TORSIONAL EFFECTS INDUCED BY AN ECCENTRIC WALL IN ONE-STOREY FRAME STRUCTURES

During the 2012 Emilia earthquake, several precast RC industrial buildings were damaged or collapsed due to a strong rotational response triggered by the presence of a single or few eccentric walls within the regular mesh of columns. The objective of this work is to analyse the torsional effects induced by an eccentric wall in one-storey frame structures, and to provide formulas for a quick estimation of the displacement of the flexible side. Two displacement amplification factors (one static and one dynamic), defined as the ratio between the translational displacement of the irregular structure over the translational displacement of the corresponding equivalent regular structure without the added wall/s, are introduced to evaluate the torsional response of the system. The static amplification factor has been obtained by means of a simple linear static analysis using the method of auxiliary constraints. The dynamic amplification factor has been obtained by applying the so-called “Alpha method” to predict to rotational response of the irregular eccentric structure. The two factors shed light on the seismic behaviour of existing precast RC industrial buildings characterised by such a configuration, and provide indications for the retrofitting design of regular buildings based on the stiffness/strength concept (rather than on the ductility/dissipation concept) with the insertion of a set of walls. The results indicate that the presence of shear walls, even though significantly stiffer than the columns with respect to lateral action, does not always lead to a reduction of the seismic displacement demand in the columns.