Horizontal Lifeline Tension and Anchorage (OSHA 1926.502)
A horizontal lifeline MULTIPLIES the force it is asked to catch, and the multiplier is set by SAG.
Example
You enter
- Span between anchors (ft) 30
- Midspan sag under load (ft) 1
- Arrest force applied at midspan (lb) 1800
- Workers attached 1
- Safety factor (2 is the OSHA minimum) 2
- Anchorage capacity available (lb; 0 = not checked) 5000
- Target cable tension, to solve for sag (lb; 0 = skip) 5000
You get
- Cable tension (lb) 13530
- Horizontal pull at each anchor 13,500 lb
- Tension multiple 7.517
- Angle (deg) 3.814
- Anchorage demand (lb) 27059.9
- Sag needed for the target tension 2.74 ft
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method.
Details, formula, and sources
ScopeAn 1,800 lb arrest at midspan of a 30 ft line sagging 1 ft puts over 13,500 lb into each anchor -- 7.5 times the arrest force -- which is why a cable strung between two roof-hatch handles is not a lifeline. Halve the sag and the tension roughly doubles, so the line pulled drum-tight is the dangerous one and the slack that looks sloppy is what keeps the anchors alive. It also costs clearance below, which is the counterweight. The engineered demand at OSHA's factor of two routinely exceeds the 5,000 lb per worker everyone quotes.
Formula or tablestatic midspan cable: T = W x sqrt((L/2)^2 + s^2) / (2 s); horizontal pull at each anchor H = W x L / (4 s). Anchorage demand = T x the safety factor, compared against the prescriptive 5,000 lb per employee attached; the larger governs. Inverse: the sag for a target tension is s = W (L/2) / sqrt(4 T^2 - W^2), which has no solution for a target at or below W/2.
Source and editionCable statics, with the regulatory requirements from OSHA 29 CFR 1926.502(d)(8), (d)(15), and (d)(16). A US federal regulation in the public domain, quoted directly.
Free access29 CFR is published in full at no cost by OSHA and the eCFR.
What governsEstimate. AHJ and licensed professional govern.
API field namesspan_ft, sag_ft, arrest_force_lb, workers, safety_factor, anchorage_capacity_lb, target_tension_lb, cable_tension_lb, horizontal_pull_lb, tension_multiple, angle_deg, anchorage_demand_lb, sag_for_target_ft
- Load case a single static load at midspan; dynamic effects and off-midspan loading are not modelledstated scope limit
- Horizontal lifelines designed, installed, and used under the supervision of a qualified person, maintaining a safety factor of at least two29 CFR 1926.502(d)(8)
- Prescriptive anchorage 5,000 lb per employee attached, unless engineered to the factor of two29 CFR 1926.502(d)(15)
- Maximum arresting force 1,800 lb with a body harness, 900 lb with a body belt29 CFR 1926.502(d)(16)(i) and (ii)
- Free fall and deceleration no more than 6 ft of free fall, 3.5 ft maximum deceleration distance29 CFR 1926.502(d)(16)(iii) and (iv)
- Cable weight neglected; sag under self-weight before a fall is not computedstated scope limit