Abstract

On the 27 of February 2010, a major Mw = 8.8 earthquake occurred off the coast of central Chile in the Maule region, at a depth of 23km. The seismic event affected urban areas along 500 km of the Chilean shore, where a large number of medium-rise reinforced concrete (RC) buildings had been constructed to that date. In cities such as Concepción, Viña del Mar, and Santiago, the response of such RC buildings has been described as overall satisfactory. However, some RC structural walls suffered severe damage of a brittle nature. Therefore, it can be argued that these buildings either remained apparently in the elastic range or, if inelasticity was observed, they presented a damage pattern associated to a brittle failure mode, somehow not specifically reported in the literature before 2010. The damage developed in RC walls was characterized by: (1) buckling of the longitudinal reinforcement in the free ends and the central part of the web; (2) spalling and crushing of concrete in the free ends and along the web; (3) loss of structural integrity between the webs and flanges of asymmetric walls, in some cases; (4) fracturing of the longitudinal rebar lumped at the free ends of the walls, in some cases; (5) shear-driven damage, in some cases; and (6) out-of-plane failure vestiges, in few cases. The identified design deficiencies which are thought to have enabled the damage to occur, are: (1) the absence of boundary elements in the wall ends, (2) the absence of transverse hooks for restraining the vertical bars placed along the central part of the web (distributed rebar); (3) inadequate anchorage of the horizontal bars used for resisting shear forces; (4) the inexistence of a maximum strain for the steel in tension, which accounts for buckling after the cycle is reversed; (5) relatively high gravity axial load, in some cases, and (6) the reduced thickness of the walls, in some cases. This report covers the following aspects: (a) observations of the damage developed in several RC walls of selected buildings, identifying their location within the structure; (b) a review of the requirements for the seismic analysis and design of this type of buildings in Chile, and a summary of their prescriptions along history to date; (c) structural drawings of the cross-sections of most of the damaged walls identified in the first part of the report, and calculations of their axial gravity load ratio; (d) a comparison of the damage observed in situ with that observed in experimental work available in the literature; (e) a theoretical explanation of the mechanics behind the predominant failure mode of flanged walls; (f) a numerical study on the effect of the asymmetries introduced by asymmetric walls on the response of a 12-storey case-study building; and (g) recommendations for the further improvement of the current code provisions.