Load Capacities, Foundations, and Structural Principles
Duty ratings
OSHA groups scaffold platforms by intended load: light duty 25 psf, medium duty 50 psf, and heavy duty 75 psf. The scaffold and each component must support its own weight plus at least four times the maximum intended load without failure. Loads are not just people and tools: count stored material, debris, accumulated ice, and impact. Never exceed the rating of the weakest component - plank span, frame, coupler, or foundation.
Foundations
A scaffold fails from the bottom up. Base plates and screw jacks sit on mud sills that spread load over firm, level, drained ground. Do not set base plates on blocks, bricks, or loose fill, and do not bridge over excavations, pits, or unbackfilled trenches without engineering. Compaction, drainage, and frost matter. Keep the scaffold plumb and level; an unlevel base creates eccentric loads that couplers and frames were not designed for.
Stability
The height-to-base ratio controls overturning. Above 4:1, the scaffold must be restrained with ties, guys, or braces. Ties are typically placed at both inner and outer rows where horizontal members occur, often near 30 ft horizontally and 26 ft vertically - always confirm against the manufacturer's instructions and the project engineer. Wind on a wrapped or partially enclosed scaffold greatly increases load and must be designed for.
Worked example: a platform 5 ft wide × 28 ft long carries three workers (250 lb each), 300 lb of tools, and 400 lb of stored material. Total = 750 + 300 + 400 = 1,450 lb. Area = 140 ft², so load ≈ 10.4 psf - well within light duty, but do the arithmetic rather than guess.