Comparative application to RC buildings of the two generations of Eurocodes and proposals for seismic design

Three concrete buildings with six storeys above ground and two basements have been designed in detail according to the first generation of Eurocodes 2 (including structural fire design) and 8, as well as the official drafts of their second generation counterparts as of the end of 2023. In one horizontal direction the buildings have a wall-, frame-equivalent-dual- or frame-lateral-load-resisting system; in the other, the first two buildings have a wall-system and the third a wall-equivalent-dual. The design seismic action is in all cases according to the current generation Eurocode 8, scaled to a peak ground acceleration on rock of 0.2 or 0.3 g. Seismic design is for ductility class (DC) Medium (M) of the current generation or DC 3 of the new one, which have the same behaviour factors, q, in all structural systems considered; so, in each building seismic action effects from the analysis are the same for the two Eurocode generations. All designs are assessed through nonlinear response history analysis (NLRHA), carried out according to the pertinent Eurocode 8 rules. Designs according to the second generation meet the performance goals of Eurocode 8 much better and transparently than with the first generation, but use markedly larger steel quantities and indeed often unnecessarily so, especially for confining reinforcement—which sometimes comes out in quantities that cannot be placed. Proposals are made for a more rational linkage of local ductility demands with the q-factor and implemented in alternative second generation designs. Minor changes to the new generation’s design/detailing rules for ductile walls are also proposed and implemented in these alternative designs; they prove only partially effective in resolving certain deadlocks originating from poorly justified detailing rules that produce unnecessary and counterproductive wall flexural overstrengths. The new generation Eurocode 8 lacks design rules for the free height of ductile walls within rigid basements, which is a weak link, very likely to fail under the high shear force which balances the wall’s moment resistance at the top of the rigid basement. NLRHA confirms that the alternative provisions proposed for second generation Eurocode 8 give more cost-effective designs than the first generation, with better overall performance at about the same or even lower cost.