Learn the different types of forces acting on structures—like tension, compression, shear, bending, and torsion—with easy real-life examples. Perfect for beginner civil engineers and contractors. Subscribe to Er. Pravin Kadam for more practical teaching videos.
Introduction
Every building, bridge, beam, or column you see stands because of how forces are understood and managed. Engineering Mechanics teaches us what these forces are and how they affect structures. In this blog, we break down the types of forces in a simple way, show their real-life effects, and give relatable examples so beginners can grasp the concepts quickly.
1. Gravitational Force (Weight)
What it is:
The force due to gravity pulling all objects toward Earth. In structures, this is the weight of the structure itself plus any live load (people, furniture, water, etc.).
Effect on Structures:
Always acts downward. It creates compressive stresses in columns and bending in beams if the load isn’t directly aligned.
Example:
A slab carrying people and furniture applies its weight to supporting beams; those beams transfer it to columns, which then take it down to the foundation.
2. Normal Force
What it is:
A reactive force from a surface that prevents objects from passing through each other. It acts perpendicular to the contact surface.
Effect on Structures:
Keeps elements from penetrating supports—used in analyzing support reactions.
Example:
A beam resting on a column receives a normal reaction upward from the column to balance the downward weight.
3. Tension
What it is:
A pulling force that tries to elongate or stretch an object.
Effect on Structures:
Members under tension resist being pulled apart. Cables, ties, and rods often carry tensile forces.
Example:
Suspension bridge cables are in tension, holding up the deck by being pulled tight between towers.
4. Compression
What it is:
A pushing force that tries to shorten or squash an object.
Effect on Structures:
Columns and struts carry compressive loads—too much compression without proper design can cause buckling.
Example:
A column in a building is mainly under compression from the weight of floors above.
5. Shear Force
What it is:
A force that causes layers or parts of a material to slide past each other.
Effect on Structures:
Can cause failure along a plane; important in beam design and connections.
Example:
A short beam loaded near its center experiences shear near the supports—if shear is too high, it can cut or shear off.
6. Bending (Moment)
What it is:
A combination effect when forces cause a structural element (like a beam) to bend.
Effect on Structures:
Top fibers get compressed, bottom fibers get tensioned (or vice versa), depending on load direction. Design must ensure the beam resists bending without cracking or yielding.
Example:
A simply supported beam with a load at midspan bends downward in the middle; this creates bending moments along its length.
7. Torsion
What it is:
A twisting force that causes rotation around the axis of an element.
Effect on Structures:
Can produce shear stresses in a circular shaft or irregular member; critical in elements like drive shafts or spiral stair supports.
Example:
A circular column resisting a twisting moment from an eccentric load or wind-induced torsion on a tower.
8. Combined Forces (Real Structures)
Most real structures face a combination: e.g., a beam might have shear, bending, and axial compression simultaneously. Understanding each helps in designing safe, efficient structural members.
Simple Combined Example:
A cantilever balcony has:
-
Bending due to its own weight causing downward deflection.
-
Shear near the wall connection.
-
Tension/compression in reinforcement depending on the direction of bending.
Simplified “Quick Reference” Table
| Force Type |
Direction/Behavior |
Typical Structural Element |
Beginner Example |
| Gravity |
Downward pull |
Slab, Beam, Column |
Floor weight |
| Normal |
Perpendicular reaction |
Support interfaces |
Beam on column |
| Tension |
Pulling/stretching |
Cable, Tie rod |
Bridge suspension cable |
| Compression |
Pushing/squeezing |
Column, Strut |
Column supporting floor load |
| Shear |
Sliding layers |
Beam web, Connections |
Cut near support in a loaded beam |
| Bending |
Curvature from load |
Beam, Cantilever |
Mid-span deflection of a beam |
| Torsion |
Twisting around axis |
Shaft, Irregular member |
Twist in tower due to wind/eccentric load |
Practical Tips for Students
-
Always draw Free Body Diagrams (FBD) first. Label all forces and reactions.
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Check equilibrium: Sum of forces and moments must be zero for a static structure.
-
Know which elements take which forces: Columns (compression), cables (tension), beams (bending + shear).
-
Use real site photos in your notes—identify which force is acting where.
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Conclusion
Understanding the types of forces and their effects is the first step toward designing safe and efficient structures. Start with simple examples, practice drawing FBDs, and observe real construction sites—each teaches you how theory meets reality.
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