How to plan winter crew hours from wind chill and the ACGIH warm-up schedule, with a steel erection example

A superintendent setting steel in January has to decide how many hours the raising gang will spend in the warming trailer. That number belongs in the lookahead schedule and in the labor budget, and it can be set before the cold arrives. Two public sources cover the calculation: the National Weather Service (NWS) wind chill formula and a work/warm-up schedule from the American Conference of Governmental Industrial Hygienists (ACGIH) that OSHA posts on its winter weather pages.

OSHA has no specific standard for working in the cold. Its cold stress page says employers still have a duty under the OSH Act of 1970 to protect workers from recognized hazards, cold stress among them. The same page tells employers to know the wind chill so they can gauge exposure and plan the work, and it recommends frequent short breaks in warm, dry areas.

Compute the wind chill

Wind chill is the temperature exposed skin feels once wind is added to the air temperature. The NWS publishes the formula, with T as air temperature in degrees Fahrenheit and V as wind speed in miles per hour:

Wind chill (°F) = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16)

The formula holds only at or below 50°F and for wind above 3 mph. The NWS bases it on a model of a human face, adjusts the wind reading from the 33-foot anemometer height down to 5 feet, and assumes no sun. Bright sunshine can raise the wind chill by 10°F to 18°F, according to the agency.

Two published examples check the arithmetic. The NWS gives 0°F with a 15 mph wind as a wind chill of -19°F, where exposed skin can freeze in 30 minutes. OSHA gives 40°F with a 35 mph wind as 28°F. In Excel, with air temperature in A2 and wind in B2, the formula is =35.74+0.6215*A2-35.75*B2^0.16+0.4275*A2*B2^0.16.

Wind chill applies to people and animals. The NWS notes that a radiator or a water pipe cools to the air temperature faster in wind but never below it. Frostbite also requires an air temperature below freezing. Freeze protection for concrete and piping is planned from the air temperature, and crew exposure from the air temperature and the wind together.

Read the warm-up schedule

The ACGIH schedule is keyed to air temperature and wind speed, and it starts at -15°F. For each combination it gives a maximum continuous work period and a number of 10-minute breaks in a warm location within a 4-hour period. It assumes moderate to heavy work and an extended warm break, such as lunch, at the end of each 4-hour period. Cells at the cold, windy corner of the table read “non-emergency work should cease.” That point arrives at -25°F with a 20 mph wind, or -45°F with no noticeable wind.

A copy of the table in a U.S. Army site health and safety plan carries four ACGIH notes that OSHA’s one-page version leaves off.

1. Light work with little physical movement takes the schedule one step colder. An equipment operator or a signal person at -20°F with a 10 mph wind would get the 40-minute, 4-break row instead of the 55-minute, 3-break row.
2. Without a wind reading, a light flag that moves means about 5 mph, a fully extended flag about 10 mph, a newspaper sheet lifted about 15 mph, and blowing and drifting snow about 20 mph.
3. The schedule slightly under-compensates for wind at the warmer temperatures and slightly over-compensates at the coldest ones.
4. The limits apply only to workers in dry clothing.

The table marks its mildest rows “normal breaks” with a count of one. This example reads that row as the usual mid-period break, so the added warm-up time is the break count minus one, times 10 minutes. An 8-hour shift has two 4-hour periods.

A worked example

The first table is an example. It runs seven forecast conditions through the NWS formula, rounds to whole degrees and looks each one up in the ACGIH schedule.

Wind chill alone does not pick the row. Air at -30°F with a 5 mph wind has a wind chill of -46°F and calls for 4 breaks. Air at -20°F with a 20 mph wind has a slightly warmer wind chill of -48°F and calls for 5. Read the schedule from the air temperature and the wind, and use the wind chill for the frostbite risk and for the NWS cold weather advisories in the area.

The second table is also an example, with round numbers. A six-person ironworker crew erects steel through a three-day cold snap on a job near Duluth. The estimator carries a loaded labor rate of $100 per hour.

The crew spends 14 hours in warm-up breaks over the three days, $1,400 of labor that produces no steel. Wednesday alone accounts for 8 of those hours because the wind rose to 20 mph at the same air temperature as Tuesday. The labor burden post shows how to build the loaded rate this example rounds to $100.

New or returning workers add a second cost. New York’s Department of Labor recommends starting a worker on an 8-hour shift at two hours of full-intensity work on the first day, then four, six and eight. A new hire on a cold Monday works 12 fewer full-intensity hours across the first three days.

How to use it on your jobs

Add the warm-up hours to the weather allowance in the baseline schedule for winter work. The weather delay days post builds the allowance for lost days from NOAA climate normals. The warm-up schedule adds the shorter work periods on days the crew stays on site.

Then record it. Each morning the superintendent logs the air temperature, the wind speed and the schedule row in the daily log, along with the warm-up breaks the crew took. After a winter of entries, the hours compare directly with units installed, and the labor productivity post shows how to divide one by the other.

To start, pick one job with outdoor work scheduled for December through February. Put the Excel formula and the ACGIH table on one sheet, enter the next 10-day forecast, and price the added warm-up minutes for each crew on the lookahead.