Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 51 Figure 9.17 Effect of purlin flexibility on bracing 9.5 Bracing at plastic hinges Section 6.3.5.2 of EN 1993-1-1 recommends that bracing should be provided to both tension and compression flanges at or within 0,5 h of the calculated plastic hinges, where h is the depth of the member (see Figure 9.18). h 2 1 0.5h 0.5h 1. Hinge position 2. Member must be braced within these limits Figure 9.18 Bracing at plastic hinges EN 1993-1-1 recommends that the bracing to a plastic hinge should be designed assuming that the compression flange exerts a lateral load of 2,5% of the flange force, (taken as the plastic moment resistance/depth of section) perpendicular to the web of the member. In addition, according to § 6.3.5.2(5)B of EN 1993-1-1, the bracing system must be able to resist the effects of local forces Q m applied at each stabilised member at the plastic hinge locations, where: 100 1,5 f,Ed m m N Q   where: N f,Ed is the axial force in the compressed flange of the stabilised member at the plastic hinge location α m is a coefficient to recognise the statistical benefits of restraining a group of members compared with an individual member         m 0,5 1 1 m  in which m is the number of members to be restrained. Where the plastic hinge is braced by diagonals from the purlins (see Figure 6.3), the stiffness of the ‘U-frame’ formed by the purlin and diagonals is especially important. Where the proportions of the members, purlins or spacings differ from previous practice, the effectiveness should be checked. In