Steel Buildings in Europe

Title APPENDIX D Worked Example: Design of portal frame using elastic analysis 29 of 44 4 - 110 7.9.2. End-of-span region In this region the bottom flange is in compression and stability must be checked between torsional restraints. 2930 mm 1 1 1 End of span region 1230 1700 298 kNm 1 2 111 kNm 1 Simplified bending moment 2 Bending moment The buckling length is taken from the torsional restraint at the sharp end of the haunch to the ‘virtual’ restraint which is the point of contraflexure of the bending moment diagram, i.e. where the bending moment is equal to zero. In some countries the assumption of a virtual restraint may not be common practice. If the practice is not allowed, the buckling length should be taken to the next purlin (i.e the first restraint to the compression flange). From the analysis, the buckling length to the point of contracflexure is 2930 mm. If the tension flange is restrained at discreet points between the torsional restraints and the spacing between the restraints to the tension flange is small enough, advantage may be taken of this situation. In order to determine whether or not the spacing between restraints is small enough, Annex BB of EN 1993-1-1 provides an expression to calculate the maximum spacing. If the actual spacing between restraints is smaller than this calculated value, then the methods given in Appendix C of this document may be used to calculate the elastic critical force and the critical moment of the section. Verification of spacing between intermediate restraints In this case, the restraint to the tension flange is provided by the purlins. These purlins are spaced at 1700 mm. L m = 2 y t 2 pl,y 2 1 Ed z 235 756 1 57 , 4 1 38              f AI W C A N i EN 1993-1-1 Annex BB §BB.3.1.1