Role of timber diaphragms in the seismic response of unreinforced masonry (URM) buildings

The research presented in this thesis was focused on timber floor diaphragms in unreinforced masonry (URM) buildings. The work was divided into two phases. The first phase was aimed at the investigation of the effects of the in-plane behavior of timber diaphragms on the global seismic response of URM buildings. The second phase was dedicated to the assessment and retrofit of timber floors, with particular attention to the out-of-plane behavior. A study on the equivalent frame method, which is a more and more appreciated masonry modeling technique, is presented. Both as-built and strengthened timber floors were addressed. In order to understand the influence of the masonry modeling method on the seismic response of URM structures when flexible diaphragms are concerned, a simplified elastic no tension method was proposed. Such method is able to describe the characteristic nonlinear behavior of masonry (due to extremely low tensile strength) by means of a series of linear analyses based on a Rankine failure criterion. An in-situ testing campaign on full-scale 100 year old timber diaphragms is presented. Both mechanical and dynamic in-plane properties of wood diaphragms were investigated. Cyclic and snap back tests were carried out thanks to an innovative ad hoc loading system, developed by means of wire ropes and steel pulleys. The loading system was designed to reproduce a realistic inertial load distribution and to be lightweight, versatile and easily relocatable from one diaphragm section to the next. The effect of different refurbishment techniques was also probed during the experimental campaign. The outcomes of a testing campaign regarding out-of-plane refurbishment techniques of existing timber floors by means of timber to timber composite structures are described. A numerical model based on the theory of composite beams with incomplete interaction, was calibrated to take into account the real load distribution and connector spacing. An original procedure to camber timber beams by employing the compression pressure generated by screw fasteners is presented. The camber deflection is attained by superposing a timber reinforcement element on top of a beam and then connecting the two elements by means of screws inserted at 45° to the beam axis. Such method which is currently patent pending, was validated using data obtained from experimental testing. A mathematical formulation was also developed to describe the cambering procedure. A specific experimental campaign was therefore performed to precisely evaluate the amount of pressure that each screw is capable of yielding. Many parameters supposed to affect the compression force, were explored through 170 tests.