When we apply an external force to a building element, it tends to deform. This deformation is opposed by stresses or internal forces created within the fibers of the element. If the magnitude of these stresses is within the level which can be satisfactorily withstood by the material, then the building will remain structurally stable.
Types of stresses
The formula for calculating stress is W / A, where W = load and A = cross-sectional area. Stress is measured in N/mm2 or kN/m2.
Compressive stress is the internal force set up within a structural element when an externally applied force produces a tendency for the member to be compressed. Then the element is said to be in compression. Some building construction materials have higher resistance capacity for compressive stresses. A very common example is concrete. The amount of stress in a structural member will increase if the load is increased due to an increase in dead loading caused by the weight of the structural elements.
Tensile stress is the internal force which induced within an element that resists an external loading that produces a tendency to stretch the component. In this situation, the member is in tension. Steel is very good at withstanding tensile stress. But concrete is weaker in resisting tensile.
Different materials are better at withstanding the different kinds of stresses. Therefore, the use of two materials called composite materials to produce structural elements is common. Reinforced concrete beams are an example of this.
In a simply supported concrete beam, the top section of the beam is in compression. So, the fibers are being crushed together. The fibers in the bottom section are being pulled apart. So, this section is in tension.
Normally, steel has been placed at the bottom to withstand the tension. And the top half of the beam is concrete to withstand the compression. In reinforced concrete, both compressive resistance of concrete, as well as tensile resistance of steel, is utilized. This makes a very economical beam section.
When an external load is applied to a building element, sometimes it tends one part of the building element to slide past the adjacent layer. The resisting internal stress opposing this action created within the building element is called shear stress.
Torsional stress is the internal force created within a structural element that resists an externally applied loading which would cause the element to twist.
When tensile or compressive stress is created on a building element, it tries to increase or decrease the length of the element.
The value of change in length depends upon the length of the element, the applied loading, and the material stiffness.
The ratio between the change in length and the original length of the element is known as strain (e).
e = ∂L / L where ∂l = change in length and L = original length.
The strain has no unit.
This effect can be seen in materials subject to shear stress. However, the deformation caused in such cases tends to change the element into a parallelogram shape.
The relationship between stress and strain (subject to loading limits, i.e. within elastic limit) is directly proportional and is a measure of the material’s stiffness. The relationship or the ratio between the stress and strain is called Young’s modulus of elasticity.
When we apply a force on a building element, sometimes it tends the element to bend. The term given to such a tendency is the moment. The value of such a moment depends on the magnitude of the applied force and the perpendicular distance from the point of rotation to the point at which the loading is applied. Due to the effect of leverage, a comparatively small load applied at sufficient distances from the point of action can create large rotational force.
For the structure to remain stable, moments on the structure must be in equilibrium, i.e. clockwise moments (+) must balance anticlockwise moments (–).
When multiplied by the value of applied force by the perpendicular distance from the point of action which is called lever arm we can calculate the magnitude of the moment. It is expressed in newton millimeters or kilonewton meters. When designing a building element, it is required to calculate the maximum bending moment that will be created in a structural element by the expected loading system.