Testing and Modeling In-Plane Behavior of Retrofitted Timber Diaphragms

An in-situ experimental campaign investigating the in-plane behaviour of retrofitted timber floor diaphragms was undertaken on full scale specimens located in a two-storey clay brick unreinforced masonry (URM) building constructed circa 1913. The diaphragm retrofit strategies included re-nailing of board-to-joist connections, use of a fire-rated ceiling, use of steel chords, and the application of a plywood sheet overlay. When compared to as-built single straight-sheathed diaphragms, all tested solutions provided significant stiffening of the in-plane diaphragm response, with the sole exception being the configuration incorporating steel chords, where the chord influence was observed to be negligible. The fire-rated ceiling showed noticeable strength degradation due to cyclic loading while the plywood sheet overlay exhibited no strength loss between cycles to the same deformation levels, demonstrating the ability of the plywood overlaid diaphragm to resist multi-cyclic loading. The subsequent phase of the study included comprehensive numerical modelling of the adopted retrofit strategies and their validation against the attained experimental results. A detailed parametric study on retrofitted timber floor diaphragms was also undertaken and is reported herein. These numerical investigations allowed evaluation of the most efficient overlay pattern, and also allowed the influence of aspects such as the plywood thickness and connection properties to be considered.