A plate girder is a large steel beam built by welding or bolting together flat steel plates, rather than rolling a single piece of steel into shape at a mill. It consists of a tall, thin vertical plate (the web) sandwiched between two thick horizontal plates (the flanges) to form an I-shaped cross section. Plate girders are used when standard rolled steel beams aren’t deep or strong enough for the job, which makes them a staple of bridge construction and large industrial buildings.
How a Plate Girder Is Built Up
The three core components are simple: two flange plates and one web plate. The web is the tall vertical plate that resists shear forces, essentially handling the loads trying to slide one section of the beam past another. The flanges are the thick horizontal plates welded along the top and bottom edges of the web, and they resist bending. When a load pushes down on a plate girder, the top flange compresses while the bottom flange stretches in tension. This division of labor between web and flanges is what gives the I-shape its efficiency.
Because each plate is cut and sized independently before assembly, engineers can tailor the dimensions to the exact demands of the structure. In bridge design, for example, typical minimum flange widths run at least 20% of the web depth (and no less than 15 inches), with flange thickness starting at 0.75 inches and web thickness at 0.50 inches. A plate girder for a highway bridge might have a web 4 to 8 feet tall, far deeper than any standard rolled beam available from a steel mill.
What Stiffeners Do
A tall, thin web plate is prone to buckling, much like a thin sheet of metal will warp and fold if you push on it from the edges. To prevent this, fabricators weld additional plates called stiffeners onto the web. These come in two main types.
Transverse stiffeners are short vertical plates welded perpendicular to the web at regular intervals along the girder’s length. They act like internal braces, dividing the web into smaller panels that are much harder to buckle. Longitudinal stiffeners run horizontally along the web and are used when the girder is especially deep. While originally designed to boost bending and shear capacity, longitudinal stiffeners also improve the web’s resistance to several types of buckling at once. On many bridges, a combination of both types is used. Stiffeners also serve as connection points for cross frames and diaphragms, the lateral bracing that ties multiple girders together in a bridge system. These connection stiffeners are typically at least 0.50 inches thick.
Why Not Just Use a Rolled Beam?
Steel mills produce standard wide-flange beams (W-shapes) by passing hot steel through a series of rollers. These are cheaper per ton and faster to specify, but they top out at a certain depth and weight. The deepest commonly available rolled beam in the United States is roughly 44 inches tall. When a design needs more depth, a heavier flange, or a non-uniform cross section that varies along the span, a plate girder is the answer.
Plate girders also offer flexibility that rolled shapes can’t match. Engineers can make the flanges thicker near the middle of a span where bending forces peak, then taper them toward the supports. They can vary the web depth to follow the curve of a roadway or to create a haunch over a pier. This ability to optimize material placement means less steel overall compared to using an oversized rolled beam, even though the fabrication costs are higher.
How Plate Girders Are Fabricated
Modern plate girder fabrication revolves around submerged arc welding (SAW), a process that produces deep, strong welds efficiently. The basic sequence starts with cutting the web and flange plates to size from large steel sheets, then assembling them into the I-shape and welding the flange-to-web connections.
In many fabrication shops, the web is laid flat while the flanges are held in position using pneumatic jigs or mechanical “squeezers” with wedges to keep everything aligned. Some shops intentionally tilt the flanges by a few degrees before welding one side, knowing that the heat-induced distortion from welding will pull them back to perfectly perpendicular once the second side is complete. Others flip the girder and alternate weld passes on each side to balance out distortion. Extensive tack welding on both sides of the web happens before the full SAW passes begin.
Fully automated “pull-through” beam welders exist that can accept pre-cut parts at one end and produce a finished welded girder at the other. More common are semi-automatic setups where operators load parts into a jig, tack them together, then engage the automatic welding carriages. For very large girders (4 feet deep and taller), crawler-mounted wire feeders run down both sides of the web simultaneously. Because girders are often too long to ship in one piece, they’re fabricated in sections at the shop and then bolted together at the construction site using field splices, connections that add cost in the form of bolts, splice plates, extra crane time, and inspection.
How They Compare to Box Girders
The main alternative for long-span steel bridges is the box girder, which encloses the web on both sides to form a rectangular or trapezoidal tube. Box girders are stiffer in torsion (twisting), which makes them better suited for curved bridges and structures with eccentric loading. But they cost more to fabricate because of the additional welding, the difficulty of accessing the interior for inspection, and the tighter tolerances required to close the box shape.
Plate girders win on fabrication simplicity. Every weld is accessible from the outside, quality inspection is straightforward, and the components are flat plates that are easy to cut and handle. For straight or mildly curved bridges, a line of plate girders connected by cross frames is typically the most economical steel option. The trade-off is that plate girders are less efficient at resisting torsion, so wider bridges or those with sharp curves often justify the extra cost of box sections.
Common Failure Modes
The primary concern with plate girders is web buckling, and it takes several forms depending on how the girder is loaded and proportioned.
Shear buckling occurs when diagonal compression forces in the web (caused by shear loads) exceed what the thin plate can handle, producing visible diagonal wrinkles. Transverse stiffeners are the standard defense, breaking the web into smaller panels with higher buckling resistance.
Flange-induced buckling happens when a girder has large, heavy flanges paired with a very slender web. As the girder bends, the compression flange can push downward into the web, causing the web to buckle vertically. Design codes prevent this by setting an upper limit on the web’s slenderness ratio (its depth divided by its thickness). This limit, first proposed by researcher Konrad Basler over 50 years ago, has remained largely unchanged in structural standards since then.
Local flange buckling is also possible if the compression flange is too wide relative to its thickness. The flange plate itself can wrinkle or fold, losing its ability to carry compressive stress. Proper proportioning of flange width to thickness keeps this in check. In composite girders, where a concrete deck is bonded to the top flange, the concrete restrains the top flange against buckling, but the web can still experience a column-like buckling mode between the flanges.
Where Plate Girders Are Used
Highway bridges are the most visible application. Spans between about 100 and 300 feet are the sweet spot where plate girders outperform both standard rolled beams (too small) and truss or cable-supported designs (unnecessarily complex). Multi-span continuous plate girders, which run over intermediate piers without joints, are common on interstate highway bridges.
Beyond bridges, plate girders show up as transfer beams in buildings where columns above don’t align with columns below, as crane runway beams in industrial facilities, and as roof girders in large warehouses and hangars. Any situation where a very deep, heavily loaded beam is needed and standard rolled sections fall short is a candidate for a plate girder.