Beam and Column - Structural Forms.

The cross-section shown in Fig. 1.4 is undoubtedly the simplest  framing solution which can be used to provide structural integrity to single-storey buildings. Used predominantly in spans of up to 10m, where flat roof construction is acceptable, the frame comprises standard hot-rolled sections having simple or moment-resisting joints.

Popular purlin and frame sections
Fig. 1.3 Popular purlin and frame sections

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.

Simple wind bracing system
Fig. 1.10 Simple wind bracing system

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.

Resistance to lateral loading can also be achieved either by the use of rigid  connections at the column/beam joint or by designing the columns as fixed-base  cantilevers. The latter point is covered in more detail in the following sub-section relating to the truss and stanchion framing system.

0 comentarios:

Post a Comment