What is a plate girder?
A plate girder is a steel beam made not from a single rolled section but assembled from separate steel plates: two flanges and a web, normally joined with continuous fillet welds. The result is an I-shaped section whose depth, plate thickness and flange width can be chosen freely — tuned exactly to the loading.
Why build up rather than roll
Rolled sections are limited in depth. Once the span or the load exceeds what the heaviest available section can carry, or when a heavy section would waste steel, building up is more economical. The bending stiffness of a beam lies mainly in its depth: making it deeper achieves far more than making it thicker.
- Flanges — the top and bottom plates take the bending moment as compression and tension.
- Web — the vertical plate carries mainly shear and holds the flanges apart.
- Stiffeners — a deep, thin web can buckle under compression. Vertical stiffeners are therefore welded on, particularly at supports and beneath concentrated loads.
Applications
Plate girders are used where long spans or heavy point loads occur: bridges, crane runways, halls, industrial buildings, propping and permanent support to existing structures at a wall opening, and canopies. The tapered welded girder, whose depth varies along its length, is a plate girder too: steel is put where the moment is greatest.
Points to watch
- Lateral torsional buckling. A deep, slender girder can buckle sideways as long as the compression flange is unrestrained. Erection is the most critical moment: a girder with nothing yet bracing it must be temporarily stabilised.
- Weld details govern fatigue. In crane runways and bridges, where loading cycles constantly, the quality and shape of the weld determine service life — not the stress at mid-plate.
- Corrosion protection. Plate edges, the web-to-flange junction and the stiffeners are the vulnerable spots. Detail the girder so water cannot stand on it and every surface stays accessible for treatment.
- Fire resistance. A slender plate girder has a high surface-to-mass ratio and therefore an unfavourable section factor; it heats up faster in a fire than a stocky section. Allow for that in the fire protection.
Related terms
- Steel section
- Welding
- Span
- Beam
- Column
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