Natural Period and Seismic Idealization of Flexible Timber Diaphragms

The current equations for diaphragm fundamental period determination and for diaphragm deformation determination published in commonly used seismic assessment documents are firstly reviewed to establish their origin. Using a validated analytical model that captures diaphragm deformation mechanics, three beam idealizations (a fixed-ended flexure beam, a pin-ended flexure beam, and a shear beam) are compared against true diaphragm behavior to determine which idealization is most suitable for the seismic assessment of diaphragm performance. Wherever necessary, recommendations have been made to update and to harmonize the current seismic assessment procedures for timber diaphragms in unreinforced masonry buildings. The presented analysis is specifically focused on straight-sheathed timber diaphragm configurations that are typically found in historic unreinforced masonry buildings.