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Steel Designers' Manual - 6th Edition (2003)
predominantly in spans of up to 10 m, where flat roof construction is acceptable, the
frame comprises standard hot-rolled sections having simple or moment-resisting
joints.
Flat roofs are notoriously difficult to weatherproof, since deflections of the horizontal
cross-beam induce ponding of rainwater on the roof, which tends to penetrate
the laps of traditional cladding profiles and, indeed, any weakness of the
exterior roofing fabric. To counteract this, either the cross-member is cambered to
provide the required fall across the roof, or the cladding itself is laid to a predetermined
fall, again facilitating drainage of surface water off the roof.
Due to the need to control excessive deflections, the sections tend to be somewhat
heavier than those required for strength purposes alone, particularly if the
cross-beam is designed as simply-supported. In its simplest form, the cross-beam is
designed as spanning between columns, which, for gravity loadings, are in direct
compression apart from a small bending moment at the top of the column due to
the eccentricity of the beam connection. The cross-beam acts in bending due to the
applied gravity loads, the compression flange being restrained either by purlins,
which support the roof sheet, or by a proprietary roof deck, which may span between
the main frames and which must be adequately fastened. The columns are treated
as vertical cantilevers for in-plane wind loads.
Resistance to lateral loads is achieved by the use of a longitudinal wind girder,
usually situated within the depth of the cross-beam. This transmits load from the
top of the columns to bracing in the vertical plane, and thence to the foundation.
The bracing is generally designed as a pin-jointed frame, in keeping with the simple
joints used in the main frame. Details are shown in Fig. 1.10.
Buildings which employ the use of beam-and-column construction often have
brickwork cladding in the vertical plane. With careful detailing, the brickwork can
be designed to provide the vertical sway bracing, acting in a similar manner to the
shear walls of a multi-storey building.
gable end
Fig. 1.10 Simple wind bracing system
Design of common structural forms 17
vertical bracing