Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 35 = = = = Figure 7.4 Cross-sections to be considered when determining C n Rafter stability in Zone B Zone B generally extends from the ‘sharp’ end of the haunch to beyond the point of contraflexure (see Figure 7.2). The bottom flange is partially or wholly in compression over this length. Depending on the overall analysis, this zone may or may not contain a plastic hinge at the ‘sharp’ end of the haunch. In this zone, torsional and lateral restraint will be provided at the ‘sharp’ end of the haunch. At the upper end, restraint will be provided by a purlin beyond the point of contraflexure. Some national authorities allow the point of contraflexure to be considered as a restraint, provided the following conditions below are satisfied.  The rafter is a rolled section  At least two bolts are provided in the purlin-to-rafter connections  The depth of the purlin is not less than 0,25 times the depth of the rafter. If a plastic hinge is predicted at the ‘sharp’ end of the haunch, a torsional restraint must be provided within a limiting distance in accordance with BB.3.1.2. The limiting distance may be calculated assuming:  A constant moment – use Expression BB.6  A linear moment gradient – use Expression BB.7  A non-linear moment gradient – use Expression BB.8. In addition, the spacing between the intermediate lateral restraints (indicated as ‘3’ in Figure 7.2) must satisfy the requirements for L m as given in § BB.3.1.1. If there is no plastic hinge, and in elastic regions, the member must be verified in accordance with Expressions 6.61 and 6.62 (see Section 6.2 of this document). Rafter stability in Zone C In Zone C, the purlins can be assumed to provide lateral restraint to the top (compression) flange provided they are tied into some overall restraint system. In many countries, it is simply assumed that the diaphragm action of the roof sheeting is sufficient to carry restraint forces to the bracing system; in other