Parapet, Two-Block, and a Growing Radius
An 80-ton AT crane sets a 9,600 lb rooftop unit; rigging adds 700 lb. The chart shows 14,300 lb at the 42 ft working radius and 12,100 lb at 48 ft, so the pick sits at roughly 72% of chart. Boom is 110 ft; outriggers are fully extended on 4 ft pads, two of them on a mat over a buried vault. The unit has to clear a 24 ft parapet, so the operator booms up until the load passes 3 ft over the wall. The anti-two-block alarm sounds as the hook block nears the boom tip. Then, swinging 90 degrees to the set point, the load swings wide and the radius grows.
1. The anti-two-block alarm sounds while the load is over the parapet. What do you signal, and what is the actual hazard?
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Signal: STOP immediately. Then land or lower the load to re-establish distance between the hook block and the boom tip. Do not boom up further and do not continue hoisting.
The hazard: the alarm means the block is at or near the boom tip. If it contacts the tip, the hoist rope can be pulled over the sheave and the load drops - the rope is not designed to carry the load in that geometry. Many cranes have an anti-two-block device that will cut the hoist function outright, which leaves you with a suspended load and no hoist control until it is reset.
Why it happened: booming up to clear a tall wall consumes the headroom between the block and the tip. Add a short sling and the block is already close to the tip at the top of the lift, so the wall clearance and the two-block margin are competing for the same few feet.
Fix before the next pick: longer slings, a boom angle and radius planned so the tip stays well above the wall with margin, and a planned top-of-lift height agreed with the operator. The anti-two-block device is a last line of defense, never the primary control for the top of a lift.
2. Explain why the radius grows on the swing and what that does to the 72% figure. What are your options?
Read the worked answer
Load chart capacity is a static, level, radius-based maximum. Swinging accelerates the load outward, so the actual radius runs wider than the 42 ft you planned - heading toward the 48 ft column, where the chart gives 12,100 lb. At 10,300 lb the pick is still inside the chart there, but the margin that looked comfortable at 72% is being spent for the entire duration of the swing, and a gust or a jerky start spends it faster.
Two compounding effects:
- Boom deflection. Hoisting a load deflects the boom, which increases radius, which lowers capacity, which increases deflection. The effects stack.
- Wind and swing inertia. Both push the load outward at the moment the boom is already working.
Options: slow the swing and start/stop smoothly rather than chasing the load; stop the swing before the set and let the load settle; if the load will not stay inside the planned radius, stop, land it, and re-plan - larger crane, shorter radius, or a different rigging arrangement.
Never drag a load sideways into position with the swing. Side loading is outside the chart entirely, and no percentage of chart capacity covers it.
3. Mid-swing you notice one outrigger pad has sunk about 2 in into the mat over the vault and the boom appears to be walking. What do you do, and what do you re-verify before resuming?
Read the worked answer
Stop. Land the load. Take the load off the outrigger.
Why this is not a 'watch it' situation: the chart assumes the crane is level and the outriggers are on stable ground. A pad that has moved means the crane is no longer level, the load is no longer distributed as planned, and the boom can be side-loaded. Capacity is not valid the moment the setup changes.
Correct sequence:
- Stop and land - do not hold the load while someone works under a settling outrigger.
- Re-block: larger spreader mat, move the pad off the vault entirely, or re-set the crane.
- Verify level with the crane's level indicator, not by eye.
- Re-check the load chart for the actual radius and configuration, then re-brief the pick.
Do not shim a loaded pad, and do not assume only the sunken corner changed - the whole setup has to be re-verified. If the mat is bridging a vault, the ground bearing is the root cause; a shim just relocates the failure.