Structural Equilibrium
Definition
Equilibrium (force)
A state of balance where equal and opposite forces act on a system.
Definition
Stability
The ability of a structure to maintain its shape and position under load.
A Structure is in Equilibrium When:
- The sum of all forces acting on it is zero (No unbalanced force → no acceleration or collapse) - Transitional Equilibrium
- The sum of all moments (torques) is also zero (No rotation or tipping) - Rotational Equilibrium
Note
- A structure can be in translational equilibrium but not rotational equilibrium, and vice versa.
- Both conditions must be met for complete stability.
Example
Structures in Equilibrium
- A bridge holding a steady load with no bending or movement
- A ladder leaning against a wall with balanced forces at the base and top
- A balanced mobile sculpture where weights and arms are evenly distributed
A Structure is Not in Equilibrium If:
- There is a net force pushing or pulling in one direction
- There is an unbalanced moment causing it to tip, rotate or twist
- It begins to accelerate, collapse or deform
Conditions for Structural Failure
- Excessive Load: When the applied load exceeds the structure's capacity.
- Material Failure: When materials break or deform under stress.
- Instability: When the structure loses equilibrium, leading to collapse.
Tip
- When analyzing a structure, start by identifying all points of contact with the ground or other supports.
- These are critical for understanding how forces are distributed.
Analogy
- Consider a seesaw.
- For it to be in equilibrium, the weights on both sides must be balanced.
- If one side is heavier, the seesaw tips, similar to how a structure fails when forces are unbalanced.
