A tower rigging plan built to ANSI/ASSP A10.48-2023 comes down to four calculated numbers: tension on each sling leg, the resultant load on the top block or gin-pole sheave, the pull the capstan hoist has to develop to move the load, and the anchorage demand those forces place on the base. All four are classical statics — tension per leg = (load ÷ effective legs) ÷ sin(angle from horizontal); block load = 2 × line tension × cos(included angle between the rope legs ÷ 2); capstan pull = load × a sheave-bearing friction factor, compounded per sheave the rope passes. Which construction plan Class (II, III or IV) that lift requires is a separate question — driven by gross load and a handful of trigger conditions, broken down in full in ANSI A10.48-2023 Rigging Plan Classes Explained. This article walks through the actual load math; run your numbers instantly in the free rigging calculator.
What does a tower rigging plan actually calculate?
A rigging plan isn't a form you fill in — it's a set of equilibrium calculations run at every point along the lift path, because the geometry (and therefore every force) changes as the load climbs:
- Sling tension per leg — what each leg of the bridle under the load is carrying.
- Top block / gin-pole sheave load — the resultant force the block, its sling and shackle have to be rated for, which is higher than the rope tension alone.
- Capstan (cathead) hoist pull — the line pull the capstan has to develop, after friction losses at every sheave the rope passes over.
- Anchorage demand — the horizontal (sliding) and vertical (uplift) force the base anchor or vehicle has to resist.
A full plan runs these at the governing point of the lift — usually not the final position, but wherever the tag line, load line or trolley geometry is most unfavorable along the way.
How do you calculate sling tension and block load?
Sling tension per leg follows the same load-angle-factor math covered in our sling angle chart: tension = (load ÷ number of legs carrying it) ÷ sin(angle from horizontal), with 3- and 4-leg bridles conservatively treated as 2 effective legs.
The block or gin-pole sheave sees a different number entirely — not the rope tension, but the resultant of both rope legs passing over it: resultant force = 2 × tension × cos(included angle between the legs ÷ 2). At a 0° included angle (rope folding straight back on itself, a 180° wrap) the block sees nearly 2× the line tension — the single most common underestimate in a field rigging sketch. As the angle between the legs opens up toward 180° (rope running straight through with no wrap), the force on the block drops back toward the line tension itself. Station 04 of the rigging calculator runs this exact formula.
How do you calculate capstan hoist pull?
Capstan line pull equals the load on the line, multiplied by a bearing-friction factor for every sheave the rope crosses before reaching the capstan — roughly 1.02× per steel roller-bearing sheave, 1.045× for a bronze bushing, and 1.09× for a plain bushing, each one compounding (squared for two sheaves, cubed for three). Two sheaves with plain bushings alone adds roughly 19% to the required pull over a direct line. Once you have the required pull, the capstan and its anchorage should be verified by an operational load test to 1.5× the maximum anticipated line pull, or an engineering review with a minimum safety factor of 2.0.
What determines whether a lift is Class II, III or IV?
Gross load is the starting point, but it isn't the only trigger — under A10.48-2023 (which folded the old Class I into Class II):
- Class II — gross lift loads up to 500 lbs, rigging attached to the structure, normal construction methods that don't affect strength or stability.
- Class III — gross loads up to 2,000 lbs with rigging attached to the structure, or any hoisting with a detached lifting device not supported by the structure (a mobile crane or telehandler) regardless of weight, or lightweight hoisting with a manufacturer-rated antenna lifting device away from the primary structure (Class III minimum by the manufacturer's own rating, even under the 500 lb threshold).
- Class IV — gross loads over 2,000 lbs with rigging attached to the structure; any removal, replacement or modification of structural members; work affecting the strength or stability of the structure or foundation; a special engineering lift; a straight-tag lift whose vertical load-line angle exceeds 10° or whose horizontal tag angle exceeds 70° anywhere along the lift path; a rigging attachment point that isn't a leg panel point, a pole shaft, or a manufacturer-rated antenna lifting device; or a gin-pole lift, which is Class IV under A10.48-2023 regardless of weight.
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The full breakdown of each class, with the reasoning behind the thresholds, is in our dedicated A10.48 plan classes article — this piece focuses on the load math that feeds into that classification, since the load-line and tag-line angles above are themselves outputs of the equilibrium calculation, not inputs you guess at.
Where do the anchorage numbers come from?
The base anchor or rigging vehicle has to resist both the horizontal (sliding) and vertical (uplift) components of every line pulling on it — the hoist line, a heel-block redirect, a trolley line, each contributing its own share. A straight-tag or trolley configuration with the support force climbing past roughly 5× the actual load being lifted is a sign the tag or trolley geometry has gone unstable (the tag is pulling almost against the load line) and needs a longer anchor distance or a different offset, not a bigger anchor.
Get the numbers, then build the plan
The instant calculators on buildrightacademy.us/tools/rigging-calculator run every formula above — sling tension, block load, capstan pull — free, with no account. A free account unlocks the full Rigging Plan Calculator, which runs the complete A10.48-style lift-path analysis (configurations A through F, obstruction clearance, anchorage demand, required plan Class) and generates a printable plan.
FAQ
What is a tower rigging plan under A10.48-2023?
A rigging plan is a set of static-equilibrium calculations — sling tension, block/sheave load, hoist pull and anchorage demand — run at the governing points along a lift path, classified into Construction Plan Class II, III or IV based on gross load and specific trigger conditions.
What's the formula for capstan hoist pull?
Capstan pull = load on the line × a bearing-friction factor, compounded once per sheave the rope passes before the capstan (about 1.02 per steel roller-bearing sheave, higher for bronze or plain bushings).
Is a mobile crane lift automatically Class III?
Yes — any hoisting with a lifting device not supported by the structure (a mobile crane or telehandler) is Class III or higher regardless of the load weight, under A10.48-2023.
Does load weight alone decide the Class IV trigger?
No. Gross load over 2,000 lbs (attached rigging) is one Class IV trigger, but structural modification, work affecting stability, a steep load-line or tag-line angle, a non-standard attachment point, and any gin-pole lift all force Class IV regardless of weight.
How do I calculate the force on a gin-pole sheave?
The same block-load formula applies: resultant force = 2 × line tension × cos(included angle between the rope legs ÷ 2). At a 0° included angle the sheave sees close to double the line tension.
Does this replace a professional engineer's review of the rigging plan?
No. The calculators assist a qualified or competent rigger with the arithmetic and follow engineering methods consistent with ANSI/ASSP A10.48 and ASME B30.9 — they don't replace a licensed PE's review where one is required, and every input should be verified against actual equipment ratings before a lift.
