Two-Block Warning at the Roof Edge
An operator is landing bundles of roofing material on a 4-story roof. The anti-two-block alarm sounds as the headache ball approaches the boom tip during a blind landing over the parapet. The operator stops, but the signalperson on the roof is out of view behind the parapet and is radioing "come down slow." The load is a 2,000 lb bundle on a two-leg bridle sling with 45° leg angles. Wind is 12 mph gusting to 20 mph, and the load is drifting toward the edge.
1. What should the operator do first, and why?
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Stop and hold - do not continue booming down or hoisting while the two-block alarm is active. Two-blocking can part the wire rope or damage the sheaves. Because the signalperson is out of view and the load is drifting, the operator should stop, set the load down if a safe landing area exists, and re-establish communication with a visible signalperson or a relay. Blind signals plus an active alarm is a stop-work condition. Never rely on radio alone when the load is out of sight and drifting.
2. What is the tension in each sling leg, and does the drift change the plan?
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Each leg carries 2,000 lb ÷ (2 × sin 45°) ≈ 1,414 lb (vertical component 1,000 lb each). That's within a properly rated sling, so rigging is not the issue. The drift is: at 20 mph gust the load can swing, increasing radius and side load on the boom. Plan: lower the load to a controlled landing, add tag lines, wait out the gust or reduce sail area, and re-check the two-block limit switch before resuming. Do not chase the load with the boom.