Direct Shear Force

In a previous article we discussed the definition of a force and discussed how a normal force acts perpendicular to an area to produce a normal stress. However, there is another main type of force in addition to a normal force, this is a shear force.

A shear force is a force that is applied parallel to a plane, not perpendicular to it. Instead of trying to extend or squash an element like a normal force, a shear force tries to deform an element sideways.

A shear force is simply a force acting in a specific direction, i.e. parallel to a plane. The units of shear force are therefore the same as the units of normal force, newtons, N. Typically structural engineers analyses shear forces that are typically in the order of magnitude of kilonewtons, kN
Figure 1: A shear force is a force that acts parallel to a surface

As with a normal force, any applied force also needs to have a reaction, again this takes the form of an equal and opposite force.

Figure 2: A shear force must satisfy force equilibrium, a shear force reaction resists an applied shear force.

As with force equilibrium, shear forces applied to an element or a system need to be in equilibrium, if not an acceleration occurs. In a beam, when a load is applied vertically, it generates a shear force in the cross section of the element. In the example below the applied load generates a shear force, shear force equilibrium is provided by the resulting reaction at the beam end.

Figure 3: Shear force generated in a beam due to an applied load and the resulting reactions.

A shear force is referred to as a “direct” shear force if it is a direct result of an applied load or shear force equilibrium.

Resolving Forces on a Plane

Both normal forces and shear forces are forces that are applied at specific angles to a plane, perpendicularly in the case of a normal force and parallel in the case of a shear force. Often forces will be applied to a plane at an angle that is not parallel or perpendicular. When this is the case the inclined force can always be resolved into a normal force and a shear force, the relative magnitude of each will change as the inclined force angle changes.

Figure 4: Resolving an inclined force into its normal force and a shear force.

Shear Resistance

It is important to consider the effects of both normal forces and shear forces on an element, some elements are less strong in shear than they are in compression, tension or bending. An example of this is a deck of cards lying on a table. If you compress a deck of cards it would take a large force to crush the deck, however due to the planar nature of cards, if you apply a force acting parallel to the plane of the cards, a shear force in other words, then it is very easy to displace the deck. An force applied to the deck of cards at an angle will likely cause a shear failure, prior to crushing the pack of cards.

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