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 design principles for spreader bars including bending moment, section modulus, and buckling considerations, along with AISC allowable stress criteria, ASME B30.20 requirements for below-the-hook devices, and practical guidance on sizing common tube and pipe sections for spreader bar service. The calculator handles the structural math, but understanding the failure modes ensures you select the right bar for the job.
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.
Per AISC ASD (Allowable Stress Design), the allowable bending stress for compact sections is 0.66 × Fy, where Fy is the yield strength of the steel. For A36 steel (Fy = 36 ksi), the allowable bending stress is 23.76 ksi. For A500 Grade B tube (Fy = 46 ksi), the allowable is 30.36 ksi.
The design requirement is: Srequired ≥ M / Fb, where Fb is the allowable bending stress. Select a section with a section modulus equal to or greater than the required value. For lifting applications, most engineers apply an additional design factor of 2.0 to 3.0 beyond the AISC allowable to account for dynamic loading, impact, and the consequences of failure.
M = W × L / 4 (center-loaded beam)
σ = M / S
Srequired ≥ M / Fb
Where Fb = 0.66 × Fy (AISC ASD)
A36 steel: Fb = 23.76 ksi
A500 Gr. B tube: Fb = 30.36 ksi
Apply design factor of 2.0–3.0 for lifting service.
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))
Euler critical buckling load:
Pcr = π² × E × I / L²
Where α = upper sling angle from horizontal, E = 29,000 ksi (steel), I = moment of inertia, L = bar length
Design factor: Pallowable = Pcr / FS (FS = 2.0–3.0 for lifting)
Spreader Bar & Lifting Beam Sizing Calculator
Size spreader bars and lifting beams for overhead lifts with Euler buckling and section modulus checks.
Practical Sizing and ASME B30.20
Common spreader bar sections for light to moderate lifts include Schedule 40 and 80 steel pipe and HSS (hollow structural sections) round and square tube. For a quick field estimate, 4-inch Schedule 80 pipe handles approximately 10,000–15,000 lbs at 8–10 foot spans, while 6-inch Schedule 80 handles 25,000–40,000 lbs at similar spans. These are rough guidelines. Always perform the full bending and buckling calculation for the specific load and geometry.
ASME B30.20 (Below-the-Hook Lifting Devices) governs the design, fabrication, inspection, testing, and use of spreader bars and other below-the-hook devices. Key requirements include: design must be performed or approved by a qualified engineer, all welds must be to AWS D14.1 or equivalent, the device must be proof-tested to 125% of rated load before first use, and annual inspection is required.
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.
All spreader bars must have clear markings showing the rated capacity, bar weight, manufacturer or designer, and serial/identification number. A maintenance and inspection log must be maintained. Bars showing any sign of bending, cracking, weld failure, or deformation at the attachment points must be removed from service and re-evaluated by a qualified engineer.
• Design by or approved by a qualified engineer
• Proof test to 125% of rated load before first use
• Annual inspection by a qualified person
• Clearly marked: capacity, weight, ID number
• AWS D14.1 welding standards
• Maintenance and inspection records required
• Remove from service if bent, cracked, or deformed
Spreader Bar & Lifting Beam Sizing Calculator
Size spreader bars and lifting beams for overhead lifts with Euler buckling and section modulus checks.