In-plane cyclic load test of masonry wall with RC overlay on one face

Abstract

Unreinforced brick masonry buildings, a prevalent structural typology for residential buildings in Bangladesh, require attention to improve their seismic performance. These buildings or their wall components are rarely designed to resist lateral loads. In seismic events, they suffer severe damage causing loss of livesand properties. That is why their seismic retrofitting and strengthening is very important for the seismic resilience of Bangladesh. This research presents an experimental investigationof seismic retrofitting of unreinforced masonry walls and its findings.

In-plane cyclic loading tests,acting simultaneously withgravity loads, were conducted on a series of half-scale masonry walls of thickness 127 mm. The walls were constructed to replicate a full-scale 10 in. masonry wall with English bond conventionally used in our country. Samples with two aspect ratios (height to length ratio), namely 1.0 (1524mm height x 1524 mm length)and 0.75 (1524 mm height x 2032 mm length),were prepared and tested under lateral cyclic loadings and their response and failure modes were observed. Then the failed samples were retrofitted and the two others were directly strengthened with a 40 mm thick RC overlay on one face having 6 mm dia. rebars provided @75 mm c/c in both directions. The cage was anchored to the wall with the help of hooks and to the base using epoxy. The lateral load carrying capacity of short walls (AR=1.0) was increased by 3.2 times for both the directlystrengthened sample and the sample retrofitted after failure.For long walls (AR=0.75), the capacities increased3.43 times and 2.85 times,respectively. The capability of undergoing a large deformation prior to failure was also improved remarkably due to the incorporation of RC overlay.

The dissipated energyfor directly strengthened short wallswas11.56 timesand for short walls retrofitted after failure was 9.1 timesof that of the control specimens.For directly strengthened long walls,the dissipated energy was 12.34 times and that for long walls retrofitted after failure was 2.93 times. The ductility was also improved by a factor of 4.3 (for samplesretrofitted after failure)and 7.45 (for direct strengthened samples)for short walls and 3.4 (for samples retrofitted after failure) and 8.9 (for direct strengthened samples) times for long wallsin comparison to the control samples. The RC overlay also influenced hysteresis damping. The hysteresis damping ratio for short wall assembly ranges from 7.85% to 16.61 % when retrofitted and 10.77 % to 17.29 % when directly strengthened. In the case of the long wall control specimen, the damping ratio was 5.07 % to 13.7 % which increased to the range of 13.01 % to 19.16 % when retrofitted and 11.89 % to 31.83 % when directly strengthened. The failure modes of the walls also changed from rocking and sliding to rocking and flexural compression, and in some cases, it was the formation of diagonal cracks. The strengthened samples exhibit a rocking type of failure and there was also a sign of corner crushing as the cracks propagated towards the toe. The test results suggest that the RC overlay improved the seismic response of unreinforced masonry (URM) walls in terms of all the parameters stated above. Regarding the influence of aspect ratio, it was found that the long walls exhibited better performance compared to the short walls with an aspect ratio equal to one.