In-Plane Orthotropic Behavior of Timber Floor Diaphragms in Unreinforced Masonry Buildings

Abstract

A full-scale experimental program consisting of testing four as-built diaphragms and four retrofitted diaphragms in both principal loading directions is presented. As-built configurations were typical of those found in historic unreinforced masonry buildings in North America and Australasia, whereas retrofitted diaphragms consisted of plywood panel overlays with stapled sheet metal blocking systems (SMBS). Test results were characterized using bilinear representations to establish recognizable performance parameters such as shear strength, shear stiffness, and ductility capacity, which were then used for comparative analysis. The nonlinear and low stiffness behavior of as-built diaphragms was confirmed in each principal loading direction. The plywood overlay and SMBS dramatically improved as-built diaphragm shear strength and shear stiffness and were shown to perform satisfactorily from a serviceability perspective. The orthotropic nature of as-built diaphragms was proven, with perpendicular-to-joist shear stiffness being as low as 68% of the corresponding orthogonal value. A typical duly framed stairwell penetration and discontinuous joists with two-bolt lapped connections were shown to have no detrimental impact on tested diaphragm performance. Predicted diaphragm performance using state-of-art assessment documents was shown to be inconsistent with corresponding values established from testing. It is recommended that these assessment procedures be updated with revised performance parameters and provisions to address diaphragm orthotropic behavior.