Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 5  cr may be found using software or (within certain limits) using Expression 5.2 from EN 1993-1-1. When the frame falls outside the limits, an alternative expression may be used to calculate an approximate value of  cr . Further details are given in Section 3.3. When second order effects are significant, two options are possible:  Rigorous 2 nd order analysis (i.e. in practice, using an appropriate second order software)  Approximate 2 nd order analysis (i.e. hand calculations using first-order analysis with appropriate allowance for second order effects). In the second method, also known as ‘modified first order analysis’, the applied actions are amplified, to allow for second order effects while using first order calculations. This method is described in Section 3.3. 2.3 Design summary  Second order effects occur in the overall frame ( P-  ) and within elements ( P -  ).  Second order effects are quantified by the factor  cr .  For portal frames, the expression given to calculate  cr in EN 1993-1-1 § 5.2.1(4) may be used within certain limits. Outside the limits prescribed by the Standard, an alternative calculation must be made, as described in Appendix B.  Second order effects may be significant in practical portal frames.  Second order effects may be accounted for by either rigorous second order analysis using software or by a first order analysis that is modified by an amplification factor on the actions.