By Gregory Havel
Roofs of buildings are designed to withstand different types of live loads including snow loads. The amount of snow load that a roof is designed to hold varies according to building code requirements and on latitude, prevailing winds, and predictable winter weather.
Photo 1 shows the roof of a school in southeastern Wisconsin made up of steel bar joists supporting a roof deck (Internet search for “cementitious wood fiber roof deck”), expanded polystyrene insulation board, a rubber membrane, and gravel ballast. The steel duct in the foreground extends 36 inches [0.914 meters (m)] above the roof. The parapet wall in the background extends 30 inches (0.762 m) above the roof. A snow storm had covered the roof to the height of the parapet wall with heavy, wet snow. A few days later, another storm, followed by colder weather, added nearly an inch of rain, causing the snow to settle and become saturated and freeze.
(1) Photos by author.
Photo 2 shows the failure of one of the steel bar joists supporting this roof, as a result of too much live load imposed on it. This was not a catastrophic roof failure, of the kind that drops the roof and its overload of snow onto the floor of the building, as we see almost every winter on the national news. It was a failure that caused the roof to sag and leak, and that damaged the steel bar joists supporting the roof. The engineer’s estimate was that the 2.5 feet (0.762 m) of wet snow already exceeded the designed load of the roof; and the rain that saturated the snow a few days later exceeded the designed load plus the safety factor, causing bar joist welds to break, and the parallel top and bottom chords of the bar joists to bend.
The engineers designed a method for straightening the bent bar joists and reinforcing them with steel angles without removing the roof and replacing the bar joists. Photo 3 shows part of the repair that was done on one of the failed bar joists.
Building codes include maps and tables that define the live (snow) load that is to be designed into roof systems in different parts of North America. These tables are based on American Society of Civil Engineers Publication 7 (ASCE 7), which is incorporated by reference into both National Fire Protection Association 5000, Building Construction and Safety Code, and the International Building Code. (A copy of the map can be found by an Internet search for “ASCE 7” or by visiting http://publicecodes.cyberregs.com/icod/ibc/2012/icod_ibc_2012_16_par089.htm.)
In general, the required design snow load increases as the latitude increases. Some examples of design snow loads from the map of the eastern United States of America (Fig. 1608.2, International Building Code© 2012 by the International Code Council) are:
- 70 pounds per foot2 (psf) [3.353 kilonewton per square meter 9kNm2)]: Sault Ste. Marie, Michigan; Thief River Falls, Minnesota.
- 60 psf (2.874 kNm2): Superior, Wisconsin; Grand Forks, North Dakota.
- 50 psf (2.395 kNm2): Marinette, Wisconsin; Wausau, Wisconsin; Minneapolis, Minnesota; Fargo, North Dakota.
- 40 psf (1.916 kNm2): Green Bay, Wisconsin; LaCrosse, Wisconsin.
- 35 psf (1.677 kNm2): Manitowoc, Wisconsin; Decorah, Iowa.
- 30 psf (1.437 kNm2): Milwaukee, Wisconsin; Madison, Wisconsin.
- 25 psf (1.198 kNm2): Chicago, Illinois.
- 20 psf(0.958 kNm2): Indianapolis, Indiana; Springfield, Illinois; St. Louis, Missouri.
- 15 psf(0.719 kNm2): Louisville, Kentucky; Cairo, Illinois; Springfield, Missouri.
- 10 psf(0.479 kNm2): Memphis, Tennessee; Greenville, Mississippi; Little Rock, Arkansas.
- 5 psf(0.239 kNm2): Montgomery, Alabama; Jackson, Mississippi; Alexandria, Louisiana.
- 0 psf(0 kNm2): Savannah, Georgia; Florida; New Orleans, Louisiana; Texas Gulf Coast.
(Note: 1 kNm2 equals 101.97 kg/m2, and 20.89 pounds per foot2.)
In mountainous regions, the designed snow load will vary with the altitude of the building site, the prevailing wind direction, and the side of the mountain on which it is located. In all cases, the final determination of the design snow load requirements is set by the state or local building code officials.
Roofs do not often fail catastrophically. Usually, the failure is similar to the one in the photos and is because of a gradual overload from snow or water accumulation. When a roof does fail catastrophically, it is because of the failure of one or more critical connections in the overloaded supporting structure or from the effects of a fire below weakening or burning away the structural supports. In the event of a catastrophic roof failure, neither firefighters nor building occupants want to be on the roof or under it.
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.
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