A precision and bias study of four masonry flexural stress bond wrenches
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Prior to 1980, one of the constraints in the use of unreinforced and reinforced masonry in earthquake and high wind areas was the relative paucity of design information on the tensile properties of masonry. Wind and earthquake loads induce curvatures in a masonry wall, usually resulting in tensile stress being developed in parts of the wall. If the applied tensile stress field exceeds the capacity of the masonry to resist the stress field the wall will crack and potentially fail, often with fatal results for the inhabitants. Two Victorian researchers, at the Brick Development Research Laboratory in Melbourne in 1980, developed the masonry bond wrench. The bond wrench provides an indirect method of measuring the tensile capacity of masonry in bending. This seminal instrument significantly advanced the ability of scientists to understand the response of masonry to wind and earthquake loads. The purpose of this study is to investigate the precision and bias of four bond wrenches, two of which are commonly used in masonry research and two new wrenches which follow the design concepts of the Italian masonry researchers who study ultralow strength lime based mortars in historic buildings. The principal conclusion, from the research, is that the two international standard wrenches, the Australian Standard AS 3700 Bond Wrench and the ASTM C1072 Bond Wrench exhibit an unacceptable bias and poor precision. The two new wrenches, IB and IUB, based on the conceptual ideas from the Italian research provide a significant improvement on the precision and exhibit only a moderate bias.
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