Composite beams with web openings

In composite beams, large openings may be formed through the web. These openings are used for the passage of services within the beam depth, and are about twice the size that would be possible in non-composite beams. The openings are normally rectangular in shape, but may also be circular or square. Welded stiffeners placed horizontally above and below the openings increase the size and aspect ratio of opening that may be used. However, access for welding of the stiffeners may be difficult in shallow beams.

For the scheme design of composite beams with rectangular openings, it is recommended that:

• large openings should be located between one-fifth and one-third of the span from the supports in uniformly loaded beams to have the least impact on the structural design of the beam

• large openings may also be placed close to mid-span, but require over-design of the beam in bending

• the spacing between the edges of openings, or to the connections of secondary beams, should not be less than the beam depth, D, unless the effect of these local forces is calculated

• rectangular openings should be not located at less than 2D from the support, in order to avoid the effects of high shear and partial shear connection close to the supports

• the suggested maximum depth x length of rectangular openings are:

0.6D x 1.2D for unstiffened openings

0.7D x 1.75D for horizontally stiffened openings

• horizontal stiffeners should be extended past the opening to provide local-bending resistance at the corners of the opening

• circular openings are more structurally efficient than rectangular openings and may be placed closer together (as in cellular beams).

These detailing rules are illustrated in Figure 4.9, which also shows alternative stiffening arrangements. The use of larger openings can be justified by more detailed calculations. If the beam is over-designed in its bending resistance, large openings can be formed where shear forces are low. For example, deep openings can be provided at mid-span of

Large Opening Web Zone Steel Beam

Various stiffening arrangements above and below opening

4.9 Detailing of stiffened openings in composite beams primary beams. Elsewhere, small openings (up to 0.3D) can usually be detailed without further checks. Composite beams can also be designed with regular circular openings, as described in Section 4.1.2.

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