Load Charts, Capacities, Reach and Stability Calculations
Reading the load chart
Every boom has a load chart or rated capacity chart specific to its configuration. Read it as a matrix of boom length, boom angle or radius, and permitted load. Understand that the chart already accounts for the boom's own weight and the pipeline; what remains is the concrete load, the end hose, and any attachments. Never interpolate beyond the chart's stated values, and never assume a chart from another machine applies.
Stability fundamentals
The boom's stability depends on the support base (outrigger footprint), the counterweight, and the load moment. Load moment = load × horizontal distance from the tipping line. As the boom extends or lowers, the radius increases and the moment increases, reducing stability. The outrigger loads are highest when the boom is over a single outrigger, so setup must account for the worst-case outrigger load, not the average.
Traps in exam questions
Questions often test whether adding concrete in the pipeline increases load - it does, and a full pipeline can add significant mass. Another trap is ignoring wind load, which increases moment. Also, a chart may specify a maximum wind speed and a maximum outrigger load; exceeding either voids the rated capacity.
Worked example
If a boom is rated for a maximum outrigger load of 20 tonnes and the ground bearing capacity is 100 kPa, the required pad area is load ÷ capacity = 20 t ÷ 100 kPa. Convert tonnes to kN (1 t ≈ 9.81 kN), so 20 t ≈ 196 kN, requiring ≈ 1.96 m² of pad area. Use a pad larger than this to provide a margin.