Plate Girder Bridge: Design, Stiffeners and Failure Modes

A plate girder bridge uses steel girders built up from flat plates rather than rolled from a mill. Two flange plates, top and bottom, are joined by a deep, thin web plate, and the whole section is tailored to the span. Where rolled beams run out of depth, plate girders take over, and they carry a vast number of railway and road bridges across the world, often with little more than a concrete deck on top to show for it.

This guide explains how the plate girder works, why its slender web needs stiffeners, how fabrication has changed from rivets to welds, and the buckling and fatigue problems engineers look for on existing girders.

Why build a girder from plates?

Rolled steel sections come in a fixed range of depths and thicknesses. For short spans that is enough, but as spans grow the bending moment rises sharply and the beam needs to be much deeper. Making the girder from plates removes that limit. The engineer chooses the depth of the web, the width and thickness of each flange, and can even vary the flange size along the span so that material follows the bending moment: thick at midspan, thinner near the supports.

The section is efficient because almost all of the bending is carried by the flanges, working as a compression and tension pair separated by the depth of the web. The web holds the flanges apart and carries the shear. Since the web does relatively little bending work, it can be made very thin compared with its depth, which saves a great deal of steel. For the broader family of girders, see our guide to the girder bridge.

How a plate girder bridge carries load

A typical road bridge has several plate girders side by side, with a reinforced concrete deck slab connected to their top flanges by shear studs so that slab and steel act compositely. A railway bridge may have two deep girders with the track carried either on top (a deck girder) or between them on cross girders (a through girder), which keeps the rail level low.

Load passes from the deck to the girders, along the girders by bending to the bearings, and down into the piers. At the bearings, the whole reaction is concentrated into a short length of thin web, which is why bearing stiffeners are fitted there. Between girders, cross-frames or diaphragms share load and restrain the compression flanges against twisting sideways, particularly before the deck has been cast.

Stiffeners and the slender web

A thin web is efficient, but thin plates buckle. Under shear, a slender web panel wrinkles diagonally; under the compression part of bending it can bulge sideways; and under a concentrated load it can crumple locally. Stiffeners are the engineer’s answer.

Bearing stiffeners. Vertical plates welded to the web over supports and under heavy point loads. They carry the concentrated force into the web and stop it crippling.
Intermediate transverse stiffeners. Vertical plates at intervals along the span, dividing the web into panels and raising its resistance to shear buckling.
Longitudinal stiffeners. Horizontal plates in the compression zone of very deep webs, which restrain bending buckling.

Tension field action

Thin webs have a useful reserve. Once a stiffened web panel begins to buckle in shear, it does not simply fail. The buckled panel develops a diagonal band of tension, anchored by the flanges and the stiffeners, which together behave like a Pratt truss: the tension field forms the diagonals and the stiffeners the verticals. Design codes allow this post-buckling strength to be counted, within limits, which lets engineers use thinner webs. It only works if the stiffeners and flanges are strong enough to anchor the tension band. For the underlying instability, see our explainer on buckling.

From rivets to welds

Early plate girders were riveted. Flange angles were riveted to the web, cover plates were riveted to the angles, and stiffeners were angles riveted on either side. The nineteenth-century railway engineers developed this approach on a heroic scale: Robert Stephenson’s Britannia Bridge across the Menai Strait, opened in 1850, carried trains through tubes built up from riveted wrought-iron plates, an ancestor of both the plate girder and the box girder.

From the mid-twentieth century, welding took over. Welded girders are cleaner, lighter and cheaper to fabricate, with flanges joined directly to the web by fillet welds. But welding brought new problems, especially fatigue, because welds create stress concentrations and residual stresses and form a continuous path along which a crack can run. Riveted girders, by contrast, are built of separate plates, so a crack in one plate often stops at the next. Our comparison of bolted and welded connections explains these trade-offs in buildings.

Failure modes in a plate girder bridge

Shear buckling of the web. Panels near supports, where shear is highest, may wrinkle if under-stiffened or thinned by corrosion.
Web crippling at bearings. Missing, undersized or corroded bearing stiffeners allow the web to crumple under the support reaction.
Lateral-torsional buckling. A deep girder with an unrestrained compression flange can twist and buckle sideways, a particular risk during erection before bracing and deck are in place.
Fatigue cracking at welded details. The ends of stiffeners, cover plates, gusset attachments and cut-outs are classic locations. Distortion-induced fatigue, where cross-frames twist a short gap of web near a flange, is a well-known cause of cracking on older welded girders.
Corrosion. Water from leaking deck joints runs down girder ends and collects on bottom flanges and bearing stiffeners. Section loss there directly reduces shear and bearing capacity.
Impact. Over-height vehicles striking the bottom flange of a girder over a road can bend it and its web.

Inspecting and assessing plate girders

On an existing plate girder, an engineer focuses on girder ends, bearings and the welded details known to be fatigue-prone. Close visual inspection is supplemented where needed by magnetic particle or ultrasonic testing of welds, thickness measurement of corroded plates, and checks on bracing and bearings. Measured section loss is then fed into a capacity assessment, because a girder that has lost a few millimetres from a thin web may have lost a large share of its shear resistance.

Repairs range from cleaning and repainting, through bolted cover plates over corroded areas and drilled holes to arrest crack tips, to the replacement of stiffeners or entire girder ends. The same thinking applies to plate girders and transfer girders in buildings, which are often concealed by finishes. Any opening proposed through a web, or new load hung from a flange, should be checked first.

Frequently asked questions

What is a plate girder bridge used for?

Plate girder bridges are used for road and rail spans that are too long for rolled steel beams but do not justify a truss, arch or cable system. They are common on railways, motorway overbridges and river crossings, usually with a concrete deck acting compositely with the steel.

Why does a plate girder need stiffeners?

Its web is deliberately thin to save steel, and thin plates buckle under shear, compression and concentrated loads. Bearing stiffeners carry support reactions into the web, while intermediate and longitudinal stiffeners divide the web into panels that resist buckling.

What is the difference between a plate girder and a box girder?

A plate girder is an open I-shaped section with one web. A box girder has two or more webs and a bottom flange forming a closed tube. Box girders are much stiffer in torsion, which suits curved and wide decks, but are more complex to fabricate and inspect.

Are riveted plate girders still safe?

Many riveted plate girders remain in service after a century or more. Their safety depends on condition: corrosion, loose rivets, cracking and the loads they now carry must be assessed. With inspection and maintenance, many can continue in use.

Need an opening through a steel girder web, or an assessment of an existing steel structure? Talk to our consultancy or get in touch.