COLLAPSE OF MASONRY BAY WINDOWS UNDER HORIZONTAL ACTIONS: SEARCHING FOR KINEMATICALLY AND STATICALLY ADMISSIBLE LOAD MULTIPLIERS

The present contribution illustrates a first set of results obtained within the framework of currently on-going research aimed at evaluating the seismic vulnerability of masonry buildings in Boston’s Back Bay area. In particular, this study focuses on "bay win- dows", a typical late 19th century architectural element of the Victorian style that character- ises the façades of Back Bay’s classic buildings. The problem of determining the collapse behaviour of bay windows is complicated by the polygonal shape of their cross section, the presence of openings, and the non-negligible uncertainties regarding the actual degree of connection between the bay window, the floor slabs and the rest of façade. In order to build simplified but effective models that could enable assessing the influence of the main parame- ters on the bay windows response to horizontal loads, here we consider a single bay window isolated from the rest of the building and resting on a rigid basement. Following a simplified approach first proposed by Giuffrè [4], [5], a series of experimental tests has been conducted on five scale model bay windows of varying height free from openings. A system of horizontal actions proportional to the model mass density distribution has been imposed by placing the model on a "tilting-table". The recordings of two digital video cameras allowed the identifica- tion of the collapse mechanisms. The experimental results are interpreted by means of some simple mechanical schemes that make use of typical tools of limit analysis [2, [3]. Masonry is modelled as a material obeying an expressly developed anisotropic failure criterion. The ob- served collapse modes are used to define a suitable set of collapse mechanisms and the corre- sponding values of kinematically admissible load multipliers as well. Statically admissible stress fields are used to determine lower-bound estimates of the load multipliers according to the static theorem. The proposed approach enables obtaining results in reasonable agreement with the experiments. Moreover, it provides a first rational basis for understanding the me- chanical response of bay windows and roughly estimating the PGA limit values.