OUT-OF-PLANE STRENGTH AND BEHAVIOUR OF MASONRY INFILL WALLS

Abstract

It has been shown in previous studies that masonry infills with rigid supports through boundary frames display greater out-of-plane strength than their flexural wall counterparts. This increase in strength is due to a mechanism often referred to as “arching action”, where an infill is considered as a three-hinged arch after initial cracking separates the infill into two rotating segments under out-of-lane loading. Any further rotation is restrained by the boundary supports, resulting in in-plane compressive forces and delaying further cracking. The failure mechanism is changed from tension controlled to the inherently stronger compression-controlled failure. The literature review on arching action, however, yielded limited available research results in terms of experimental testing and numerical studies. For design, the current Canadian masonry design standard CSA S304-14 specifies that the first principle mechanics be used for calculating the out-of-plane strength of infills without providing explicit equations. The American masonry standard MSJC 2013, on the other hand, adopts a semi-empirical equation for out-of-plane strength of regular infills.

This study is part of an on-going research study to gain understanding on the out-of-plane strength and behaviour of masonry infills in general and in particular, on the effect of several influential design parameters on the infill strength. The previous phases (Phase I and II) of the study focused on the testing of RC framed concrete masonry infills whereas this study dealt with the steel bounding frames and the design parameters considered were more representative of this frame type. A total of seven steel framed concrete masonry infills were subjected to out-of-plane loading to failure and design parameters included boundary frame condition, prior damage, interfacial gap, and presence of vertical loading. The experimental results were obtained and discussed in terms of load vs. displacement curves, cracking pattern, failure mode, and the vertical and horizontal displacement profiles. The experimental results were then used to evaluate the design parameter effects and determine the adequacy of the existing analytical models.

The experimental results showed that a steel framed infill sustained a significantly lower out-of-plane capacity than a RC framed infill. This is attributed to lower friction at steel to infill interface as evidenced by top infill slip-out that initiated a pre-mature failure that was observed in multiple test specimens. The presence of top beam-to-infill gap resulted in a reduction in strength as the two-way arching was not fully developed as evidenced by different cracking pattern and failure mode observed in the test. Prior in-plane damage caused a reduction in the out-of-plane strength of infills and more severe the prior damage, the greater the reduction. Further, the reduction is more pronounced when the loading causing the damage was maintained during the out-of-plane loading stage. The presence of a vertical load applied on the infill through the frame beam resulted in a reduced out-of-plane strength.

The existing analytical methods were shown to provide inconsistently strength estimate for specimens with RC frames vs. steel frames as none of the models considered the difference in interfacial friction between two materials. In general, the analytical models under-predicted the strength of RC framed infills while over-estimating the strength of the steel framed infills.