According to 1926.451, each scaffold and scaffold component must support without failure its own weight and at least four times the maximum intended load applied or transmitted to it.
OSHA also says that a qualified person must design the scaffolds, which are loaded in accordance with that design and that scaffolds and scaffold components must not be loaded in excess of their maximum intended loads or rated capacities, whichever is less.
Load carrying timber members should be a minimum of 1,500 lb-f/in2 construction grade lumber.