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Elements
Design of beams structural steel
**Cross-section geometry**

Cross-section geometry

Cross-section size and shape influence beam performance.

Typical open section beam

The performance of any beam is dependent upon the cross-sectional geometry, not only on the physical dimensions, but also the shape. Steel beams are available in a variety of cross-sectional shapes. These include open sections and closed sections or tubes. In practice I beams are most commonly used for the beams in buildings. Rectangular hollow sections may be used for edge beams where particular edge details are required.

Beams may be standard sections or specially manufactured.

Both open and closed sections are produced in a large variety of standard (serial) sizes by a hot-rolling process. Standard sections are generally used because they are readily available and are economic.

Variations on standard sections include castellated and cellform beams which are efficient for very long spans, and provide for service runs.

Other sections can be fabricated from plate or by joining standard sections by bolting or welding.

Beams may be curved.

Beams may be curved to reduce overall deflections (precambering) or simply to create more interesting shapes. This is achieved by a specialised bending process which can be performed at a modest surcharge to fabrication costs. For small degrees of curvature, the beam design calculations are no different from those for a straight beam. However for significantly curved beams, the structural action becomes that of an arch.

Bending behaviour is related to the moment of inertia and the section modulus of the beam.

The main bending behaviour of a beam is determined by two geometric quantities. These are the *moment of inertia* and the *section modulus* which depend on the size and shape of the cross-section. These quantities are given in standard tables for all rolled sections.