Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 34 plastic hinge is predicted at this position, a restraint must be located within h /2 of the hinge position, where h is the depth of the rafter. In Figure 7.2, a hinge is predicted at point 7, and a restraint to the bottom flange has been provided. The restraints to each flange in the haunch region are shown in Figure 7.3. 1 2 4 5 3 6 1. Zone A 2. Depth of haunch 3 Intermediate restraint between torsional restraints 4. Torsional restraints 5. Depth of rafter 6. Restraints to flange Figure 7.3 Restraints in the haunched region of a portal frame It is necessary to check that the distance between torsional restraints (in Figure 7.2 this is indicated as ‘1’ in zone A) on both sides of a plastic hinge does not exceed L s as given in § BB.3.2.2. In zone A, the member is tapered, and the bending moment is not constant. L s is given in § BB.3.2.2 Expression BB.11 for a three flange haunch and Expression BB.12 for a two-flange haunch. In both cases, a factor C n (given in BB.3.3.2) takes account of non-linear moment gradients by calculating relevant parameters at the five cross-sections, as shown in Figure 7.4. The parameter c is a taper factor, given in § BB.3.3.3(1)B. § BB.3.2.2 also demands that the spacing of intermediate lateral restraints satisfies the requirements for L m given in § BB.3.2.1. In Figure 7.2, both lengths indicated ‘2’ must satisfy this check. Expression BB.9 is used for a three flanged haunch and BB.10 for a two-flanged haunch. A three flanged haunch would be the common situation when the haunch is fabricated from a section cutting and welded to the underside of the rafter.