Beam shear is the internal stress of a beam as caused by the shear forces applied to that beam. Shear forces, or shear stresses, are caused by forces applied parallel to a material, potentially causing deformation of that material. Beam shear can be caused by horizontal or vertical stresses, as well as by bending. Each type of stress affects a beam differently.
In horizontal shear stress, forces may cause a beam to slide from side to side. If the beam is secured, disallowing any movement, the internal shear stress will then attempt to find ways to accommodate the movement, which can sometimes result in the beam bending or fracturing along the internal horizontal layers. If the beam has non-attached layers, which allow a slight amount of movement, it is less likely to fracture or bend.
In vertical beam shear stress, forces are applied to parallel surfaces of the beam. These forces can include parallel sides or the top and bottom ends of the beam. If one of the surfaces experiences greater stress than another, the material will buckle or twist. This action causes a weakening of the overall structure.
Beam shear failure occurs when stresses applied to the beam are greater than the strength of that beam. Failures often result in the collapsing or cracking of the structure surrounding the beam, as is often seen in earthquake damage. The most common type of failure, however, is bending. This occurs when the top surface of a beam becomes compressed, while the bottom surface expands and cracks along vertical axes. This results in a sagging or bending of the beam.
In many cases, to avoid structural failure, a building or structure will be retrofitted. Retrofitting involves creating a secondary framework that serves to support the initial structure, while alleviating the load-bearing forces on that initial structure. Most of the time, this takes the form of external bracing.
To determine shear, a small cross section of beam must be examined and a series of mathematical calculations run based on the measurements and observations of that cross section. The calculations used today are credited to Leonard Euler, a mathematician from the 18th century. The true origins of beam shear studies, however, can be traced back to the work of the 16th-century scientist Galileo Galilei.