Dynamic performance of timber diaphragms in the 1903 Nathan House

The flexibility of timber diaphragms in unreinforced masonry buildings has been reported to have a significant effect on the seismic performance of the complete structure. The current absence of New Zealand specific data has resulted in practitioners adopting assessment guides developed overseas to assist in seismic assessment and the design of seismic retrofit solutions. To address this issue a series of modal tests were conducted on the third floor timber diaphragm in Nathan Building located in Auckland’s Britomart Precinct, which is a building typical of New Zealand historic unreinforced masonry construction. Preliminary analysis indicates that the fundamental horizontal natural frequency occurs at 20.5 Hz which reasonably matches the finite element model, which predicted a frequency of 18.49 Hz. Further testing, system identification and finite element updating is intended to be conducted to refine these preliminary results.