What Is Shear Force and Bending Moment? A Simple Guide for Civil Engineers

When a beam carries a load, it does not simply remain straight and unchanged. The load creates internal forces inside the beam, and two of the most important forces engineers study are shear force and bending moment.

If you are a civil engineering student or work in construction, understanding these concepts is essential for analyzing beams, slabs, bridges, and other structural members.

In this guide, we will explain what is shear force and bending moment, what is shear force in construction, and what is shear force and bending moment in civil engineering using simple language and practical examples.

What Is Shear Force and Bending Moment?

Shear force and bending moment are internal forces that develop in a structural member when external loads act on it.

Imagine placing a beam on two supports and putting a load in the middle. The load pushes the beam downward while the supports provide upward reactions. As a result, internal forces develop inside the beam.

Shear Force

Shear force is the internal force that tends to make one part of a beam slide or move relative to another part.

In simple words, shear force acts somewhat like a cutting force.

For example, imagine cutting a piece of paper with scissors. The scissors create a shearing action. A similar type of internal action develops in a structural beam when loads are applied.

Shear force is commonly represented by V and is usually measured in newtons (N) or kilonewtons (kN).

Bending Moment

A bending moment is the internal moment that causes a structural member to bend.

When a load is applied to a beam, the beam tends to curve. The internal resistance against this bending is represented by the bending moment.

Bending moment is commonly represented by M and is usually measured in N·m or kN·m.

In simple terms:

Shear Force = Tendency to slide or cut

Bending Moment = Tendency to bend

What Is Shear Force in Construction?

If you are wondering what is shear force in construction, it is the internal force developed in structural elements due to applied loads and support reactions.

Shear force is important in the design of:

• Beams

• Slabs

• Columns

• Foundations

• Bridges

• Retaining structures

• Other structural members

For example, consider a concrete beam supporting the floor of a building. The weight of the slab, walls, furniture, and people creates loads on the beam. These loads produce shear forces and bending moments within the beam.

Engineers calculate these forces to determine whether the beam has enough strength to safely carry the expected loads.

What Is Shear Force and Bending Moment in Civil Engineering?

In civil engineering, shear force and bending moment are fundamental concepts used in structural analysis and design.

A structural engineer uses loading information, support conditions, and material properties to determine how a member will respond to applied loads.

The results can then be represented using:

Shear Force Diagram (SFD)

and

Bending Moment Diagram (BMD)

These diagrams help engineers understand how shear force and bending moment vary along the length of a beam.

Simple Beam Example

Consider a simply supported beam with a downward point load at its center.

The beam has two supports:

Support A → Beam → Support B

A downward load is applied somewhere between the supports.

The supports generate upward reactions to maintain equilibrium. These reactions and the applied load produce internal shear forces and bending moments along the beam.

Close to the supports, the shear force can be significant. As we move along the beam, the shear force changes depending on the applied loads.

The bending moment also changes along the beam and reaches a maximum value at a location determined by the loading and support conditions.

What Is a Shear Force Diagram?

A Shear Force Diagram (SFD) is a graphical representation showing how shear force changes along the length of a structural member.

The horizontal axis generally represents the length of the beam, while the vertical axis represents the shear force.

An SFD helps engineers identify locations where shear force is high.

For example, a typical simply supported beam carrying a central point load will show a change in shear force at the point where the load is applied.

What Is a Bending Moment Diagram?

A Bending Moment Diagram (BMD) shows how the bending moment changes along the length of a beam.

The diagram helps engineers identify locations where the bending moment reaches important values, particularly the maximum bending moment.

For many simply supported beams carrying downward loads, the bending moment is zero at the simple supports and reaches a larger value somewhere between them.

The exact shape and values of the diagram depend on the loading and support conditions.

Difference Between Shear Force and Bending Moment

Although shear force and bending moment are related, they represent different internal actions.

Shear ForceBending MomentTends to cause sliding or shearingTends to cause bendingRepresented by VRepresented by MUnit is usually kNUnit is usually kN·mShown using an SFDShown using a BMDImportant for shear strengthImportant for flexural strength

Both are required for proper structural analysis.

Relationship Between Shear Force and Bending Moment

Shear force and bending moment are mathematically related.

For a beam:

dM/dx = V

This means the rate of change of bending moment with respect to distance along the beam is equal to the shear force.

Similarly, the rate of change of shear force is related to the distributed load acting on the beam.

This relationship is useful when constructing and checking shear force and bending moment diagrams.

Why Are Shear Force and Bending Moment Important?

Understanding these forces helps engineers design structural members that can safely resist applied loads.

If shear forces are too high, a beam may experience shear-related failure. If bending moments are too high, the beam may experience excessive bending, cracking, yielding, or other structural problems depending on its material and design.

For reinforced concrete beams, engineers therefore consider both shear and flexural behavior when determining reinforcement and member dimensions.

Shear Force and Bending Moment in Reinforced Concrete

In reinforced concrete construction, beams are designed to resist different types of internal forces.

Concrete is generally effective in resisting compression, while reinforcing steel provides significant tensile capacity.

When a beam bends under load, tension develops in one region and compression develops in another. Reinforcement is placed accordingly based on the structural design.

Shear reinforcement, commonly in the form of stirrups, may also be provided to help resist shear forces.

The amount and arrangement of reinforcement should be determined through structural calculations and the applicable design code.

Common Types of Loads That Produce Shear and Bending

Different loads can create shear forces and bending moments in a beam, including:

• Point loads

• Uniformly distributed loads

• Uniformly varying loads

• Self-weight of the beam

• Floor loads

• Wall loads

• Equipment loads

• Environmental loads where applicable

The resulting shear force and bending moment depend on the magnitude, position, and distribution of these loads, as well as the beam's support conditions.

A Simple Way to Remember

A useful way to remember the concepts is:

Load → Shear Force → Bending Moment → Structural Response

Or simply:

Shear = Sliding/Cutting

Moment = Bending

This basic understanding makes it easier to study structural analysis and reinforced concrete design.

Final Thoughts

Understanding what is shear force and bending moment is one of the foundations of structural engineering. These concepts help engineers understand how beams and other structural members respond to different loads.

If you are learning what is shear force in construction, remember that shear force represents an internal action that tends to cause sliding or shearing between sections of a structural member.

Similarly, when studying what is shear force and bending moment in civil engineering, it is important to understand both concepts together because they are closely connected and are essential for structural analysis.

Once you understand support reactions, shear force diagrams, and bending moment diagrams, you have a strong foundation for moving on to more advanced topics such as structural analysis, reinforced concrete design, steel design, and structural engineering.