Static and Dynamic Forces in Structural Systems
Forces
Factors affecting strength when compressing, pushing, pulling, twisting, or squeezing an object.
Forces can be:
- Static – loads that do not move (e.g. weight of a building)
- Dynamic – loads that change over time (e.g. wind, people walking, vehicles)
Static Forces
Static Force
A stationary force acting on a structure, including its own weight and the load it carries.
- Static forces are constant or slowly changing forces acting on a structure.
- They include:
- Compression – squashing force
- Tension – pulling force
- Torsion – twisting force
- Bending – curving under load
- Shear – sliding force
Compression
Compression (force)
A force that squeezes a material or causes it to shorten.
Common in: columns, legs of a chair, arches
Tension
Tension (force)
A pulling or stretching force that acts to extend a material.
Common in: shafts, door handles, bike pedals
Torsion
Torsion (force)
A twisting force.
Common in: shafts, door handles, bike pedals
Bending
Bending (force)
A load applied at a distance from a fixed point, causing a material to bend.
Bending Moment (M): Force (F) × Distance (d)
- Force (F): The perpendicular force applied to the beam.
- Distance (d): The perpendicular distance from the force to the point of calculation (lever arm or moment arm).
Common in: beams, floorboards, bookshelves
Shear
Shear (force)
Two equal and opposite forces acting on an object that are not aligned, creating a cutting effect.
Common in: scissors, riveted joints, bolts in wind loads
Static forces are often predictable and can be calculated during the design phase.
Dynamic Forces
Dynamic Force
A moving force acting on structures, often caused by wind, vehicles, or other external motion.
Dynamic forces change over time, often suddenly or repeatedly. These include:
- Wind Loads – e.g. gusts hitting a tall building
- Seismic Loads – e.g. ground shaking during an earthquake
- Impact Loads – e.g. a car hitting a barrier or bridge
Dynamic forces are often unpredictable and require special consideration in design.
Static vs Dynamic Forces
| Type of Force | Description | Examples |
|---|---|---|
| Static | Constant, unchanging | Building weight, furniture resting on a floor |
| Dynamic | Changing over time or location | Wind, footsteps, traffic, earthquakes |
Skyscrapers
- Static Forces: Weight of the building materials.
- Dynamic Forces: Wind and earthquakes.
