrete and sever strands.
Loss of prestress. In concrete girders, tendon corrosion or long-term losses can reduce capacity and increase deflection.

Girders in buildings

The same principles govern transfer girders in towers, long-span roof girders and the steel beams that carry link bridges between blocks. When a building owner wants to remove a column, hang a new load from an existing girder or cut an opening through a web for services, the questions are the same as for a bridge: what is the load path, where is the compression flange restrained, and what happens to the shear capacity around the opening? A structural engineer should answer those before any cutting begins.

Frequently asked questions

What is the difference between a girder and a beam?

Every girder is a beam, but the word girder usually means a large primary beam that supports other beams or a deck. In bridges, girders are the main longitudinal members, shaped as I-sections or boxes to carry bending efficiently over the span.

What span can a girder bridge achieve?

Rolled steel and precast concrete girders suit shorter spans. Steel plate girders and box girders extend the range considerably, and prestressed concrete box girders built by balanced cantilever reach longer spans still. Beyond that, trusses, arches and cable systems usually become more economical.

Why are box girders used for elevated railways?

Box girders are stiff in both bending and torsion, which keeps deflection and twist low under moving trains and on curves. They also suit precast segmental construction, where short box segments are made off site and stressed together on the viaduct.

What caused the West Gate Bridge collapse?

The span collapsed in October 1970 during construction, after attempts to correct a misalignment between two halves of a steel box girder led to buckling. The Royal Commission identified failings in both design and construction methods. It killed 35 people.

Need a check on a transfer girder, link bridge or a new opening in a steel beam? Talk to our consultancy or get in touch.