Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 24 Practical design addresses this interaction in several ways:  Out-of-plane stability near plastic hinges is generally addressed by the concept of stable lengths, L stable , L m , L k and L s . These are assumed to be independent of any interaction with in-plane stability effects (see Section 6.4.).  Interaction between bending moment and axial load is addressed by simultaneously satisfying Expressions 6.61 and 6.62 of EN 1993-1-1. This is usually undertaken by considering the most onerous out-of-plane check (from any part of the member) with the relevant in-plane check. 6.2 Buckling resistance in EN 1993-1-1 The verification of buckling resistance of members is addressed by several clauses in EN 1993-1-1. The clauses of primary interest in portal frame design are described below. 6.3.1 Uniform members in compression. This clause covers strut buckling resistance and the selection of buckling curves. The clause is primarily concerned with flexural buckling, but also addresses torsional and torsional-flexural buckling. These latter modes of failure will not govern the IPE sections and similar cross-sections adopted for portal frames. 6.3.2 Uniform members in bending. This clause covers lateral-torsional buckling of beams. The distribution of bending moments along an unrestrained length of beam has an important influence on the buckling resistance. This is accounted for by the choice of C 1 factor when calculating M cr (See Appendix C). 6.3.3 Uniform members in bending and axial compression. This clause addresses the interaction of axial load and moment, in-plane and out-of-plane. The clause requires the following checks to be carried out unless full second order analysis, including all member imperfections ( P –  , torsional and lateral imperfections), is utilised. 1 M1 z,Rk z,Ed z,Ed yz M1 y,Rk LT y,Ed y,Ed yy M1 y Rk Ed           M Δ M M k M Δ M M N k N (6.61) 1 M1 z,Rk z,Ed z,Ed zz M1 y,Rk LT y,Ed y,Ed zy M1 z Rk Ed           M Δ M M k M Δ M M N k N (6.62) For Class 1, 2, 3 and bi-symmetric Class 4 sections, 0 z,Ed y,Ed   M M   It is helpful to define M1 y.Rk y   N as N b,y,Rd and  LT M1 y,Rk  M as M b,Rd . M z.Ed is zero because the frame is only loaded in its plane.