Spreader Beam Design Calculator — the guide

A spreader beam hangs from two inclined top slings that meet at the hook. The slings' horizontal components squeeze the beam from both ends, so the beam works as a compression strut — its enemy is buckling, not bending. A lifting beam hangs from a single lug (or a pair close together) and carries its load points in bending — its enemy is the moment diagram. EN 13155's own definitions (§3.8) make the relationship explicit: a lifting beam whose loading is purely compressive is the same device the trade calls a spreader beam.

The practical trade is headroom. A spreader needs vertical room for its sling triangle — steeper slings mean a taller triangle — and rewards you with a light, efficient strut. A lifting beam works right under the hook where a crane's height is exhausted, and pays for it in bending steel. When the headroom exists, the spreader usually wins on weight; when it does not, the lifting beam is the honest choice, and its moment diagram deserves respect.

For a symmetric two-sling spreader with hook load $W$ and sling angle $\alpha$ from the horizontal, statics gives each leg $T=(W/2)/\sin \alpha$ and puts $N=(W/2)\cot \alpha$ of compression into the beam. Both denominators collapse as the slings flatten:

At 90° each sling carries half the load and the beam carries nothing axially. At 30° each sling carries the full hook load and the beam carries almost 87 % of it in compression. At 20° the numbers are 1.46 W per leg and 1.37 W in the beam — the rig is fighting itself. This is why flat slings punish both components: the same trigonometry sits in both denominators.

The constraint that flattens slings in practice is headroom: a hook that cannot go high enough forces a wide, shallow triangle. That is a real constraint, not a mistake — but it should be priced, and the tool prices it: enter the geometry you actually have, and the solved angles, tensions and compression land in the checks, with your declared minimum angle enforced and a written warning when the flattest sling drops below 30°.

An I-section is superb at bending about its major axis and mediocre as a strut: its weak-axis radius of gyration $r_y$ is typically a small fraction of $r_x$. A column in a building often gets away with this because girts, purlins or slabs brace the weak axis at intervals. A spreader hanging in the air has no such friends: nothing braces a suspended beam between its end slings, so the full span, times the same effective-length factor, works on the weakest radius. The Euler stress falls with $(KL/r{)}^2$ — halve the radius and the elastic buckling resistance drops fourfold.

A circular tube has one radius of gyration in every direction and is exempt from lateral-torsional buckling on every route this tool computes — which is why the market's default spreader is a CHS. An I-section spreader is not wrong; it is simply a design whose governing check is usually weak-axis flexural buckling, and whose lateral-torsional behaviour depends on a question most tools never ask.

Solved tilt, not assumed level. An off-centre centre of gravity tilts the hang until the hook sits over the effective CoG. The solver finds that attitude as a constrained minimum of potential energy, reports the tilt against your declared limit, and resolves self-weight and every sling force at the solved angle — so a CoG surprise shows up in the numbers, not on the site.

Tension detection. Splay the bottom slings outboard of the beam's lugs and the bottom rigging pulls the beam ends outward: the beam goes into tension, and the compression checks are the wrong checks. Most spreader tools cannot represent this arrangement at all. This one detects the sense from the solve and runs the right checks, spelling the sense out in words — for the default spreader, 30.7 kN of compression.

The eccentricity ledger. A padeye on a flange face cannot have its pin on the beam's neutral axis. Each pin's horizontal force component acting at its offset applies a transfer couple $H\cdot e$ that jumps the moment diagram — on the default spreader, 4.61 kN·m at each end against a self-weight-only moment of 3.60 kN·m. Zeroing the eccentricities halves this beam's real midspan moment out of the model — the methodology page draws it.

The story in that figure is worth retelling in words, because it is the best single argument for solving the hang. Level, the centred lifting beam is as benign as rigging gets — the hook force and the load force share one line, and the beam carries only its own weight in bending. Tip the assembly six degrees, as EN 13155 requires you to assume it can be tipped, and the closure walks 31 mm away from the pin: suddenly a 103-kilonewton force pair acts across a short lever arm, shear jumps 42-fold, and the peak moment grows by 42 %. The clause exists precisely for this effect, and a spreadsheet that assumes its angle will never see it.

• ASME. Designs to ASME BTH-1-2020 — design categories, service classes, allowable stresses, fatigue, and the suspended-beam lateral-torsional reduction. The one fully self-contained route, with B30.20's marking and proof obligations reported alongside.
• EN hybrid. EN 13155:2020 load conditions — withstand 2 × without permanent deformation, 3 × without releasing the load, plus the mandatory ±6° tilt case — on EN 1993 resistances. Labelled EN HYBRID because EN 13155's own calculation annex nominates the EN 13001 chain, which is outside this assessment; the label is printed wherever the route appears.
• CSA hybrid. CSA S16-09 factored resistances against demands carrying a load factor you declare, because S16 takes its load factors from the NBCC and Canada has no below-the-hook standard. The route gates until the factor is declared — no default is invented.
• DNV overlay. For marine lifts to DNV-ST-N001: DAF from the standard's table at the solved hook load, the skew factor under its own validity rules, and a fixed consequence factor of 1.30 for spreaders. It multiplies demands on any route, with a written warning where the combination double-counts dynamics.

• The status strip. PASS, FAIL or INDETERMINATE. INDETERMINATE is not a soft fail: it means a check refused to answer outside its provision — a designed gate with a named reason and remedy, listed in full on the methodology page. A FAIL always outranks a gate.
• The governing check, named by station. Not just “utilisation 0.51” but which check, at which pin or station, in words. And the governing check is not always steel: on the default spreader the overall governing item is the sling-angle margin at 0.759 — the solved 59.3° against the declared 45° floor — while the largest strength utilisation is the padeye attachment weld — bth-1 §3-3.4 at 0.557. Both are named, which is exactly the kind of thing worth knowing before ordering steel.
• The factor ledger. Every demand-side and resistance-side factor, each with its clause, printed in the report. An auditor reads the ledger instead of reconstructing the arithmetic.
• The capacity envelope. WLL against span at utilisation 1.00, every point a genuine re-solve. A flat plateau means a connection governs — span is free until a member check takes over, and the redesign target is the padeye, not the beam. See the methodology page for the self-consistency test behind the curve.
• Warnings, and accept-and-record. Warnings that are engineering judgements — an unusually flat sling, a CoG above the lift points — can be accepted. An acceptance is an input: it rides the share link and prints in the report with its timestamp while its condition holds. Nothing is dismissed into silence.

The rig: a symmetric 6 m spreader in CHS 273×12.7 (S355), 10 t rated load below the beam plus 0.5 kN of bottom rigging, hook 5.2 m above the beam axis, end lugs with 150 mm pin offsets, vertical 2 m bottom slings.

1. The solve. Beam self-weight 4.80 kN brings the hook load to 103.40 kN. The top slings solve to 60.13 kN each at 59.3° from the horizontal — comfortably above the 45° floor — with a horizontal component of 30.71 kN per sling.
2. The forces. The beam carries 30.71 kN of compression between the lugs. The moment diagram jumps by the H·e couple of 4.61 kN·m at each end and peaks at 8.20 kN·m at midspan; shear peaks at 2.40 kN.
3. ASME route (Category B, Service Class 0). Member utilisations are small — axial 0.037 at 3.17 MPa, bending 0.104 at 13.52 MPa, combined 0.141 — and the connection carries the largest strength utilisations: weld 0.557, pin bearing 0.508, pinhole tension 0.251. The tube's manufacture is declared unknown, so the route designs on the §3-1.7 reduced wall.
4. The same steel on the EN hybrid route tells the same story in different factors: the yielded (3 ×) condition governs through the padeye pin at 0.624, with the weld at 0.596. The member barely notices; the connection is the design.
5. And on the CSA hybrid route at a declared load factor of 1.50: compression utilisation 0.021, bending 0.045, interaction 0.066, pin chain up to 0.295 — governed by the padeye weld at 0.310. Three routes, three factor systems, one conclusion: this design is connection-governed.

1. Rated load, rigging weight and beam self-weight all in the hook load.
2. Centre of gravity position confirmed from drawings or weighing — not assumed centred.
3. Padeye pin eccentricities taken from the fabrication drawing, not set to zero.
4. Sling angle floor declared, and the solved angles checked against it.
5. Declared shares entered for any arrangement with more than two bottom points.
6. End twist restraint declared honestly — a shackle braces nothing.
7. Route factors declared: category and service class, cycle count, or the project load factor.
8. Hollow-section manufacture declared, so the right wall thickness designs the ASME route.
9. Proof-load and marking obligations for the jurisdiction identified and assigned.
10. Every accepted warning reviewed in the report — an acceptance is a recorded decision.

Why does the tool refuse my 4-point lift?

Four bottom points on rigid bodies are statically indeterminate: the sharing depends on sling stretch and fabrication tolerance, which the model deliberately does not include. Equal sharing is an assumption, not a solution — real 4-point lifts routinely carry most of the load on a diagonal pair. Enter the declared shares from your rigging analysis and the tool computes with them, printed as your declaration.

Why is my result INDETERMINATE instead of FAIL?

FAIL means a computed check exceeded its capacity. INDETERMINATE means a check could not honestly compute — a provision's validity limits were exceeded, a required declaration is missing, or the arrangement did not solve. The result names the gate and the remedy. Treating a refusal as a soft pass, or as a fail, would both be wrong: it is a question the tool is handing back to you.

Why can't I edit the DNV consequence factor?

Because DNV-ST-N001 does not offer a choice: Table 16-5 row 1 assigns 1.30 to spreader frames and beams not subjected to load testing, and a load-tested spreader keeps 1.30 as a lift-point attachment under §16.8.5.1. Either reading lands on 1.30, so it is printed, sourced, and not a knob.

Why does the EN route run a ±6° case I didn't ask for?

EN 13155 §5.1.2.3 requires attachments not intended to tilt to be designed for at least 6° of tilt — and §5.2.6.3.1 says a lifting beam for horizontal use shall tolerate 6°. The tool re-closes the statics at that attitude and envelopes the demands, because on some geometries the tilt case adds nothing and on others it dominates the shear diagram entirely. Section 4 of this guide shows one of the second kind.

Why doesn't my capacity drop when I increase the span?

Because a connection governs. A padeye weld's capacity does not know the span, so the WLL-versus-span curve runs flat until a member check — buckling or bending — takes over. The plateau is the design insight: to lift more, fix the connection, not the beam.

Which route should I pick in Canada?

Canada has no below-the-hook device standard, so there is no route that removes the decision from you. The CSA hybrid route judges the steel to S16-09's factored resistances, and asks you to declare the demand-side load factor your project basis requires — because S16 itself takes load factors from the NBCC, which is a building code, not a lifting standard. Many projects also accept a BTH-1 design; that is a specification question for the lift plan's owner.

The methodology page documents every check, clause, factor and gate, with the live check registers and the cross-route comparison. The verification record re-solves the hand-calculated validation cases as the page renders and states the open items plainly. And the calculator opens on the worked example from section 7 — change one number and watch the whole chain re-solve.