Seismic retrofit of masonry walls using timber strong-backs

A high proportion of New Zealand’s clay brick unreinforced masonry (URM) structures have not been retrofitted to resist earthquake forces, and in particular to prevent out-of-plane failures which are the most critical deficiencies of URM buildings. Despite a number of seismic improvement techniques having been applied previously there is a significant lack of experimentally validated simple and cost-effective solutions that also consider the impact on the building tenants, aesthetics and heritage building fabric. The main objectives of the research presented herein were to develop and validate seismic securing techniques for URM solid- and cavity-walls that satisfied the above conditions. Full-scale shake-table testing of two cavity and three double-leaf solid clay brick URM walls was undertaken. The vertical timber framing that is typically considered to be a non-structural support for the inner wall lining was used as part of the retrofit solution and was fixed to the wall using mechanical screw-ties in order to form a strong-back. The intended outcomes of the research reported herein included (i) measuring via laboratory testing the improvement of seismic capacity in terms of peak ground acceleration (PGA) achieved and maximum out-of-plane displacement experienced by the URM solid- or cavity-walls due to strong-back retrofit installation; (ii) comparing the performance of different strong-back configurations; (iii) providing construction details, and providing analytical formulations for response prediction.