Inelastic shears in ductile RC walls of mid-rise wall-frame buildings and comparison to Eurocode 8

Nonlinear response-history analyses (NLRHA) are performed for 16 five- or eight-storey prototype regular reinforced concrete wall-frame ("dual") buildings designed to Eurocode 8 for ductility class (DC) M (Medium) or H (High), in order to evaluate the higher-mode inelastic magnification of shear forces in the walls. The percentage of the total base shear taken by the walls in the 16 buildings covers evenly a range from 37 to 90 %. The NLRHA results show that there is indeed post-elastic amplification of wall shear forces due to higher modes, but that it is overestimated by the current approach for DC H in Eurocode 8. A recently proposed rational modification of that approach, which suits better modal response spectrum analysis (MRSA), is also evaluated; it is found to be in better overall agreement with NLRHA in an analysis context, but, in general, to underestimate inelastic shears above the ground storey in a design context. The design envelope of wall shears in "dual" buildings prescribed in Eurocode 8 is found to cover such shortfalls; it should be extended to wall systems as well, even in case the current approach in Eurocode 8 is modified per the recent, more rational proposals. The current design envelope of wall moments in Eurocode 8 does not safeguard against plastic hinging at upper levels; even if the linear design envelope is anchored to the moment resistance of the base section, instead of the moment from elastic analysis with the design (i.e., reduced) spectrum, it does not preclude flexural plastic hinging in upper storeys. Although for the design of walls safe-sided envelopes and approximations may be appropriate, for the purposes of evaluation of seismic performance and vulnerability of walls, NLRHA seems to be the only means to estimate with certain confidence the post-elastic higher mode effects on wall shears.