In-Plane Behavior of Timber Diaphragms Retrofitted with CLT Panels

Traditional un-reinforced masonry (URM) buildings often comprise flexible wooden floor diaphragms where a layer of floorboards is laid perpendicularly over the timber joists and fixed by means of nails. It is not rare that such floor structures need to be improved in order to satisfy serviceability and ultimate limit states criteria, as concerns their behavior both under out-of-plane and in-plane loading. A common retrofit technique, mainly aimed at improving out-of-plane floor performance, consists in the application of a layer of cross-laminated timber (CLT) panels laid over the existing floorboards and oriented parallel to the joists. Appropriate panel-to-joist shear connection determines a joist-slab composite behavior that results in a significant improvement of both the of out-of-plane strength and stiffness. In addition, if adequate connections between adjacent panels are provided, CLT elements can also increase the in-plane strength/stiffness, a key-aspect in defining the seismic response of URM buildings. Both force distribution among vertical resisting piers and out-of-plane deformation of masonry walls are in fact affected by the diaphragm in-plane behavior. Inadequate in-plane response from the diaphragms can lead to the out-of-plane collapse of large masonry portions (1st mode failures). Non-linear static and non-linear dynamic simulations proved that the above mentioned retrofit strategy can effectively increase diaphragm in-plane stiffness and limit the in-plane displacement demand of diaphragms subjected to seismic shaking.