Steel Buildings in Europe

Part 4: Detailed Design 4 – 7 Relevant action effects are:  Externally applied horizontal loads, e.g. wind, H Ed  Equivalent horizontal forces (EHF) used to allow for frame imperfections, V Ed   Other possible sway effects. (These are rare in low rise multi-storey frames but could occur, for example where the building is on a sloping site with differing levels of foundations. In such circumstances, axial shortening of the different lengths of columns will cause overall sway.)  cr < 3.0 For  cr < 3.0, EN 1993-1-1, § 5.2.2 requires such structures to be analysed by second order analysis. This approach is not recommended for low or medium rise buildings. Second order analysis may lead to marginal economies in the mass of bracing systems but this advantage is more than offset by the increased design costs and the difficulty of optimising such structures. Such flexible structures are anyway likely to fail horizontal serviceability criteria. 2.5 General design procedure Unless the simple approach described in Section 2.6 for low rise buildings is adopted, the general design process is as follows: 1. Determine the ULS vertical actions. 2. Calculate the equivalent horizontal forces (EHF) to allow for imperfections (see Section 2.7.1 of this guide). 3. Determine the ULS horizontal loads. 4. Determine the total horizontal loads (from 2 and 3 above). 5. Choose bracing configuration and choose bracing members, based on the total horizontal loads. (Note that the wind forces and EHF are usually distributed to individual braced bays by simple load allocation techniques, thus avoiding the need for a three-dimensional analysis). 6. Carry out first order analysis of the braced frames to determine both the forces in the bracing system and the sway stiffness of the frames. (This two dimensional analysis of each vertical bracing system is usually carried out by computer to provide ready access to displacements). 7. For each floor of each braced bay, determine the ‘local’  cr from:  cr = H,Ed Ed Ed  h V H       8. Determine the governing  cr as the lowest value obtained from the analysis above. 9. If  cr > 10, second order effects are small enough to be ignored. If 3,0 <  cr < 10, calculate the amplification factor and increase all relevant action effects (the bracing may need to be re-designed).