Calculating Leg Load for a Loaded Lift
A scaffold designer is checking a 1.8 m wide, 2.0 m long bay of a frame scaffold. The working lift will carry a distributed load of 3.0 kN/m² over the full bay plus two point loads of 1.5 kN each from a mortar tub and a block stack. The scaffold has two standards per corner (four legs per bay corner arrangement), and the designer must decide whether the base can take the load on the existing 150 mm square base plates on a firm clay. The bay is one of a continuous run, so loads share between adjacent bays. A colleague argues that because the scaffold is 'medium duty' the load is automatically acceptable without calculation.
1. Estimate the total load on one bay and explain how it transfers to the legs.
Read the worked answer
Plan area is 1.8 m x 2.0 m = 3.6 m². Distributed load = 3.0 kN/m² x 3.6 m² = 10.8 kN. Add the two point loads: 10.8 + 1.5 + 1.5 = 13.8 kN total imposed load on the bay, before self-weight of decking, ledgers and standards.
Transfer is through the transoms to the standards. In a continuous run, adjacent bays share some load through the ledgers, so the worst leg does not simply take a quarter of 13.8 kN. A conservative check assigns half the bay load to each pair of standards at the bay ends, then splits between the two standards of each pair. The point loads should be positioned where they actually sit, because a tub near one standard increases that leg's share. The lesson is that duty rating is a permit, not a calculation; the actual load and its position govern.
2. Advise on the base plate bearing check and respond to the colleague's 'medium duty' argument.
Read the worked answer
Base plate area = 0.15 m x 0.15 m = 0.0225 m². If a leg carries, say, 5 kN, bearing pressure = 5 / 0.0225 = 222 kN/m², which firm clay may not sustain without settlement, so a sole board or designed pad is needed to spread load. The check must use the worst leg load including self-weight and any wind effects, and compare it with the assessed ground bearing capacity.
The 'medium duty' argument is wrong. Duty ratings describe the intended use and allowable imposed load for the platform, not a guarantee that a particular bay, base or ground condition is adequate. A medium-duty scaffold can still fail if point loads are concentrated, if the ground is weak, or if the base plates are too small. The designer must calculate, not assume.