Seismic response of gravity-load designed frame systems with masonry infills
Comprehensive experimental-analytical studies on the seismic vulnerability of existing reinforced concrete frame buildings, designed for gravity-loads only as typically found in most seismic prone countries before the introduction of adequate seismic design code provisions, confirmed the inherent weaknesses of these systems, due to inadequate detailing and the lack of capacity design principles. Controversial effects on the global inelastic mechanism can be expected depending on the infills properties (mechanical characteristic and distribution) and the joint damage mechanism. In this contribution, the interaction between un-reinforced masonry infills and r.c. frame systems, when appropriately considering the joint zone non-linear behaviour, is investigated through pushover and nonlinear time-history analyses on 2-D frame systems. Preliminary results are critically discussed. A simplified and reliable analytical model based on a concentrated plasticity approach, validated on different experimental tests on beam-column joints and frame systems (with and without infills), is used and proposed for extensive studies on seismic vulnerability assessment. Limit states, based on interstorey drift and related to joint or infills damage levels are also tentatively suggested.