Experimental shear testing of unreinforced masonry wall panels 2016 NZSEE Conference H. Qiu, R. Chin, J. Ingham, and D. Dizhur The University of Auckland, Auckland, New Zealand ABSTRACT: An experimental program was undertaken with the principal aim of determining the transition point between the stair-step failure mode and diagonal tension failure mode of eight 1200 mm x 1200 mm URM wall panels when subjected to simulated earthquake lateral loads. Preparations included the formulation of mortar compositions and then subsequent pairing with bricks of varying strengths to replicate the range of material characteristics of existing URM structures found throughout New Zealand. Diagonal shear tests were conducted with experimental results indicating two distinct failure mechanisms. It was concluded that the transition between failure modes occurs when the mortar to brick compressive strength ratio is approximately 0.4. In addition, following the failure of the wall panels, three panels were repaired using 8 mm steel wire rope placed in differing orientations and quantities in order to investigate the feasibility and performance of this repair technique. Steel wire rope proved to be a simple and cost effective remediation method with improvements in diagonal shear strength and displacement capacity of up to double and fifty times respectively that of the as-built counterparts. 1 INTRODUCTION According to Magenes and Calvi (1997) the in-plane failure modes of unreinforced masonry (URM) piers can be categorised as either: rocking, diagonal shear, or bed joint sliding. Rocking and bed joint sliding types of failure modes typically allow for the dispersion of energy in cycles through displacement. Comparatively, diagonal shear failures are typically more critical and may be relatively more brittle in nature. Diagonal shear failure may develop as one of two failure mechanisms (see Figure 1), as cracks may either develop through both brick units and mortar joints or through the mortar joints alone in a stepping pattern depending on the ratio between mortar and brick strengths (Dizhur & Ingham, 2013). Failure through both brick units and mortar joints is recognised as being relatively brittle in nature, as the shear strength capacity of the wall deteriorates heavily after the maximum shear stress has been achieved. In the NZSEE (2015) assessment guidelines, URM pier diagonal tension failure modes that are dominated by brick splitting correspond to a force reduction factor, K R of 1.0. In contrast, stair- stepped failure though the mortar bed and head joints creates multiple sliding planes analogous to the bed joint shear sliding failure mode, where additional energy from seismic forces can be subsequently dispersed through sliding. In the NZSEE (2015) assessment guidelines, the stair-step failure mode in URM piers corresponds to a force reduction factor, K R of 3.0. When calculating the URM spandrel capacity in Section 10.8.6.3 (NZSEE, 2015), it is also important to be able to distinguish between stair-step failure modes and diagonal tension failure modes that are dominated by brick splitting, see Figure 1. Consequently, determining the transition point between the two mechanisms of diagonal shear failure is paramount for understanding the behaviour of URM piers and spandrels subjected to in-plane loading. Currently limited guidance is provided in the NZSEE (2015) document on distinguishing the occurrence of the two failure mechanisms. This information would allow engineers to more accurately assess the seismic vulnerability and shear capacity URM buildings. The experimental program reported herein was undertaken to address and attempt to provide such valuable information for
Experimental shear testing of unreinforced masonry wall panels
May 19, 2023
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