Beams

A beam is a horizontal or near horizontal member that usually supports gravity loads applied perpendicular to its length. These loads can include the self weight of the structure and its contents. The loads are usually directly applied by joists, trusses, slabs, etc. onto the top of the beam, but beams may also carry suspended loads. A beam transfers the load it supports into supports such as columns, bearing walls, piles, etc. The most common materials used for beams are wood, steel, and reinforced concrete.

In beam design, the primary considerations are bending, shear, and deflection. Bending and shear are the two primary modes of potential structural failure. Deflection may not represent a safety issue by itself, but can result in cracking in attached finishes and a perception of instability to the building’s occupants. Bending results in compression at the top of the beam and tension at the beam’s bottom. Bending stress is usually maximum near the midspan of a beam, where shear is often maximum near the beam’s supports. Deflection is primarily a function of the beam’s length and depth, but the beam’s material, sectional properties, loading, and support conditions also play a role.

Although beams are primarily associated with bending, or flexural behavior, they can also act as struts. When a beam is subjected to axial compression in addition to bending, it may function as a compression member within a larger framing system. For example, in a braced frame or truss configuration, a horizontal member can develop axial compressive force and contribute to resisting lateral loads such as wind or seismic forces. Beams are also used to brace compression members such as columns.

In homes and smaller buildings, beams often consist of sawn lumber or engineered wood members. However, steel is also used especially where a longer, unsupported span or shallower beam is needed. Commercial or heavier construction typically use concrete or steel. In masonry construction, precast concrete and steel beams placed over window and door openings are referred to as ‘lintels’.

ADDITIONAL RESOURCES:

ACI 318

AISC Steel Construction Manual

National Design Specifications for Wood Construction

Timber Construction Manual