Field Testing of Flexible Timber Diaphragms in an Existing Vintage URM Building

It is well known that the in-plane behavior of flexible timber diaphragms has a large influence on the global earthquake response of unreinforced masonry (URM) buildings. The type and the activation of out-of-plane URM wall failure mechanisms are strictly related to diaphragm stiffness, which is a property that also governs the effectiveness of the diaphragm to distribute earthquake-induced loads between lateral load resisting wall elements. However, there is a lack of reported experimental campaigns pertaining to these issues, and in particular, few experimental data have been published on the in situ behavior of existing vintage flexible timber floor diaphragms such as single straight sheathed diaphragms that are typically encountered in New Zealand URM buildings. To address this paucity of available in situ test data, an experimental campaign was executed to investigate the as-built cyclic and dynamic behavior of full-scale vintage flexible timber floor diaphragms with the outcomes being presented herein. Two sections of a diaphragm located in a vintage two-story URM building, measuring 6.4×9.6 m and 4.6×9.6 m, were subjected to a series of cyclic and snap back tests in the direction orthogonal to the floor joists. In order to reproduce a realistic inertial load distribution to the test specimens, an ad hoc loading system using wire ropes and steel pulleys was developed. The loading system was designed to be lightweight, versatile, and relocatable between test specimens. From the obtained test results, it was identified that the equivalent stiffness of existing timber diaphragms (in the direction orthogonal to the joists) is predicted with a certain degree of conservativeness using the most recent procedures currently available in literature, while reliance on the ASCE standard can lead to an overestimation of the floor diaphragm stiffness, even at small displacements. Due to the nonlinear behavior of the tested flexible timber diaphragms, the measured natural period proved to be highly dependent on the target displacement.