Short Answer:
Shear force is the internal force that acts along the cross-section of a structural member when an external load tries to cause one part of the member to slide past the other. It acts parallel to the surface or section where the load is applied. In simple words, shear force resists the tendency of the material to shear or deform under the action of transverse loads.
When a beam or structural member is subjected to external loads, reactions develop inside it to balance those loads. The force that acts tangentially along the section, trying to cut or shear the member, is known as shear force. It plays an important role in the design and strength analysis of beams and frames.
Detailed Explanation :
Shear Force
Shear force is a type of internal force that arises in a body or structural element when external loads are applied in such a way that they tend to slide one layer of the material over another. This force acts parallel to the cross-section of the body. When a beam or any other member is loaded transversely, the internal forces develop to resist these loads. The part of the internal resistance acting parallel to the cross-section is termed the shear force.
To understand it more clearly, imagine a beam supported at its ends and loaded in the middle. The applied load will try to push the beam downward, but the internal shear force will develop at every cross-section to resist this downward action and maintain equilibrium. The direction of the shear force depends on how the beam is loaded and supported.
Shear force is usually represented by the letter V and is expressed in newtons (N) or kilo-newtons (kN). It is a very important parameter for structural analysis because if the shear force exceeds the material’s capacity, the structure may fail due to shearing. Therefore, engineers calculate shear forces at different sections of beams to ensure safety and stability.
Explanation through Beam Example
Consider a simply supported beam carrying a uniformly distributed load. The load tries to bend the beam and produce both bending moment and shear force. At any given cross-section of the beam, if the left portion tends to move upward and the right portion tends to move downward (or vice versa), there will be a tangential internal force at that section. This tangential component is the shear force.
At any section of a beam, the shear force is equal to the algebraic sum of all the vertical forces acting either to the left or right of the section.
Mathematically,
where = Shear Force and = Vertical Forces acting on one side of the section.
If the algebraic sum of vertical forces on the left side of the section is upward, then the shear force is positive; if downward, then the shear force is negative. The sign conventions are used to maintain uniformity while analyzing structures.
Importance of Shear Force
Shear force plays a significant role in determining the safe design of beams, girders, shafts, and other load-bearing members. It helps to locate the points of maximum shear and predict where shear cracks may occur. The shear force distribution along a beam is not constant; it varies depending on the type of loading and support conditions.
For example:
- In a simply supported beam under a point load, the shear force remains constant between the supports but changes suddenly at the point of load application.
- In a uniformly distributed load (UDL) condition, the shear force decreases or increases linearly along the beam length.
Understanding shear force distribution is essential for drawing Shear Force Diagrams (SFD), which visually show how shear varies along the beam.
Applications and Effects
Shear force is not limited to beams only; it appears in shafts, plates, bolts, and rivets as well. For example:
- In bolts and riveted joints, shear force acts to cut the fastener along its cross-section.
- In shafts, it arises due to torsional loads.
- In plates or thin structures, excessive shear force may cause distortion or shearing failure.
Shear force is also considered while choosing the material for construction because different materials have different shear strengths. Ductile materials like mild steel can resist higher shear before failure, while brittle materials like cast iron fail more suddenly under shear loads.
Conclusion
In conclusion, shear force is an internal tangential force that resists the sliding of one part of a material over another when external transverse loads are applied. It plays a vital role in structural and mechanical design, ensuring that components like beams, shafts, and joints can safely withstand applied loads without shearing or collapsing. Proper analysis of shear forces helps engineers in predicting structural behavior, maintaining balance, and ensuring long-term durability of mechanical systems.