Dynamic performance of timber diaphragms in the 1903 Nathan House

Abstract

To date researchers and practitioners have assessed expected unreinforced masonry (URM) building response and formulated seismic retrofit solutions based on limited laboratory data, and inadequately validated modelling techniques. A problem exists that throughout the world almost no in-field testing has been undertaken to generate data with which to validate the accuracy of models and laboratory based studies of retrofit solutions. It is widely recognised that the behaviour of timber diaphragms in URM buildings is crucial to the response of the structure as a whole but little is known about the actual response and dynamic characteristics of these diaphragms in existing buildings. This includes both the properties of the diaphragm itself, and the dynamic characteristics of the diaphragm-wall connection when cyclically loaded either normal or tangential to the plane of the wall. A current research programme has been initiated to investigate the dynamic behaviour of the timber diaphragms in the Nathan House, located in Auckland’s Britomart precinct. The timber diaphragm was tested in-situ using a dynamic mass shaker. The forced vibration test enabled determination of dynamic properties such as natural frequencies, damping ratios and modal shapes. These modal parameters were used to calibrate stiffness and boundary conditions of an FEM model of the diaphragm, enabling estimation of the stiffness of the diaphragm and diaphragm-wall connections. The results of this test will improve the understanding of timber diaphragm response, including stiffness characteristics and connection performance to the URM walls. The data collected from this test will aid the development of effective retrofit solutions and is essential in the accurate finite element modelling of URM structures.