Suspended platform rescue: anchor angles and haul force
At 10:05 you are the rescue sector commander at a 42-storey building under facade cleaning. A gondola has jammed at level 18. One worker is unconscious inside; the second is conscious with a suspected wrist fracture. The platform is 55 m above the podium roof. Access from the roof is possible via a parapet anchor: two independent steel columns 4 m apart, each rated 12 kN. Your team proposes a two-leg webbing sling with legs at an included angle of 90° to a master point, then a 3:1 rope haul system. The combined load of casualty, rescuer, stretcher and tools is 160 kg; use g = 10 m/s². Two rescuers on the roof can each exert a steady pull of about 250 N. Assume 10% friction loss in the haul system. The parapet edge is sharp and the rope runs over it. Wind is 15 km/h and visibility is good.
1. Calculate the static tension in each sling leg at a 90° included angle and at 120°. Are the 12 kN columns adequate?
Read the worked answer
Load W = 160 kg x 10 m/s² = 1,600 N = 1.6 kN.
For a symmetric two-leg sling, each leg tension is T = W ÷ (2 cos θ), where θ is half the included angle.
- At 90° included angle, θ = 45°: T = 1.6 ÷ (2 x 0.707) = 1.13 kN per leg.
- At 120° included angle, θ = 60°: T = 1.6 ÷ (2 x 0.5) = 1.6 kN per leg.
Against a 12 kN column rating, the static tension is only about 9% to 13% of the rating, so the columns are adequate on the static figures alone. That is not the whole judgement. The calculation ignores shock loading as the load is eased over the parapet, the extra tension from the haul rope at the master point, sling wear, and the condition of the columns. A 12 kN rating is a nominal figure and must be verified as an engineered anchor, not assumed from appearance. The sling angle should be kept as narrow as practical; opening it from 90° to 120° increases leg tension by about 41% for the same load.
2. Assess whether the 3:1 haul system is adequate for two rescuers and recommend an alternative.
Read the worked answer
Ideal 3:1 mechanical advantage:
Input force = 1,600 N ÷ 3 = 533 N.
With 10% friction: 533 x 1.1 = 587 N required at the haul rope.
Two rescuers can produce about 2 x 250 = 500 N of steady pull. Even before adding friction, 533 N exceeds their combined steady effort; with friction, 587 N is clearly beyond it. The 3:1 system is therefore not adequate for a 160 kg load with two rescuers on a continuous haul.
A 5:1 system gives:
1,600 ÷ 5 = 320 N; with 10% friction = 352 N.
That is about 70% of the rescuers' combined capacity, leaving margin for uneven pulling and edge drag. The recommendation is a 5:1 system, a powered winch, or a 3:1 system with at least one additional rescuer and a progress-capture device. The haul should also be belayed independently so that a slip does not load the main system.
3. Identify three critical safety checks before committing the rescue, and explain the consequence of omitting each.
Read the worked answer
Anchor verification and independence. Confirm that the two columns are structurally sound, truly independent, and rated for the intended load. If one anchor fails, the remaining leg and master point may be shock-loaded beyond design, dropping the platform.
Edge protection and rope management. Pad the sharp parapet and ensure the rope runs freely without abrasion. A cut or abraded rope under load can fail suddenly; even partial damage reduces strength and makes the haul unpredictable.
Independent belay and progress capture. Fit a separate belay rope and a progress-capture device. Without them, a rescuer losing grip or a prusik slipping allows the load to run back, causing impact injury to the casualty and shock-loading the anchor.
Additional checks that should not be skipped include the casualty's suspension trauma risk, the rescuer's fall protection, radio communication, and a load estimate that includes the stretcher and tools. Each omission changes the rescue from a controlled lower to an uncontrolled fall risk.