What is Shear Stress? – Definition, Equation & Units

Definition of Shear Stress
Have you ever tried to bend and even break a wooden stick but failed because it was too thick? Then you probably leaned it against a wall and stepped on it really hard and broke it. What made it break is shear stress.
Shear stress is the amount of force per unit area perpendicular to the axle of the member. When you stepped on the wooden stick really hard, the impact load on the stick caused two types of stresses:
Bending stress, also called flexural stress, is parallel to the axle of the member
Shear stress is perpendicular to the axle of the member.
Examples of Shear Stress
As a matter of fact, whenever you cut something you are applying shear stress on it. A few other examples of shear stress include stress exerted on the pipeline by a flowing fluid and shear stress on soil exerted by a normal load from the top. Shear is what makes a sand castle collapse when someone steps on it instead of just settling. Geo-technical engineers account for shear stress in the soil when designing structure foundations to avoid failure under shear. When a pair of scissors is used to cut a peace of wood, the two parts of the scissors exert lateral loads, which cause shear stress on the member and cause it to cut.
Shear stress is not to be confused with shear force. Shear force is an internal force caused by an applied force, and it’s represented by shear diagrams for all sections along a member. However, shear stress is in the unit of force over unit of area.
Units and Equations of Shear Stress
The units of shear stress are like the units of any other type of stress. The unit for shear stress is the unit of load (or weight) divide by the unit of area; i.e. N/m^2 or Pa (Pascal) for the SI system and lbf/ft^2 for English system.
Unlike in the case of flexure, structural members encounter sudden failure under shear. That is why the American Concrete Institute (ACI) requires a 0.75 reduction factor of the nominal strength for designing members for shear; for flexure it is 0.9. This is due to the fact that reinforced concrete is very weak in tension. In a reinforced concrete beam, vertical reinforcement is provided in order to resist shear stress caused by the normal loading exerted on the beam.
 
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