Load Capacities, Weights, and Forces
Load types
Understand dead load (self-weight of scaffold, boards, guardrails), live load (workers, tools, materials), environmental load (wind, snow), and dynamic/impact effects. Scaffolds are classified by duty: light duty (e.g., inspection, ~150 kg per bay or 75 kg/m²), general purpose (~450 kg per bay or 150 kg/m²), and heavy duty (~675 kg per bay or 200 kg/m²). Recognize that a working platform is rated for its duty and that overloading is a leading cause of collapse.
Force paths
Loads travel from boards → transoms → ledgers → standards → base plates → ground. Ties resist wind and overturning. Braces provide stability and transfer horizontal forces. Eccentric loading (materials piled at one edge) increases moments on standards.
Traps
- Mixing up duty ratings; assuming all scaffolds carry heavy materials.
- Forgetting that point loads (a pallet of blocks) exceed uniform area loads.
- Ignoring wind uplift on sheeted scaffolds.
- Confusing safe working load with breaking load; never design to failure load.
- Overlooking that cantilever and suspended scaffolds have much lower capacities.
Rule of thumb
Estimate bay load: count workers (say 100 kg each with tools), add material weight, and compare with duty rating. If in doubt, reduce load or upgrade duty - never guess upward.
Worked example
A general-purpose bay rated 450 kg holds three workers (3 × 100 = 300 kg) and two bundles of boards at 60 kg each (120 kg) = 420 kg, close to the limit. Adding a third bundle (60 kg) gives 480 kg - overloaded; remove material or use a heavier-duty scaffold.