Analytical and numerical modelling of the in-plane response of timber diaphragms retrofitted with plywood panels

In the region of Groningen (NL), human-induced earthquakes initiated by gas extraction are causing structural damage. In that area, the building stock is mainly composed of unrein- forced masonry (URM) buildings with light and flexible timber floors and roofs. Thus, an ex- perimental campaign was arranged for assessing the in-plane response of these diaphragms, and a retrofitting method was developed, consisting of an overlay of plywood panels screwed to the existing sheathing around their perimeter. This light, reversible stiffening measure showed a great increase in the in-plane strength, stiffness, and energy dissipation of the floors. Subsequently, an analytical model was developed, showing very good agreement with experimental results, and enabling the design of retrofitting interventions with this technique. Starting from the formulated model, a user-supplied subroutine was implemented in the finite element software DIANA FEA, allowing to represent the in-plane response of the diaphragms, including their energy dissipation. Finally, the impact of this retrofitting intervention on a case study of an existing building was evaluated by means of nonlinear time-history analyses. The results of numerical analyses show that the user-supplied subroutine accurately describes the in-plane behaviour of the retrofitted timber floors. Besides, the proposed retrofitting tech- nique greatly increases the global seismic performance of the building, compared with both its as-built configuration and to stiffer and less reversible strengthening measures.