What a Spreader Bar and a Lifting Beam Each Do
Why this matters
A spreader bar and a lifting beam get treated as the same tool with two names, and they are not. They carry the load through different internal paths and only one of them buys you headroom. Reaching for a spreader because the ceiling is low is the specific mistake this card exists to stop: a spreader does not shorten the stack under the hook, and in a tight bay it can force your sling legs to a shallower angle than the direct bridle you were trying to avoid. Every leg tension figure in the worked example below is derived in front of you, and each number that has to come from a document is named.
Two different load paths
A spreader bar is a strut. Slings run from the hook down to each end of the bar, and separate slings run from those same ends down to the load. The bar is in compression along its length, pushed outward by the horizontal component of the upper legs. It is a long slender member loaded end to end, so its governing limit is buckling, and a spreader that has been struck, bent or repaired is not a spreader any more.
A lifting beam has a single bail on top, directly over the beam, and lift lugs along its underside. There are no angled legs above it. The beam carries the load in bending, like a simply supported beam with a point support at the bail, so its governing limit is bending stress and the deflection that goes with it. That is why a beam of the same rated load is heavier and shorter than a spreader: bending is a much less efficient way to move force than pure compression.
SPREADER BAR LIFTING BEAM
hook hook
/\ |
/ \ upper legs | single bail
/ \ still need | directly above
/ \ height +--------+
+============+ bar in | beam | in bending
| | compression +--------+
| | | |
+--+ +--+ lower legs | | short stack
+------+ near vertical +----------+
| load | | load |
+------+ +----------+
The angle convention, stated once
Every angle in this card is measured from horizontal, which is the convention on sling tags and in the sling capacity tables at 29 CFR 1910.184 and 1926.251. Half the trade quotes the angle from the vertical instead, and the two conventions give reciprocal answers, so a 30 degree leg is either the shallow dangerous one or the steep safe one depending on which convention the person speaking is using. Say "from horizontal" out loud when you say the number.
The relationship you need is one line. For a symmetric two-leg bridle, the tension in one leg is that leg's vertical share of the load divided by the sine of the angle from horizontal, and it holds only while the load hangs free and the legs are symmetric about the hook. At 90 degrees the sine is 1 and the leg carries its share. At 30 degrees from horizontal the sine is 0.5 and the leg carries twice its share. Tension climbs as the angle falls, and it climbs faster the lower you go, which is why shallow bridles surprise people. Many employer rigging programs set a floor at 30 degrees from horizontal and stop there; the sling manufacturer's own rating table is what actually governs, and it is the document to open.
What a spreader actually buys you
Two things, neither of which is headroom.
It takes the inward crush off the load. In a direct bridle the same angle that raises leg tension also pulls the pick points toward each other. That horizontal component is real force applied to the load's own structure, and a sheet-metal cabinet, a long skid or a thin-wall vessel can be squared up by a crane and never be square again. A spreader replaces the angled lower legs with near-vertical ones, so the inward component goes to the bar instead of the load.
It puts the pick points where you want them. With a bar you choose the spread, so you can pick a long object at the two points its own frame was designed to be supported at rather than at whatever two points give a workable sling angle.
What it does not do is shorten the assembly. The angled legs simply move from below the bar to above it, and the height they need does not change with position.
What a lifting beam actually buys you
A beam buys headroom, and it is the only one of the two that does. Because the bail sits directly over the beam, the whole overhead stack is the beam depth plus a shackle plus whatever master link you use. Nothing above the beam is angled, so nothing above the beam demands vertical height.
You pay for that twice. First in weight: a beam sized for a given load and spread is heavier than a spreader of the same rating, and the whole of that weight is a deduction from the crane's net capacity, not a free ride. Second in spread sensitivity: the bending moment a beam sees is set by the distance from the bail to the lug the load actually hangs from, so moving to a wider set of holes raises the moment even if the load got lighter. The device's marked rated load is stated at a marked spread. That relationship is the subject of the below-the-hook rating card in this library and is not re-derived here.
The headroom gate
Run the gate before you argue about which device is better. Measure the vertical distance from the top of the load to the underside of the hook at the height you will actually be lifting to, with the load on the ground and nobody between the load and the machine, and treat that as your entire budget. Then:
- If the budget is larger than half the pick spread multiplied by the tangent of your target angle from horizontal, a direct bridle fits and you may not need a device at all.
- If it is not, and the load can take the inward component, a longer sling and a steeper hook position is the cheap answer.
- If it is not, and the load cannot take the inward component, you need a spreader, and you must re-run the gate for the upper legs above the bar.
- If it is not, and you cannot make the upper legs work either, you need a beam and you must find the crane capacity to carry it.
Worked example: a 20 ft skid with 9 ft of headroom
A 20 ft skid weighing 6,000 lb, with two rated lift lugs 16 ft apart and its center of gravity centered between them. The building steel leaves 9 ft between the top of the skid and the underside of the hook. Every figure below is either derived here or labelled as coming from a document.
Vertical share per pick point. Symmetric load, centered center of gravity, two points: 6,000 lb divided by 2 equals 3,000 lb per point.
Option one, direct bridle. Half the spread is 8 ft. With 9 ft of height the leg angle from horizontal is the arctangent of 9 over 8, which is 48.4 degrees.
- Vertical share per leg: 3,000 lb
- Angle factor per leg, one over sine 48.4 degrees: 1.34
- Leg tension: 3,000 lb x 1.34 = 4,010 lb per leg
- Inward horizontal at each pick point, 3,000 lb divided by tangent 48.4 degrees: 2,667 lb
To reach 60 degrees from horizontal instead you would need 8 ft x tangent 60 degrees, which is 13.9 ft of height. You have 9 ft. The 60 degree bridle does not exist on this job. And that 2,667 lb of inward force at each lug is the number to take to whoever owns the skid frame, because nothing on the sling tag knows about it.
Option two, spreader bar at 16 ft. The lower legs go vertical, so each carries its share with no angle factor and no inward component:
- Lower leg tension: 3,000 lb x 1.00 = 3,000 lb per leg
- Inward horizontal at each pick point: zero
Now re-run the gate above the bar. Shackles and a master link at each end eat about 1.5 ft, and the bar itself is about 1 ft deep, so the upper legs get 9 minus 1.5 minus 1, which is 6.5 ft.
- Upper leg angle: arctangent of 6.5 over 8 = 39.1 degrees from horizontal
- Angle factor per upper leg, one over sine 39.1 degrees: 1.59
- Upper leg tension: 3,000 lb x 1.59 = 4,770 lb per leg
- Compression in the bar, 3,000 lb divided by tangent 39.1 degrees: 3,691 lb
The spreader protected the skid and made the sling angle worse, from 48.4 degrees down to 39.1 degrees, and raised the leg tension from 4,010 lb to 4,770 lb. That is the whole point of this card. It is still a defensible choice if the skid frame cannot take 2,667 lb of inward force, but you chose it for load protection, not for height.
Option three, lifting beam at 16 ft. Beam depth about 1 ft, plus shackle and master link about 1.5 ft, so the overhead stack is about 2.5 ft and you have 6.5 ft of slack instead of a problem.
- Leg tension below the beam, vertical: 3,000 lb x 1.00 = 3,000 lb per leg
- Bending moment at the bail: 3,000 lb x 8 ft = 24,000 lb-ft, which is the figure the beam's rated load at 16 ft spread has to cover
- Beam weight, from the device data plate: 900 lb in this example
The deduction line nobody prints. The crane does not carry 6,000 lb in any of the three cases. It carries the load plus everything hanging below the hook:
- Direct bridle: 6,000 lb load + 120 lb slings = 6,120 lb
- Spreader: 6,000 lb + 400 lb bar + 200 lb slings = 6,600 lb
- Beam: 6,000 lb + 900 lb beam + 120 lb slings = 7,020 lb
The beam costs about 15 percent more suspended weight than the direct bridle, and that difference comes straight off the crane's net capacity at the working radius. On a lift already near the chart, the beam that solved your headroom problem is what puts you over.
What getting this wrong looks like. The common field version is a spreader hung on short upper legs because the bay is tight, ending up somewhere near 25 to 30 degrees from horizontal, where the angle factor is 2.0 or worse. Nothing looks wrong from the ground, the load lifts, and the failure shows up on a later lift when a leg that has been cycled at double its assumed tension parts. The catch is arithmetic, not eyesight.
How to verify you picked the right one
Do these with the load landed, the hoist stopped and nobody inside the swing radius. Nothing here is done while the load is in the air.
- Measure, do not eyeball, the height budget. Tape from the top of the load to the underside of the hook with the hook at working height and the crew clear of the swing radius. An eyeballed 12 ft that is really 9 ft is the whole error.
- Compute the angle from horizontal for every angled leg, including the legs above a spreader, and print the angle factor as its own line next to the vertical share before you multiply. If the factor never appears on paper, the crew will rig to the vertical share.
- Check the device rating at the spread you are actually using, not at the spread on the sticker, and confirm the data plate is legible and matches the device.
- Add the device weight into the suspended load before you compare against the crane's net capacity, and confirm you counted it once, not twice.
- Inspect the bar for straightness before it is rigged, with it on cribbing and unloaded, because a spreader with a bow in it has already used part of its buckling capacity and there is no way to see that under load.
References
- 29 CFR 1926.251, rigging equipment for material handling, which applies to construction work and requires inspection of slings and fastenings each day before use by a competent person
- 29 CFR 1910.184, slings, the general-industry counterpart, including the sling capacity tables whose angles are stated from horizontal
- ASME B30.20, below-the-hook lifting devices, and ASME B30.9, slings, in the edition your employer's rigging program or your contract adopts, which is how these consensus standards bind you rather than on their own
- Manufacturer documentation for the specific spreader, beam, sling and shackle, which owns every rated capacity, removal-from-service criterion and marked spread
- See related: Why a Below-the-Hook Device Carries Its Own Rating; How to Read a Crane Load Chart Without Getting It Wrong