A spreader bar (also called a lifting beam or strongback) is a below-the-hook device that distributes a crane load to two or more pick points on the load being lifted. The primary purpose is to increase the sling angle. Instead of slings running at shallow angles from a single hook point to widely spaced lift lugs, the spreader bar moves the upper pick points outward so the slings hang more vertically. This dramatically reduces sling tension and eliminates the inward crushing force that angled slings apply to the load.
This guide covers the structural ideas behind spreader-bar screening: bending moment, section modulus, Euler buckling, source boundaries, and the handoff to current AISC and ASME B30.20/BTH-1 review. The calculator screens basic arithmetic for nominal round-pipe rows, but a qualified person must complete and approve the actual below-the-hook device design before use.
Bending Moment and Section Modulus
A spreader bar under load acts as a simply supported beam with a concentrated load at the center (from the crane hook) and reactions at each end (from the sling connections to the load). The maximum bending moment occurs at the center of the bar: M = W × L / 4, where W is the total load weight and L is the bar length between the end attachment points.
The bending stress in the bar is: σ = M / S, where S is the section modulus of the bar cross-section. The section modulus depends on the shape: for a round tube, S = π × (D4 − d4) / (32 × D), where D is the outer diameter and d is the inner diameter. For a wide-flange beam, S is tabulated in the AISC Steel Manual.
The ToolGrit calculator uses a local A36 material prompt of Fy / 1.67 for its lifting-beam bending screen, which is about 21.6 ksi. That is a screening value only; actual below-the-hook device design still depends on the adopted ASME B30.20/BTH-1 requirements, AISC member checks where applicable, service category, fatigue, impact, connections, and the verified material certificate.
The screening requirement is: Srequired ≥ M / Fscreen. Meeting that local screen does not approve the device, set a rated load, or replace lateral-torsional buckling, local buckling, shear, deflection, weld, lug, shackle, proof-test, marking, or inspection review.
M = W × L / 4 (center-loaded beam)
σ = M / S
Srequired ≥ M / Fscreen
ToolGrit prompt: Fscreen = Fy / 1.67 for A36
A36 prompt: about 21.6 ksi
Use current ASME B30.20/BTH-1, AISC, material, connection, proof-test, marking, and inspection requirements for actual design.
Spreader Bar & Lifting Beam Sizing Calculator
Size spreader bars and lifting beams for overhead lifts with Euler buckling and section modulus checks.
Compression and Euler Buckling
When slings connect from the crane hook to the ends of a spreader bar and then down to the load, the horizontal component of the upper sling tension puts the spreader bar in compression. The compressive force is: P = T × cos(α), where T is the upper sling tension and α is the angle of the upper slings from horizontal. For a symmetric two-point lift: P = (W/2) × cos(α) / sin(α) = W / (2 × tan(α)).
A long, slender bar under compression can fail by Euler buckling, a sudden lateral bowing failure that occurs at a stress well below the material yield strength. The critical buckling load is: Pcr = π² × E × I / L², where E is the modulus of elasticity (29,000 ksi for steel), I is the moment of inertia of the cross-section, and L is the unsupported length.
The AISC allowable compressive stress depends on the slenderness ratio KL/r, where K is the effective length factor (1.0 for pin-pin ends), L is the length, and r is the radius of gyration. For KL/r below the critical slenderness ratio Cc, the allowable stress uses the AISC column formula. For KL/r above Cc, the Euler formula governs.
In practice, buckling is the controlling failure mode for most spreader bars because they are long relative to their cross-section. Always check both bending and buckling, and design for whichever governs. Round tubes and pipes are excellent spreader bar sections because they have equal moment of inertia in all directions, preventing weak-axis buckling.
P = W / (2 × tan(α))
Ideal Euler critical buckling load:
Pcr = π² × E × I / (K L)²
Where α = upper sling angle from horizontal, E = 29,000 ksi (steel prompt), I = moment of inertia, K = 1.0 in the ToolGrit screen, and L = unsupported length
ToolGrit compares Pcr / P against a local 3.0 screen. BTH-1 design factors and real end conditions govern actual design.
Practical Sizing and ASME B30.20
Common spreader bar sections may include Schedule 40/80 pipe or HSS round and square tube, but there is no universal canned capacity for a pipe size. Span, sling angle, material certificate, section tolerance, connection geometry, lug/weld design, fatigue, and owner requirements can govern. The ToolGrit app only provides nominal round-pipe screening rows computed from OD and wall.
ASME B30.20 and ASME BTH-1 govern below-the-hook lifting device design, fabrication, marking, testing, inspection, maintenance, and use through current source documents and qualified-person review. Do not rely on a web calculator, guide paragraph, or rough pipe-size rule as the rated-load basis.
Fixed vs. adjustable spreader bars: Fixed bars are simpler, lighter, and more rigid but only work for one specific lift geometry. Adjustable bars (telescoping or multi-hole) accommodate different load widths but add weight, complexity, and potential failure points at the adjustment mechanism. For production lifts (same load repeatedly), fixed bars are preferred. For rigging companies that handle varied loads, adjustable bars provide flexibility.
Rated-load marking, device identification, maintenance records, inspection intervals, removal-from-service criteria, repair control, and proof-test documentation are source-controlled requirements. Resolve those items from the adopted standard, manufacturer/designer records, owner policy, and qualified review before any lift.
• Current ASME B30.20/BTH-1 design basis and adopted edition
• Qualified-person or PE design responsibility
• Verified material certificate and section tolerances
• Lug, shackle, end-fitting, weld, fatigue, and connection checks
• Required proof testing, marking, inspection, maintenance, and records
• Owner, insurer, site, AHJ, and lift-plan requirements