Mass-eccentric building structures: effects of asymmetric distribution of axial forces in vertical resisting elements

Torsional behavior of asymmetric buildings is one of most frequent sources of structural damage and failure during strong ground motions. Plan irregularity of buildings is frequently due to asymmetric distribution of mass that results in rotational motions of the floor slab in addition to translations, even for stiffness and strength symmetric systems. Moreover, asymmetric distributions of mass lead to an asymmetric distribution of axial forces in resisting elements, so that they present different lateral strength capability because of the influence of interaction phenomena. However, simple single storey asymmetric models used so far are not capable to evidence the influence of uneven distribution of axial forces; in fact, they have been developed under the assumption that resisting elements are able to sustain uni-directional horizontal forces only and no allowance for vertical forces is usually made. Therefore, a refined advanced numerical model of one-storey asymmetric building structure has been developed which is able to overcome limitations of the above-mentioned simplified models: this new idealization can take into account the presence of vertical forces due both to gravity loads and to vertical input ground motion as well as the effects of inelastic interaction between axial force and bi-directional horizontal forces. Results obtained from this new model are compared to those from previous models, thus evidencing a significant effect of plan-asymmetry of axial forces due to gravity loads on the overall system lateral-torsional inelastic behaviour.