Structural design for extreme loadings generally requires specialist methods, since non-linear analysis or empirical formulae arc frequently necessary, and in some cases specially commissioned testing is required to fully validate the design.

12 Steel frames

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7'€K}”'CX■WELDED JOINT

12.1 Steelwork

Structural steelwork is mainly used for the frames of power station buildings. The beams, columns and other members are fabricated at the works, delivered by road or rail transport to the site and erected on prepared foundations. The structural engineer designs the frame using rolled-steel joist sections, universal beams and column sections, channel and angle sections, flat plates and tubes. In addition to these sections, members such as welded plate girders up to 4 m deep and welded box columns up to 2 m x 1 m in section, with plates up to 90 mm thick, are used to support the heavy plant load­ ing associated with the main power station buildings. Castellated beams (Fig 3.40) which consist of rolledsteel joists or universal beams cut through the centre

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NOTE: THE DEPTH OF A CASTELLATED STEEL BEAM IS APPROXIMATELY V/: • THE DEPTH OF THE ORIGINAL STEEL SECTION

Fig. 3.40 Arrangement of castellated steel beam

line of the webs to form a sawtooth pattern, and then welded with the points together, are particularly useful when light loads arc carried on a long span and deflection rather than bending stresses is the design criterion (c.g., in roof designs as rafters and purlins to support roof decking). Compound sections, lattice members, box girders and heavy and light trusses are also used.