Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 36 countries any purlins providing restraint must be connected directly to the bracing system. The out-of-plane checks require the verification of the member in accordance with Expressions 6.61 and 6.62 (see Section 6.2 of this document). Normally, if the purlins are regularly spaced, it is sufficient to check the rafter between restraints assuming the maximum bending moment and maximum axial load. If a plastic hinge is predicted to form adjacent to the apex, it must be restrained. In addition, the usual requirements for stability near a plastic hinge must be satisfied:  The distance between the restraint at the plastic hinge and the next lateral restraint must not exceed the limiting distance L m .  The distance to the next torsional restraint each side of the hinge must not exceed the limiting distance L k , or L s , with the spacing of intermediate restraints satisfying the requirements for L m , all as described for zone B. Even if there is no plastic hinge adjacent to the apex, it is normal practice to provide a torsional restraint at this point, as this will be necessary when considering the uplift combinations of actions – the bottom flange will be in compression. 7.3.2 Rafter and haunch stability for uplift conditions Under uplift, most of the bottom flange of the rafter is in compression. A typical reversal bending moment diagram is shown in Figure 7.5. 1 1 2 E F 3 1 Torsional restraint 2 Torsional restraint to column 3 Possible additional torsional restraint required for the uplift condition. Figure 7.5 Typical purlin and rafter stay arrangement for wind uplift This type of bending moment diagram will generally occur under internal pressure and wind uplift. Normally, the bending moments are smaller than the gravity load combinations and the members will remain elastic. The stability checks recommended below assume that plastic hinges will not occur in this uplift condition.