Seismic out-of-plane retrofit of URM walls using timber strong-backs

Inadequate seismic performance of unreinforced masonry (URM) buildings is frequently encountered in earthquake prone countries such as Italy and New Zealand and consequently, effective and economical retrofit strategies need to be adopted in order to guarantee safety and the preservation of the URM building stock. The retrofit technique studied herein consisted of connecting vertical timber elements (strong-backs), to the interior surface of the walls of a building using mechanical screws or bolts. The vertical timber framing is typically a non-structural support for the inner wall lining of URM buildings and its use, as a part of the retrofits, resulted in being a cost effective and low impact solution. The out-of-plane behaviour of as-built and retrofitted masonry walls was 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 self-weight 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 developed using the finite element software SAP2000. Its effectiveness was validated by means of a comparison between the experimental and the numerical results as well as a sensitivity analysis of the model on parameter variations, and further considerations on the retrofit solution were drawn. © 2020 Elsevier Ltd. All rights reserved.