Wind, Radius Creep, and a Pick Near Chart Capacity
A 90-ton crawler with 100 ft of boom is setting a 12,000 lb rooftop unit with about 120 sq ft of sail area. The delivery truck can only park where the pick is at 60 ft radius. The chart for the 100 ft boom reads 14,100 lb at 60 ft, 13,600 lb at 61 ft, and 12,800 lb at 65 ft; rigging weighs 800 lb. Wind at the boom tip is 19 mph gusting to 27 mph, and the manufacturer's in-service wind limit is 20 mph. The chart notes require a level, stationary crane. The operator plans to boom up, swing 90 degrees, then boom out to 60 ft and set the unit. Two taglines are rigged.
1. Is the pick within capacity as planned? Show the numbers and state your decision.
Read the worked answer
Gross load = 12,000 + 800 = 12,800 lb against 14,100 lb at 60 ft - a 1,300 lb margin, about 9%. On paper it passes. In practice it does not.
- Wind is out of limits. 19 mph sustained is one mph under the 20 mph ceiling; 27 mph gusts are well over it. The manufacturer's in-service limit is a hard stop, not a suggestion, and a sail area of 120 sq ft makes this load a sail.
- The wind moves the radius. Approximate wind pressure = 0.00256 x V^2. At 27 mph: 0.00256 x 729 = ~1.87 psf x 120 sq ft = ~224 lb horizontal on the load. Against 12,800 lb hanging on a 60 ft load line, the hook goes off plumb by about arctan(224/12,800) = ~1 degree, displacing the load outward roughly 1 ft. At 61 ft the chart drops to 13,600 lb - margin now about 800 lb, 6%.
- Add boom deflection. The chart radius is measured on the loaded, deflected boom; at 100 ft of boom with 12,800 lb on the hook the actual radius is greater than the plumb-line radius the operator booms to. The 60 ft number is optimistic before the wind is even considered.
Decision: do not make the pick as planned. Options, in order of preference: reposition the truck so the set radius is 50-55 ft and the margin is real; wait for sustained wind comfortably below 20 mph (many manufacturers require lower limits for high-sail loads - check the manual); or reconfigure to a stronger boom/chart.
2. Mid-swing, a gust pushes the unit and it starts to drift. The operator swings after it to catch it. Evaluate that response and state what should happen.
Read the worked answer
Swinging to catch a drifting load is the wrong response and creates the hazard it is trying to fix.
- Chasing the load induces side loading on a lattice boom, which is a buckling mode the chart does not rate. Lattice sections tolerate very little side load.
- It also produces a shock load when the load line snaps taut, and it accelerates the pendulum instead of damping it.
- If the swing brake is released to chase, the crane loses control of the load's position; if the load swings out, radius increases and capacity falls at exactly the moment the crane is already near its limit.
Correct sequence:
- Stop the swing; hold the swing brake. Do not boom down or boom out with the load hanging.
- Lower the load - to the ground or the deck, whichever is closer and can take it - while holding position.
- Let the pendulum decay before re-hoisting. Keep taglines tensioned from two directions, with handlers positioned outside the load's fall/swing zone and never with a wrap around a hand or arm.
- If the load cannot be landed and the crane is at or beyond the wind limit, treat it as an emergency: clear personnel, hold the load, and follow the manufacturer's/employer's emergency procedure.
- Do not resume until wind is measured within limits at the boom tip (not at ground level) and the radius has been re-verified on the loaded crane.