Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 37 Haunch stability in Zone E In Zone E, (see Figure 7.5) the top flange of the haunch will be in compression and will be restrained by the purlins. The moments and axial forces are smaller than those in the gravity load combination. The members should be verified using Expression 6.62 (see Section 6.2 of this document). By inspection, it should be clear that the rafter in this zone will be satisfactory. Stability in Zone F In Zone F, the purlins will not restrain the bottom flange, which is in compression. The rafter must be verified between torsional restraints. A torsional restraint will generally be provided adjacent to the apex, as shown in Figure 7.5. The rafter may be stable between this point and the virtual restraint at the point of contraflexure. If the rafter is not stable over this length, additional torsional restraints may be introduced, and each length of the rafter verified. This verification may be carried out using Expression 6.62. The beneficial effects of the restraints to the tension flange (the top flange, in this combination) may be accounted for using a modification factor C m , taken from § BB.3.3.1(1)B for linear moment gradients and from § BB.3.3.2(1)B for non-linear moment gradients. If this benefit is utilised, the spacing of the intermediate restraints should also satisfy the requirements for L m , found in § BB.3.1.1. 7.4 In-plane stability In addition to the out-of-plane checks described in Section 7.3, in-plane checks must be satisfied using Expression 6.61. For the in-plane checks, the axial resistance M1 y Ed   N is based on the system length of the rafter. The buckling resistance M1 y,Rk LT   M should be taken as the least resistance from any of the zones described in Section 7.3. 7.5 Design summary  Rafters should be IPE or similar sections with Class 1 or Class 2 proportions under combined moment and axial load. Sections containing plastic hinges must be Class 1.  Cross-sections should be checked to Section 6 of EN 1993-1-1.  Detailed checks must be carried out to ensure adequate out-of-plane stability under both gravity and uplift conditions – see Sections 7.3.1 and 7.3.2.  In-plane stability of the rafters and interaction with out-of-plane stability must be verified, using Expressions 6.61 and 6.62 – see Section 6.2.