Static forces remain constant or change slowly enough that a structure can be treated as being in equilibrium. Dynamic forces change rapidly with time or arise from motion, producing acceleration, impact, vibration, or repeated loading.
In a structural system, an external force creates internal stress and strain. Under static loading, the structure is analysed using the equilibrium conditions:
These conditions mean that the resultant force and resultant moment are zero. For example, a stationary load supported equally at two ends of a beam produces two upward reactions of each.
Under dynamic loading, the force or its direction changes with time. The structure may accelerate, so the relationship becomes relevant. A falling object can exert a force much greater than when it strikes a beam because its momentum changes over a short time.
| Feature | Static force | Dynamic force |
|---|---|---|
| Behaviour | Constant or applied gradually | Changes rapidly or involves motion |
| Structural response | Mainly steady deformation | Acceleration, impact, or vibration |
| Examples | Structure's own weight; stationary equipment | Wind gusts; earthquakes; moving vehicles |
| Main design concerns | Strength, stiffness, and stability | Impact resistance, resonance, and fatigue |
Repeated dynamic loading can cause fatigue, in which cracks develop after many loading cycles even when each force is below the material's static failure strength. If the loading frequency approaches a structure's natural frequency, resonance may produce dangerously large vibrations.
A common misconception is that a static force causes no deformation. Static means that the loading is constant or slowly applied, not that the structure remains undeformed.
For an IB Design Technology A3.2 response, define both force types, give a structural example of each, and explain their different effects. If asked to distinguish them, do not merely state that one “moves” and the other does not; refer to variation with time, equilibrium, and dynamic effects.