Optimization of timber-based strengthening techniques for the seismic retrofit of existing masonry buildings

The present study aims at investigating the effectiveness of various timber-based solutions for the seismic and energy retrofit of unreinforced masonry (URM) buildings considering various case scenarios and evaluating both the mechanical and the energetic performance of the intervention. Timber engineered products were employed as base components for the retrofit intervention. The products include both one-dimensional vertical elements (strong-backs) and two-dimensional panels (cross-laminated timber panels (CLT), laminated veneer lumber panels, and oriented strand board panels). The strengthening techniques studied herein have a common root, which consists of connecting such timber-based elements to the surface of the walls of a building by using mechanical or adhesive point-to-point connections. The analysed solutions see the use of strong-backs and timber panels combined with membranes and insulation layers to obtain integrated and highly efficient interventions. The first part of this work focused on the study of retrofit solutions which aim at reducing the out-of-plane vulnerability of existing masonry walls, to avoid the collapse of local masonry portions when an earthquake occurs. The out-of-plane behaviour of URM walls and masonry walls retrofitted with timber strong-backs was experimentally investigated by conducting full-scale static airbag tests. The walls were subjected to semi-cyclic out-of-plane loading through the application of uniformly distributed loads, using inflated airbags in order to simulate the inertia force induced by an earthquake. Five different wall specimens were tested in the as-built and retrofitted conditions, the improvement of out-of-plane seismic capacity was recorded and the performances of different retrofit configurations were investigated. A simplified numerical model to predict the response of the masonry wall retrofitted using timber strong-backs was developedand validated.A sensitivity analysis of the model on parameter variations was performed, and further considerations on the retrofit solution were drawn. An analytical formulation to reproduce the out-of-plane behaviour of masonry wall retrofitted with timber strong-backs was developed. Such formulation is based on the limit analyses theorems to predict the lateral load - displacement curves and the internal actions on all the components (masonry wall, timber elements and fasteners) of the retrofitted walls, with the aim of designing the retrofit intervention and performing linear and non-linear verifications. The analytical results were validated building on the outcomes of the previous experimental campaign and by means of a numerical model developed using a simplified micro-modelling approach. The outcomes of various retrofitted walls are reported as a reference and some consideration on the retrofit solution were drawn. The application of the retrofit solution was extended to the infills of existing reinforced concrete structures.The out-of-plane behaviour of URM infill walls and infill walls retrofitted using combineded timber strong-backs with a ventilated cladding system (with insulated facade) was investigated by conducting full scale static airbag tests. The infill walls were subjected to semi-cyclic out-of-plane loading through the application of uniformly distributed loads, using inflated airbags to simulate the inertia force induced by an earthquake, using a test set-up similar to the one of the experimental campaign conducted previously on URM and retrofitted masonry walls. Ten different wall specimens were tested in the as-built and retrofitted conditions, the improvement of out-of-plane seismic capacity was recorded and the performances of the retrofit on different types of infill walls were investigated. The seismic behaviour of the non-structural layers was also analysed with specific experimental tests. The second part of the work focused on investigating the performances of timber-based strengthening solutions for the in-plane retrofit of unreinforced masonry walls, considering both structural and energetic aspects. The in-plane behaviour of walls retrofitted with CLT panels was investigated numerically, building on previous preliminary analyses and in the lights of the results of experimental evidence collected by the research team at the University of Trento. The effectiveness of this retrofit solution was analysed on several masonry types, considering different masonry properties and geometries. The influence of various parameters (such as masonry-to panel connection typology, panel anchoring connection properties, panel-to-panel side connections, connectors number and position) on the increase in the lateral capacity of the retrofitted walls was studied by means of a series of non-linear quasi-static simulations. The in-plane behaviour of masonry walls retrofitted with timber strong-backs, timber panels and hybrid solutions obtained combining those timber elements has been numerically investigated. Two different modelling approaches were used to model masonry walls characterised by regular and irregular texture, simulating the masonry as an equivalent homogeneous material (macro-modelling approach), or considering the masonry units (micro-modelling approach). The retrofits were designed to be applied either inside or outside the building, and insulation layers and membranes were used to guarantee the durability and the hygrothermal performance of the solutions. The thermophysical properties of the retrofitted walls were evaluated by means of analytical and numerical analyses, considering the heat transmission in both steady and unsteady state conditions and the thermal bridge in correspondence with the corner of the wall. Following the investigation of the seismic performance of retrofitted piers and walls with openings, the study advanced to the analysis of an entire structure. The seismic behaviour of a full-scale building retrofitted with timber-based coating was investigated numerically. A detailed modelling approach was developed, building on previous analyses, to assess both the global and the local behaviour of a retrofitted masonry building and to provide data that could be used as reference for the calibration of less refined, yet computationally more efficient, macro-element models. Various retrofit configurations were analysed, considering the timber panels applied to the entire perimeter walls of the building or to selected elements (e.g., retrofit applied to masonry piers alone or both piers and spandrels) and involving different geometries influenced by the maximum panel transportable size. The effectiveness of the retrofit configurations was evaluated in terms of increased lateral load and displacement capacity through non-linear simulations.