What Wind Does to a Lift and When It Stops It

Why this matters

Wind does not push on weight, it pushes on area, and it pushes with the square of its speed. That single mismatch is why two loads on the same site at the same hour get opposite answers: the dense compact one is unaffected at a speed that has already made the broad light one uncontrollable. Crews that judge wind by how it feels on the ground get this backwards, because what they feel is the speed and what matters is speed squared acting on the sail they hung on the hook. This card runs one stated gate against two real loads and lets them resolve in opposite directions.

The gate

Stop the lift at the lower of two numbers: the manufacturer's in-service wind speed for that machine in that configuration, and the wind speed at which the load will not hang under the hook well enough to control. Measure at the working height where you can, not at ground level, and judge on the gusts rather than the average.

The first number is not yours to derive. It lives in the load chart and the operator's manual, it changes with boom length, jib configuration and counterweight, and 29 CFR 1926.1417 requires the equipment be operated in accordance with the manufacturer's procedures, which is where that limit sits. For work under 29 CFR 1926 Subpart CC, that is the binding route. ASME B30.5 covers the same ground for mobile cranes and reaches you through whichever edition your jurisdiction, your contract or your employer's programme has adopted; it does not bind on its own.

The second number you can compute, and this card shows how.

The relationship, and what it holds constant

Wind pressure on a surface facing the wind is proportional to the square of the wind speed, at constant projected area and constant air density:

q = 0.00256 x V squared, with q in pounds per square foot and V in miles per hour.

That constant is derived at standard sea-level air density, about 0.0765 lb per cubic foot. At higher elevation the air is thinner and the real pressure is lower for the same indicated speed, and in very cold dense air it is slightly higher. Do not take credit for altitude in the safe direction; treat 0.00256 as the working value and let the elevation error sit where it protects you.

The force on the load is then F = q x Cd x A, where A is the projected area facing the wind and Cd is a shape factor. For a flat panel normal to the wind, Cd is around 1.2 to 1.3 for roughly square shapes and higher for long thin ones; for a partly open frame it is lower because air passes through. These are illustrative planning values. Where a shape factor is load bearing on a real lift, it belongs to the engineer who stamped the lift plan or to the crane manufacturer's own wind-area guidance, not to a rule of thumb.

Two consequences fall straight out of the square law. Going from 15 mph to 25 mph is not a two-thirds increase in force, it is (25 divided by 15) squared, or 2.78 times. And a load's vulnerability scales with area divided by weight, which is a property of the object and can be worked out days ahead of the pick.

The two loads

Same site, same afternoon, a steady 25 mph with gusts. Both loads are inside the machine's capacity by a wide margin and neither is anywhere near the chart. The wind gate is the only thing in play.

Load A, a pump skid. 6,000 lb, presenting roughly 24 square feet to the wind, Cd taken at 1.2.

• q at 25 mph = 0.00256 x 625 = 1.60 psf
• F = 1.60 x 1.2 x 24 = 46 lbf
• F divided by weight = 46 / 6,000 = 0.0077
• Hang angle off plumb = arctangent of 0.0077 = 0.4 degrees

Load B, a wall panel. 900 lb, presenting roughly 200 square feet, Cd taken at 1.3.

• q at 25 mph = 1.60 psf, the same number, because it is a property of the wind and not of the load
• F = 1.60 x 1.3 x 200 = 416 lbf
• F divided by weight = 416 / 900 = 0.46
• Hang angle off plumb = arctangent of 0.46 = 25 degrees

Load A hangs four tenths of a degree off plumb, which nobody would see. Load B hangs twenty-five degrees off plumb, which means it is not under the hook at all, it is out to one side and it will stay there as long as the wind holds and swing to the other side when the wind drops. The panel weighs one seventh of what the skid weighs and takes nine times the force.

What the panel's number actually means

Twenty-five degrees off plumb is not a control problem, it is a machine problem. Load charts are computed for a load hanging in line with the boom, straight down from the boom point. A load standing out to one side puts a horizontal force at the boom tip that is not on any chart, and a lattice or telescopic boom is far weaker against side load than against the compression it was designed for. The 416 lbf is not a rounding error at the tip of a long boom, and no chart tells you what it costs, which is exactly the point: you are off the published condition and there is no published answer for where you now are.

Then look at what it would take to hold it. A tag line running from the panel down to a worker on the ground at roughly 45 degrees below horizontal would need about 416 divided by the cosine of 45 degrees, or 588 lbf of tension, to cancel the wind force. A person cannot hold that, and if they could, the vertical component would be another 416 lbf pulling down and adding straight to the hook load. Tag lines are for controlling rotation and making fine positioning possible from outside the fall zone. They are not a wind restraint, and a crew that treats them as one has put a person on the end of a rope attached to something the wind is winning against. Nobody takes a wrap of a tag line around a hand, a wrist or a body, ever, and tag lines are tended from outside the load's swing radius rather than under it.

Where the gate lands, and what changes it

At 25 mph the panel does not fly. The finding is not "be careful", it is a number: back-solve the same relationship for the speed that keeps the hang angle inside what the plan allows. If the plan's limit is a 5-degree hang, the allowable force is 900 x tangent(5 degrees) = 79 lbf. Working back, q = 79 / (1.3 x 200) = 0.30 psf, and V = square root of (0.30 / 0.00256) = 11 mph. Round that in the conservative direction and the panel's own stopping speed is 10 mph, which is well below anything the machine cares about and well below what the crew would notice on the ground.

Run the identical arithmetic on the skid and the answer is above the machine's own in-service limit, so for the skid the manufacturer's number governs and the load-specific one never binds. Same gate, same afternoon, two answers, and which term governs flipped between them.

Three things change these answers.

Presented area changes with orientation. A panel picked flat, lying in the plane of the wind rather than across it, presents a fraction of the area. Reorienting the pick is often the whole fix, and it is free. It is also fragile: the load rotates on the hook, so the reduced area only holds while the tag lines hold the orientation, and that is only credible in light wind.

Height changes the speed. Wind at 60 feet is faster than wind at the ground, and the reading that matters is at the load. A ground-level anemometer reading of 15 mph is not a statement about the working height, and the correction between them belongs to the engineer rather than to a rule of thumb.

Gusts, not averages, break things. A steady 15 mph with gusts to 30 mph delivers four times the force in the gusts as at the mean, and the gust arrives while the load is at maximum radius or halfway into a swing. Plan against the gust.

The failure mode

The way this goes wrong in the field is not a crew ignoring a gale. It is a crew watching the wind against the machine's number and never running it against the load. The machine's in-service limit was written for the machine and it assumes the load hangs where the chart put it. A broad light load can be out of control at a third of that speed, which means the machine's limit will read comfortably green the entire time the load is behaving worst.

The tell is visual and it is available before the load leaves the ground: break it free by a few inches, hold it, and watch where it hangs and whether it stays there. If it is standing off plumb or hunting while it is still inches up, the wind has already beaten the geometry and the rest of the lift only makes the lever longer. Set it back down. Everyone stays out from under the load and out of the swing path during that check, including the person watching it, and the check is called by the same person who will call the lift.

How to verify you got this right

Compute the load-specific stopping speed before the day, not on it. It takes the projected area, an assumed shape factor and the weight, all of which you have while planning, and it produces a number the crew can hold an anemometer against.

Then check three things on site. That the measured speed came from the working height or carries a stated correction. That the gate you applied was the lower of the two numbers rather than whichever one you happened to look at. And that the hang check was done and the load hung plumb, because that check is the only one that tests the arithmetic against the actual object rather than against your estimate of its area.

Finally, re-read your own shape factor. If you assumed an open frame and the load arrived shrink-wrapped, your area assumption is wrong by whatever fraction of the frame the wrap closed off, and the correction runs the wrong way.

References

• 29 CFR 1926 Subpart CC, cranes and derricks in construction, including 1926.1417 on operating in accordance with manufacturer procedures, which is where in-service wind limits live
• ASME B30.5 for mobile and locomotive cranes, in the edition adopted by your jurisdiction, your contract or your employer's programme
• The specific machine's load chart and operator manual for in-service wind speed by configuration
• See related: Why a Published Weight and an Actual Weight Diverge; What a Tandem Lift Adds Beyond Two Cranes