Shear strength

Shear is a type of stress in which an applied force causes a structure to 'slide' in two or more directions. Shear can cause a structural member to split vertically or diagonally. For example, a cantilever beam built into a wall may shear at the point of support due either to its own weight or the applied forces, or both.

Shear strength is the ability of a material or component to resist shear forces without failing or, put another way, the maximum shear force that can be accommodated before failing.

Shear strength is analogous to ultimate tensile strength (UTS). The difference is that in shear the strain is parallel to the face (or cross-section) of the element in question whereas tensile strain is at right angles.

In structures, knowing the shear strength of materials is critical to be able to design or specify structural components (e.g beams, plates, bolts etc) economically whilst still withstanding shear forces.

In timber, shear strength tends to be affected by the direction of loading in relation to the grain. The shear strength tends to be around 10-15% of its tensile strength (in the direction of the grain). But shear strength will be reduced by the presence of knots, cracks and faults.

Adhesives tend to have high shear strengths. This can be measured by bonding two strips together then pulling them apart under a constant load. Typically, an epoxy resin adhesive can have shear strengths in the region of 26MPa, however, this is an idealised test and in use the shear strength of an adhesive can depend on many variables including surface preparation, conditions and so on.

Bolts can have a critical function in structures, for example, if they are used to connect a steel frame together or fix steel beams to a concrete core. In such instances, bolts may be required to withstand significant shear forces.

Typical shear strengths include:

• Concrete – 6-17 MPa
• Epoxy resin adhesive – 26MPa
• Aluminium – 55.2 MPa
• Mild steel – 345-525 MPa.

In soil mechanics, the shear strength of soil is the shear force which can be sustained by the soil. This is dependent on numerous variables such as the friction between particles and the degree of interlock between them, whether they are cemented together or bonded at contact surfaces and so on.

Determining the shear strength of a cohesionless soil can be achieved by using either a direct shear test or a box shear test. Using the direct shear method allows the cohesion and angle of internal friction of soil to be established, which can be useful in the engineering design of elements such as foundations and retaining walls.

The direct shear test can be applied to the following soil conditions:

• Unconsolidated-undrained.
• Consolidated-undrained.
• Consolidated-drained.

See also: Shear.

[edit] Related articles on Designing Buildings Wiki

• Compression.
• Defects in construction.
• Failure of cast iron beams.
• Latent defects.
• Metal fabrication.
• Rebar.
• Shear.
• Stainless steel.
• Structural failures.
• Structural steelwork.
• Tensile structures.
• Types of bolt.
• Types of steel.