Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 40 1 2 3 4 1 Torsional restraint 2 Stay from side rail forming torsional restraint 3 Segment must satisfy L s (if elastic) or L m (if plastic) 4 Segment must satisfy elastic buckling checks Figure 8.4 Typical portal frame column with plastic hinge at underside of haunch 8.3.2 Out-of-plane stability under gravity combinations If there is a plastic hinge at the underside of the haunch, the distance to the adjacent torsional restraint must be less than the limiting distance L s as given by EN 1993-1-1 § BB.3.1.2. Expression BB.7 should be used when the moment is linear, and BB.8 when the moment is not linear. In addition, the spacing between intermediate lateral restraints should satisfy the requirements for L m as given in BB.3.1.1. If the stability between torsional restraints cannot be verified, it may be necessary to introduce additional torsional restraints. In Figure 8.4, the check between the torsional restraint (indicated as ‘1’ in the figure) and the base was not satisfied – an additional torsional restraint was introduced at location ‘2’. If it is not possible to provide additional intermediate restraints, the size of the member must be increased. In all cases, a lateral restraint must be provided within L m of a plastic hinge. If there is no plastic hinge, the stability of the column should be checked in accordance with Expression 6.62 (See Section 6.2 of this document) Account may be taken of the benefits of tension flange restraint as described in Appendix C of this document.