Preventing Structural Damage From Excessive Snow Loads

April, 2003

With the recent heavy snowfall, High Concrete Structures noticed unusually high piles of snow on several local parking structures. We have contacted the owners of these structures, but also want to remind owners in general of the hazard of overloading a structure with excessive snow loads. Please evaluate and monitor the snow loading on your structures, as well as removing ice dams from roof drains and scuppers.
Understanding the risk: Snow loading is a concern with large accumulations and can cause damage to a structure. Garages are normally designed for a roof live load of 60 to 80 pounds per square foot. When snow is plowed into piles on the structure, or if ice accumulates due to clogged drains, this load can be exceeded. Snow piles can become far heavier if rain occurs after the pile is in place, and as noted, snow that is compacted and piled by a plow can easily exceed the design load of the garage. Also, blocked drains can cause loads to increase as snow melts during warm periods and then re-freezes. Consider these weights:
One cubic foot of dry, fluffy snow: 7 pounds.
One cubic foot of wet snow: 30 pounds.
One cubic foot of snow and ice mixture: 60 pounds.
One cubic foot of plowed, wet snow: 45 pounds.
Hence, if 3 feet of plowed, wet snow were piled onto a structure: 3 feet x 45 pounds per cubic foot = 135 pounds per square foot of load! This is well above the design load of a garage and is cause for immediate concern.
Here are some tips to remove the risk:
* Remove the snow from the structure with appropriately sized equipment rather than piling it onto the structure. Small pieces of equipment, which do not have wheel loads in excess of 2500 pounds, can be used on most structures, but care must be taken to ensure that the equipment weight is considered in combination with already existing snow loads. If there are questions on this, a design professional should be consulted.
* Clear the roof drains and scuppers so that melting snow or rain can drain from the structure. Also be certain that there are clear paths for water to reach the drains.
* In cases of existing overload, have a design professional examine the structure for signs of distress and determine a method of safe removal.
(Editor's Note: Clip this out and save for next winter.)