7.3 Wind, Storms, Lightning, Ice & Cold: Weather Limits for Crane Work

The rules that apply

The manufacturer's manual is where the actual numbers come from: maximum operating wind speed for each configuration, the reference height for measuring it, out-of-service wind limits, cold-temperature limits and any capacity reductions.

Wind force: the square law

Dynamic wind pressure on a surface is commonly estimated as:

$$P = 0.00256 \times V^2 \times C_d$$

where $P$ is pressure in pounds per square foot, $V$ is wind speed in miles per hour and $C_d$ is the drag (shape) coefficient: about 1.2–2.0 for flat panels and forms, and lower for round shapes. The total lateral force is the pressure times the projected area:

$$F_{\text{wind}} = P \times A_{\text{sail}}$$

Doubling the wind quadruples the force. With $C_d = 1.2$:

• At 15 mph: $P = 0.00256 \times 15^2 \times 1.2 \approx 0.69$ psf
• At 30 mph: $P = 0.00256 \times 30^2 \times 1.2 \approx 2.76$ psf

Worked example. A 10 ft × 40 ft precast wall panel (400 ft²) is flown broadside in a 25 mph gust with $C_d = 1.5$:

$$P = 0.00256 \times 25^2 \times 1.5 = 2.40 \text{ psf}, \qquad F = 2.40 \times 400 = 960 \text{ lb}$$

That is nearly half a ton of sideways force, applied at the hook and transferred to the boom as side load. Taglines cannot reliably hold it.

Wind speed also generally increases with height, so the wind at a long boom tip can be much stronger than a ground reading. That is why manufacturers specify where wind speed is measured, and why tower cranes carry a wind speed indicator at the top.

Sail area and the manufacturer's wind limit

A crane's published wind limit assumes a standard load, a compact object with a limited wind-exposed area per unit of weight. Many crane manuals state the assumed load wind area per ton (1.2 m² per metric ton is a common figure). Large, light loads such as wall panels, curtain-wall units, plywood bundles, empty containers and HVAC units present far more area per ton. Most manuals then require a lower permissible wind speed, often calculated as:

$$V_{\text{permissible}} = V_{\text{chart}} \times \sqrt{\frac{A_{\text{standard}}}{A_{\text{actual}}}}$$

where $A_{\text{standard}}$ is the standard area allowed for that load's weight and $A_{\text{actual}}$ is the load's real projected area (including the drag factor if the manual says so). Example: the chart allows 30 mph for the configuration; the load's actual area is 4 times the standard area, so $V_{\text{permissible}} = 30 \times \sqrt{1/4} = 15$ mph. Follow the formula and definitions in your own manual. They vary by manufacturer.

High-sail loads cause three problems:

1. Side load on the boom tip that the boom is not built to resist (Section 5.1).
2. Radius growth when the load is blown away from the crane, reducing capacity.
3. Loss of tagline control and danger to the people holding the taglines.

Recognizing wind speed

The manufacturer's limits govern. This Beaufort-based guide only helps with judging wind:

Storms and securing the crane

• Storm warning (OSHA 1926.1417(h)): a competent person decides whether to carry out the manufacturer's securing procedure. Typical steps for mobile cranes include landing the load, retracting and lowering the boom, or parking it at the manufacturer's recommended angle, and setting swing and drum brakes and locks.
• Tower cranes: manufacturers normally require the slewing brake to be released when out of service, so the jib can weathervane with the wind. Never lock a tower crane against the wind unless the manual allows it.
• Lightning: neither 230 CMR nor OSHA Subpart CC sets a lightning distance, but a crane boom is a tall conductor. Most contractors adopt a lightning policy, for example stopping lifts and moving people to shelter when thunder is heard or lightning is seen, and waiting 30 minutes after the last thunder before resuming. Lower the load and boom first if it can be done safely. Never shelter under the boom or next to outrigger pads.

Ice, snow and cold

• Ice and snow add weight. Ice on a lattice boom or snow on telescopic sections adds uncalculated dead weight at a long lever arm and can jam limit switches, A2B weights and sheaves. OSHA 1926.1417(n) makes the competent person responsible for adjusting operations for these effects. Clear the boom, or reduce loads as the manufacturer directs.
• Cold affects hydraulics and steel. Cold, thick hydraulic oil makes controls sluggish and can delay brake release. Warm up the hydraulics per the manual before working. Structural steels lose toughness at low temperature, so avoid shock loading. Crane manufacturers publish minimum operating temperatures and any cold-weather capacity reductions for their machines, and those figures govern.
• Slings and rope: synthetic web slings have temperature limits (Section 8.1), and frozen, wet fiber slings need the manufacturer's guidance.