Where the Zone Under a Suspended Load Actually Is

Why this matters

Ask a crew to stand clear of a suspended load and watch where they go. They step out of its shadow. The shadow is the one part of the zone that is easy to see and it is the smallest piece of it, because a load that lets go does not fall straight down onto its own outline, and because most of the people hurt around lifts were never under the load at all. They were in the arc it swept, in the circle the machine's tail rotated through, or on the line the rigging snapped back along. The zone is a set of regions with different shapes and different owners, and marking the wrong one gives everybody the feeling of a controlled site with none of the control.

What the shadow is not

Start with what the footprint leaves out, because that is where the injuries are.

It leaves out the arc. A load at a working radius does not move in a line when the machine rotates, it moves along a circle centred on the machine, and the ground it passes over is a band, not a point. It leaves out the overshoot, because a suspended load is a pendulum and it does not stop when the boom stops. It leaves out the tail, the circle swept by a rotating superstructure and its counterweight, which is a crush hazard that exists with nothing on the hook at all. It leaves out the spread, because a load that drops from height topples, rolls, slides and skids, and long objects reach their own length outward from where they land. And it leaves out the snap path, the line along which a parted sling, a failed shackle or a released tag line travels, which is straight, fast, and roughly along the axis the tension was acting on.

Five regions, and the footprint is a subset of one of them.

The plan view, drawn

        tail swing circle
         .-----------.
        /             \
       |    machine    |         load at radius R
        \      +      /               [ ]
         '-----|-----'                 |
               |   swept band as the   |
               |   superstructure      |
               |   rotates through     |
               +-----------------------+
                        arc

    exclusion line: outside the tail circle AND
    outside the swept band AND outside the load's
    own spread distance beyond the band

The line you paint is the outer envelope of all three, held for the duration, not just while the load is moving. It does not shrink when the lift pauses.

Working the arc and the overshoot with numbers

Two pieces of this are computable on the tailgate, and computing them is what turns "stand back" into a distance.

The arc. Ground swept equals the working radius times the swing angle in radians. A load at a 40 ft radius swung through 90 degrees, which is 1.571 radians, sweeps 40 x 1.571 = 62.8 ft of ground, call it 63 ft of band. That band is not a walkway that opens up behind the load. It is closed for the lift.

The overshoot. A load hanging below the hook behaves as a pendulum, and for small swings on a single-point suspension the period is 2 x pi x the square root of the effective length divided by 32.2 ft per second squared, where the effective length runs from the pivot to the load's centre of gravity. With 12 ft of rigging below the hook, that is 2 x pi x the square root of (12 / 32.2), which is 2 x pi x 0.610 = 3.84 seconds for a full cycle, so roughly 1.9 seconds to travel from one extreme to the other.

Read what that relationship holds constant, because the useful part is what is missing from it. The period does not contain the weight and it does not contain the amplitude. A heavy load does not swing slower than a light one and it does not swing less far, which is the exact opposite of what most crews assume when they watch a big pick creep along. How far it swings is set by the energy that got put in, which is the operator's stop, the wind and the tag lines, and that is not a number you compute on a tailgate. Route it: the amplitude belongs to the lift plan and the qualified person, and the practical control is to swing slowly and to stop slowly, because both the input and the overshoot scale with how abruptly the motion changed.

What you do with the 1.9 seconds is decide how far outside the band the line goes. A load that arrives over its target still swinging has an outer excursion beyond the stop point, and a person standing at the band's edge is inside that excursion for about two seconds at a time, repeatedly, until the swing damps.

The tail is a separate zone with a separate owner

The counterweight circle of a rotating superstructure is a fixed radius published in the machine's own data, and it is the one region in this article whose number you take from a document rather than derive. Get it from the machine's load chart and dimension sheet, not from pacing it out, and barricade it. For construction crane work, 29 CFR 1926.1424 in Subpart CC requires the employer to control the area within the swing radius of the rotating superstructure so employees are not struck or crushed, and that duty is independent of whether anything is on the hook.

This is the region where the "I stepped out of the shadow" habit kills people, because the tail sweeps toward the person who moved away from the load.

The spread, and why a long load needs a bigger circle

A dropped load does not stay where it lands. It topples about the corner that hits first, and a long object lying down reaches roughly its own longest dimension away from the point below the hook. A 20 ft length of duct or pipe dropped from height has a 20 ft reach before you account for any bounce, roll or slide, and on a hard flat deck it will not stop there.

Treat the load's greatest dimension as a floor on the spread, not an estimate of it. The bounce and slide are genuinely not computable from anything on site: they depend on the surface, the shape and the height, and there is no honest coefficient to hand you. Where the consequence of being wrong matters, that distance is a question for the lift plan and, on an engineered lift, for the professional the plan routes it to. Where you have to make a field call, add margin outward, never inward, and say plainly that you are bounding it rather than calculating it.

Keeping clear is a duty, not a courtesy

Two things follow from that, and they are the parts crews argue about.

Riggers may be near the load; nobody is under it. For construction crane work, 29 CFR 1926.1425 under Subpart CC sets out keeping clear of the load, and the narrow allowances it makes are for the people whose task requires them to be there for a specific operation, not for a general licence to work in the zone. In general industry, 29 CFR 1910.179 covers overhead and gantry cranes and includes the operator's duty regarding carrying loads over people. Cite whichever Part covers your work, in the plan, by name.

A customer or a homeowner is outside OSHA entirely. The duty you owe a bystander who is not your employee is the ordinary duty of care, not a citation, and the practical consequence is that you cannot instruct them, you can only exclude them physically. Rope, cone and body-block the approach before the load leaves the ground, because a verbal warning to someone who does not work for you is worth exactly nothing at the moment they walk toward their own building.

Setting the line, one worked case

A packaged unit is coming off a flat roof onto a trailer in the parking lot. Machine set at one end, working radius to the roof pick 40 ft, swing to the trailer 90 degrees, rigging below the hook 12 ft, unit's longest dimension 9 ft, tail swing radius taken off the machine's dimension sheet as 15 ft.

• Tail circle: 15 ft radius about the centre of rotation, barricaded before the machine is set up, held all day.
• Swept band: 40 ft radius, 90 degrees, which is 63 ft of ground along the arc. Band width is the load's own width plus its spread on both sides.
• Spread: floor of 9 ft, the load's longest dimension, added outward on both sides of the band and around the pick and set points. Bounded, not calculated, and stated that way on the plan.
• Overshoot allowance: the load is stopped slowly, but the line is set outside the band by the spread distance anyway, which covers the excursion at the ends of the swing where the load is moving fastest.
• Result: the parking lot is closed across a 63 ft arc plus 9 ft either side, and the tail circle is closed independently. The pedestrian route the building manager offered "behind the crane" runs through the tail circle and is refused.

The failure mode this catches is the one that happens every time: somebody asks for a walkway to be kept open, everybody looks at the load, and the walkway that gets approved is behind the machine, which is the region with the least warning and the shortest reaction time of the five.

How to check you got it right

Walk the line before the lift and ask three questions at each point. Can I see the load from here, and if the answer is yes, can the load reach me. If the machine rotates 180 degrees from where it is now, does this point end up inside the tail circle. And if the load released at the highest point of the lift, does anything within its longest dimension of the landing point matter, including the machine's own outrigger, a gas meter, or the customer's entrance.

Then check the line during the lift, not just before it. A zone that is set once and never re-walked will be occupied within twenty minutes, usually by somebody who has a legitimate reason to be there and no idea the geometry moved when the boom did.

References

• 29 CFR 1926.1424 (work area control) and 29 CFR 1926.1425 (keeping clear of the load), Subpart CC, construction
• 29 CFR 1910.179 (overhead and gantry cranes), general industry
• The machine's own load chart and dimension sheet for tail swing radius and working radius, which is the governing source for both
• ASME B30.5 (mobile and locomotive cranes), in the edition your jurisdiction, contract or employer programme has adopted
• See related: Stopping a Lift and What Happens Next; What a Two-Blocking Event Is and Why It Is Sudden