Beams are structural members that support loads which are applied
transverse to their longitudinal axes. They are assumed to be placed
horizontally and subjected to vertical loads. Beams have a far more complex
load-carrying action than other structural elements such
as trusses and cables.
The load transferred by a beam is primarily by bending and shear. Any
structural members could be considered as a beam if the loads cause bending of
If a substantial amount of axial load is also present, the member is referred to as a beam-column.
Type of beams
Beams may be classified as follow:
- Floor beams (fig. 1.) – a major beam of a floor system usually supporting joists in buildings, a transverse beam in bridge floors.
Fig. 1. Floor beam
- Girder (fig. 2.) – in buildings, girders are the same as floor beams, also a major beam in any structure. Floor beam are often referred to as girders.
Fig. 2. Grider
- Joist (fig. 3.) – a beam supporting floor construction but not a major beam.
Fig. 3. Joist
- Lintels (fig. 4.) – beam members used to carry wall loads over wall openings for doors, windows etc.
Fig. 4. Lintel
- Purlin (fig. 5.) – a roof beam usually supported by roof trusses.
Fig. 5. Purlin
- Rafter (fig. 6.) – a roof beam, usually supporting purlins.
Fig. 6. Rafter
- Spandrels (fig. 7.) – exterior beams at the floor level of buildings, which carry part of the floor load and the exterior wall.
Fig. 7. Spandrels
- Girt (fig. 8.) – a horizontal member fastened to any spanning between peripheral columns of an industrial building, used to support wall cladding such as corrugated metal sheeting.
Fig. 8. Grit
- Stringers (fig. 9.) – members used in bridges parallel to the traffic to carry the deck slab.
Fig. 9. Stringer
Design of beams
The design of laterally supported beams consists of selecting a section on the basis of the modulus of the section and checking it for shear, deflection, and web crippling. The steps to be followed are as follows.
- The loads that may be acting on the beam are ascertained. The design loads are obtained by summing up the loads multiplied by the appropriate partial load factors.
- A trial beam section is assumed and the distribution of the bending moment along the length of the beam is determined by an elastic analysis (if the beam is statically indeterminate) or by statics (if the beam is statically determinate). The maximum bending moment and shear force are calculated.
- The required section modulus may be determined using Eqn.
- From section tables (IS 808-1989), a suitable section is selected, which has a section modulus equal to or more than the calculated modulus. ISMB sections may be preferred as they are readily available in the market.
- The beam is checked for shear as per Eqn.
- The beam is checked for deflection.
- The web is checked for web crippling as per Eqn.
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