Theory of Built-up Sections

• A built-up beam is also known as compound beam.
• The built-up beams are used when the span, load and corresponding bending moment are of such magnitudes that rolled steel beam section becomes inadequate to provide required section modulus.
• Built-up beams are also used when rolled steel beams are inadequate for limited depth.
• In building construction, the depth of beam is limited by a space provided by the architect.
• Drawing beam of small depth do not provide required section modulus. Therefore, plates are attached to the beams.
• The strength of rolled steel beams is increased by adding plates to its flange which is one of the method forming built-in section.
• The other method is to compound a number of rolled steel sections themselves.

The built-up sections shown in figure’ A’ and ‘B’ are used for heavy loads and small spans.

The built-up section ‘C’ is also used for heavy loads and small span.

The built-up section ‘D’ is used for light loads and large spans.

The built-up sections ‘E’ & ‘F’ are provided for Gantry girders.