A shear force is a force applied perpendicular to a surface, in opposition to an offset force acting in the opposite direction. This results in a shear strain. In simple terms, one part of the surface is pushed in one direction, while another part of the surface is pushed in the opposite direction.
This is different to compression, which occurs when the two opposing forces are pushing into each other at the same point (ie they are not offset), resulting in compressive stress.
When a structural member experiences failure by shear, two parts of it are pushed in different directions, for example, when a piece of paper is cut by scissors.
Large or high-rise buildings must be designed with shear walls to provide resistance to shear forces, which might otherwise push over parallel structural elements of a building. For more information, see Shear wall.
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