Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 78 depth at the shallower end of the member and limited to members where 1 ≤ h max / h min ≤ 3. Note that the expression for c was derived in reference 4 for elements with    1.05, which is the common case for haunches in portal frames C m accounts for linear moment gradients. The value is given by the Expression BB.13 of EN 1993-1-1 Annex BB. It is recommended that C m ≤ 2,7 C n accounts for non-linear moment gradients. The value is given by the Expression BB.14 of EN 1993-1-1 Annex BB. It is recommended that C n ≤ 2,7 When using EN 1993-1-1 Annex BB.3.3.2, the following points need clarification: The same definition of ‘positive’ and ‘negative’ moments applies as in BB.3.3.1: Moments that produce compression in the non-restrained flange should be taken as positive. This is fundamental as only positive values of R should be taken. BB.3.3.2 assumes that the loads are applied at the shear centre. C.2.2 Calculation of M cr0 For uniform sections, symmetric about the minor axis, restrained along the tension flange at intervals:            t 2 t w 2 2 t 2 z 2 cr0 2 1 GI L EI L EI a a M   but z 2 t 2 z w 2 z 2 cr0 π EI s GI I I s EI M    where: a is the distance between the restrained longitudinal axis (e.g. the centroid of the purlins) and the shear centre of the member. This takes account of the fact that the effective restraint is provided slightly away from the flange L t is the length of the segment along the member between torsional restraints to both flanges s is the distance between the restraints along the restrained longitudinal axis (e.g. the spacing of the purlins). For tapered or haunched members, M cr0 is calculated using the section properties of the shallow ends. The parameters a , L t and s are shown in Figure C.1