Estimating the dynamic properties of wall-frame structures
DOI:
https://doi.org/10.5459/bnzsee.1741Abstract
This study outlines a robust method to approximate the dynamic properties of wall-frame structures with a reasonable degree of reliability based on simple mechanics. The seismic response and drift demand of a structure are largely influenced by its first translational period and mode-shape. This study develops a method to estimate the storey stiffness of a wall, which can be combined with the storey stiffness of a frame to estimate the fundamental period and mode-shape for wall-frame structures. The fundamental period and mode-shape are calculated using this effective storey stiffness and Rayleigh’s principle. A total of 301 wall-frame structures were sized to evaluate the reliability of the proposed method. Structures ranged from 2 to 25 storeys tall. The fundamental period and mode-shape estimated using the proposed method were compared with results from eigenvalue analysis of a detailed linear structural model. The proposed method leads to approximately 4% error on average for estimating the fundamental period of regular structures. The proposed method leads to 5% error on average for irregular structures with partial height walls, as well as variations in storey height, and lateral stiffness. For regular wall-frame structures, the proposed method led to average errors of 4% when estimating the roof mode-shape factor and 2% when estimating the maximum difference in mode-shape factor from one floor to the next, a proxy for storey drift. These errors were 5% and 15% for irregular structures with partial height walls. Results were also compared with estimates obtained from existing empirical equations to approximate the period of wall-frame structures, highlighting that empirical equations lead to greater error, between 15 and 70% depending on the equation. The method outlined in this paper enables users to estimate or corroborate the fundamental period, mode-shape, and lateral displacement for a dual wall-frame structure with a reasonable degree of reliability, suitable for preliminary design and linear analysis. Tools have been developed in MathCAD and python to automate the procedure for estimating the dynamic properties of wall-frame structures and are available here.
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