Load Charts and Rigging
Reading the tower crane chart
A tower crane chart is a matrix, not a single number. Capacity depends on jib length, counterweight/ballast configuration, number of hoist falls, and working radius measured horizontally from the slew center to the hook (not the jib tip, not the trolley). Find the row for the actual radius and the column for the actual jib and reeving, then read the capacity. Charted capacity already accounts for the crane's own structure and stability; you then deduct the weight of everything hanging below the chart reference - hook block, slings, spreader, shackles.
Rigging fundamentals
Every rigging component has a working load limit; the lowest WLL in the system governs. Sling leg tension rises as the angle from horizontal drops. Hook latches must function, shackle pins must be fully seated, and edge protection is required on sharp corners. Locate the center of gravity so the load hangs level.
Traps
Using the wrong jib-length column; reading radius as jib length; deducting only the payload; mixing single-part and two-part reeving; and treating the 30°-from-horizontal sling angle as efficient when tension nearly doubles.
Worked example
A chart shows 4,000 lb at 120 ft radius. Hook block is 500 lb and slings 100 lb, so maximum payload is 4,000 − 600 = 3,400 lb. Sling tension: a 10,000 lb load on two legs each 60° from horizontal - each leg carries 5,000 ÷ sin 60° = 5,000 ÷ 0.866 = 5,774 lb. At 30°, each leg would carry 5,000 ÷ 0.5 = 10,000 lb.