Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 33 7.3 Rafter out-of-plane stability 7.3.1 Rafter and haunch stability under maximum hogging moment Both in-plane and out-of-plane checks are required. Initially, the out-of-plane checks are completed to ensure that the restraints are located at appropriate positions and spacing. B A 2 3 1 4 5 6 7 7 8 M 2 C M p p 1 Tapered length between torsional restraints 2 Tapered length, between lateral restraints 3 Length between lateral restraints 4 Length between torsional restraints 5 Elastic section of rafter 6 Elastic section of rafter 7 Torsional restraint to the rafter 8 Torsional restraint to the column Figure 7.2 Typical portal frame rafter with potential plastic hinges at tip of haunch and first purlin down from apex Figure 7.2 shows a typical moment distribution for permanent plus variable actions and typical purlin positions and typical restraint positions. Purlins are placed at about 1,8 m spacing but this spacing may need to be reduced in the high moment regions near the eaves. Three stability zones are noted on Figure 7.2 (zones A, B, and C), which are referred to in the following sections. The presence of plastic hinges in the rafter will depend on the loading, geometry and choice of column and rafter sections. The selection of the appropriate check depends on the presence of a plastic hinge, the shape of the bending moment diagram and the geometry of the section (three flanges or two flanges). The objective of the checks is to provide sufficient restraints to ensure the rafter is stable out-of-plane. Haunch stability in Zone A In Zone A, the bottom flange of the haunch is in compression. The stability checks are complicated by the variation in geometry along the haunch. The junction of the inside column flange and the underside of the haunch (point 8 in Figure 7.2) should always be restrained. The ‘sharp’ end of the haunch (point 7 in Figure 7.2) usually has restraint to the bottom flange, from a purlin located at this position, forming a torsional restraint at this point. If a