Last edited 21 Dec 2020

#  Introduction

A material under stress is in a state that has resulted from the application of a force or forces. These forces can also be called stresses. The effect on the material will depend on the type of stress that is applied.

Stress patterns in structural elements can be complex but they usually comprise just three basic types of stress:

• Tensile.
• Compressive.
• Shear.

#  Tensile stress

If the stress acting on a structural member tends to make it longer, it is said to be under tensile stress or 'in tension'. The load carried by each unit area of the member’s cross section is the tensile stress in the member. This will make the member a ‘tie’. Steel is ideally suited to resist tensile stresses and is used widely in construction for this purpose, for example to reinforce concrete, or in the form of cables, wires and chains.

#  Compressive stress

If the stress acting on a member tends to result in it shortening (its components are pushed together), it is said to be under compressive stress or 'in compression'. The load per unit area of the member is the compressive stress. This makes that member a strut, or if the member is large, it might be a column, pier or stanchion, depending on its position in a structure. Most materials can carry some compressive stresses – other than cables, wires, chains and membranes.

#  Shear stress

Shear stresses make the particles of a material slide relative to each other and usually result in deformation. An example is a riveted connection which can shear when excessive force is applied. Vertical forces acting on a cantilever can make it shear off at the wall junction. Shear forces produce shape deformation in materials eg, a rectangular element can be contorted into a skewed parallelogram. The shear stresses are those acting on the planes along which the sliding takes place and are measured across a unit area.

#  Units of stress

The N/m2 is the basic SI unit of stress but is very small for most purposes. As a result, MN/m2 may be used, often expressed as N/mm2 in structural codes for steel, concrete and timber.

#  Related articles on Designing Buildings Wiki

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