A box girder is a beam with a closed, hollow cross-section — a rectangular or trapezoidal “box” of steel or concrete. Instead of an open profile with two flanges and a web (as in an I-beam), the top and bottom flanges plus two web plates form a closed tube.
Why a closed section
The great advantage of a closed section is torsional stiffness. An open profile twists readily when load is applied off-centre; a closed box is many times stiffer in torsion. That makes the box girder the obvious choice when:
- the load is eccentric, for example traffic in one lane of a bridge;
- the girder is curved in plan (a sweeping viaduct);
- a long span must be achieved within a limited structural depth;
- the structure needs a slim, clean appearance — a box reads quieter than a truss.
Applications
| Application | Material | Characteristic |
|---|---|---|
| Road bridges and viaducts | Steel or prestressed concrete | Often one wide box beneath the deck |
| Crane runways | Steel | Large torsional forces from lateral crane loads |
| Columns and portal frames | Steel (RHS/SHS hollow section) | Equal buckling stiffness in both directions |
| High-rise cores | Concrete | The building itself acts as a tube |
On smaller jobs — a carport, a canopy, a steel lintel over a wide opening — the rolled hollow section is the DIY version of the same principle.
Points to watch
- Internal corrosion — a closed box cannot be inspected or painted inside. Large box girders are therefore given manholes and sometimes a controlled internal climate; small hollow sections are welded airtight so moisture cannot enter.
- Condensation — a box that is not fully sealed warms up by day and cools at night, drawing in moist air. This is the most common cause of rust from the inside out.
- Connections — a closed profile offers no internal access for bolts. Joints are therefore usually welded or made with plates passing through the section.
Related terms
- Girder
- I-beam
- Plate girder
- Rolled beam
- Span
- Steel
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