LAUSR

Abstract In seismic codes, the force strength reduction factor is proposed to transform elastic to inelastic strength. The ductility reduction factor, R μ , plays a key role on R factor if no overstrength is present. The R μ is determined by SDOF systems. But the higher mode (HM) and Multi-Degree-of-Freedom (MDOF) effects need to be considered to extract the R factor . These effects are studied by proposing α v-MDOF via at least 1764 Nonlinear dynamic Analysis of 2D-frames. Also a sensitivity study has been performed on R μ and α v-MDOF . Results obtained from studies conducted on the frames, indicate that HM and MDOF effects have a considerable influence on the base shear. Also the α v-MDOF is typically higher than unity both for ordinary and for near-field earthquakes and it is affected by period, span number and ductility level. These effects are remarkable for near-field motions. Finally, a simplified practical expression is proposed to estimate the α v-MDOF .