Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 44 1 3 2 1 Eaves strut/tie 2 Position of plan bracing 3 Vertical bracing acting as strut/tie Figure 9.4 Bracing in a single end bay with an eaves strut 9.2.5 Single central braced bay The concept of providing a single braced bay near the centre of a structure (Figure 9.5) is unpopular because of the need to start erection from a braced bay and to work down the full length of a building from that point. However, bracing in the middle of the building has the advantage that it allows free thermal expansion of the structure, which is particularly valuable in locations such as Southern Europe and the Middle East where the diurnal temperature range is very large. In most of Europe, the expected temperature range is more modest, typically  5°C to +35°C, and overall expansion is not generally considered to be a problem. If a central braced bay is used, it may be necessary to provide additional temporary bracing in the end bays to assist in erection. 3 1 1 2 1 Free expansion 2 Eaves strut 3 Position of plan bracing Figure 9.5 Typical cross bracing at centre of the structure to allow free thermal expansion 9.2.6 Bracing using moment-resisting frames Where it is difficult or impossible to brace the frame vertically by conventional bracing, it is necessary to introduce moment-resisting frames in the elevations. There are two basic possibilities:  A moment-resisting frame in one or more bays, as shown in Figure 9.6.  Use of the complete elevation to resist longitudinal forces, with moment resisting connection often located in the end bays, where the end column is turned through 90° to provide increased stiffness in the longitudinal direction, as shown in Figure 9.7. This arrangement is only possible if the end frame (the gable) is constructed from a beam and column arrangement, rather than a portal frame. Gable frames are discussed in Section 10.