Resistance to Sway Forces - Steel Structures.

Most of the common forms provide resistance to sidesway forces in the plane of the frame. It is essential also to provide resistance to out-of-plane forces; these are usually transmitted to the foundations with a combination of horizontal and vertical girders. The horizontal girder in the plane of the roof can  be of two forms  as shown in Fig. 1.6. Type (a) is formed from members, often tubes, capable of  carrying tension or compression. One of the benefits is in the erection stage as the braced bay can be erected first and lined and levelled to provide a square and safe springboard for the erection of the remainder.

Roof bracing
Fig. 1.6 Roof bracing

Type (b) uses less material but more members are required. The diagonals are tension-only members (wire rope has been used) and the compression is taken in the orthogonal strut which has the shortest possible effective length. It may be possible to use the purlins, strengthened where necessary, for this purpose.

Similar arrangements must be used in the wall to carry the forces down to foundation level. If the horizontal and vertical girders are not in the same bay, care must be taken to provide suitable connecting members.

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