Overstrength and ductility factors for steel frames designed according to BS 5950

Abstract

Buildings sited outside the seismic zones such as those in Singapore are not designed for seismic loads. However, the buildings in these regions vibrate due to tremors caused by faults in Sumatra, Indonesia. Therefore, in order to assess the vulnerability of these buildings under low-seismic excitation, it is important to determine the overstrength and ductility of these buildings designed for gravity loads and wind loads or notional horizontal load. In this study, steel frames of three-, six- and ten-story three-bay moment resisting frames (MRFs), concentrically braced frames (CBFs), and semirigid frames are designed according to BS 5950, which does not have any seismic provisions. A nonlinear push-over analysis involving P-Δ effect has been performed on these steel frames, considering the effects of various combination of dead load (DL) and imposed load (IL), viz 1.0DL+0.4IL (most common state), 1.0DL + 1.0IL (serviceability state), and 1.2DL + 1.2IL (ultimate state). By using the N2 method, the seismic performance of the steel frames are investigated when they are subjected to the "design earthquake" for Singapore. The results show that MRP, CBF, and semirigid steel frame would be subjected to a base shear up to 4.2% of seismic gravity load, while the corresponding value for infilled frames is up to 7.8%. These values are much less than the strength capacity of the steel frames with considerable response modification factors which are the combination of the overstrength and ductility.