Seismic analysis of in-plane loaded walls in unreinforced masonry buildings with flexible diaphragms

Yasuto Nakamura,Michael C Griffith,Jason M Ingham,Hossein Derakhshan

doi:10.5459/bnzsee.47.4.275-289

Yasuto Nakamura, Michael C Griffith + Show 2 more

Open Access

https://doi.org/10.5459/bnzsee.47.4.275-289

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Abstract

It is well recognised that the dynamic response of unreinforced masonry buildings with flexible timber diaphragms typically contains multiple dominant modes associated with the excitations of the diaphragms and the in-plane walls. Existing linear analysis methods for this type of structure commonly account for the multi-mode behaviour by assuming the independent vibrations of the in-plane loaded walls (in-plane walls) and the diaphragms. Specifically, the in-plane walls are considered to be rigid and the unmodified ground motion is assumed to be transmitted up the walls to the diaphragm ends. While this assumption may be appropriate for many low-rise unreinforced masonry buildings, neglecting the dynamic interaction between the diaphragms and the in-plane walls can lead to unreliable predictions of seismic demands. An alternative analysis approach is proposed in this paper, based on the mode properties of a system in which (1) the mass ratios between the diaphragms and the in-plane wall are the same at all levels, and (2) the periods of the diaphragms are the same at all levels. It is proposed that under these conditions, two modes are typically sufficient to obtain the peak seismic demands of the in-plane walls in elastically responding low-rise regular buildings. The applicability of the two-mode analysis approach is assessed for more general diaphragm configurations by sensitivity analysis, and the limitations are identified. The two-mode approach is then used to derive a response modification factor, which may be used in conjunction with a linear static procedure in the seismic assessment of buildings with flexible diaphragms.

Highlights

In older unreinforced masonry (URM) buildings, the floor and roof diaphragms are often constructed of flexible timber systems
The excitations of the diaphragms themselves provide feed-back effects on the in-plane walls, potentially modifying the behaviour of the walls. As these older structures are highly vulnerable to seismic action, there is a need for a practical procedure to evaluate the seismic demand imposed on the inplane walls for the global assessment of URM buildings with flexible diaphragms
This study proposes an alternative, and more rational, method to account for diaphragm flexibility by explicitly considering the interaction between the elastically responding in-plane walls and the diaphragms

Summary

Introduction

In older unreinforced masonry (URM) buildings, the floor and roof diaphragms are often constructed of flexible timber systems. The flexibility of the diaphragms introduce two dynamic effects that are absent in rigid diaphragm structures. The excitations of the diaphragms themselves provide feed-back effects on the in-plane walls, potentially modifying the behaviour of the walls. As these older structures are highly vulnerable to seismic action, there is a need for a practical procedure to evaluate the seismic demand imposed on the inplane walls for the global assessment of URM buildings with flexible diaphragms. The simplest seismic analysis procedure, and likely the first choice of analysis in practice for typical low-rise regular URM buildings, is the linear static method. In the context of the performance-based assessment guideline of ASCE 41-13 [1], the equivalent static base shear is determined using a corresponding linear system,

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