Gary Brown is a writer and engineer for Gale Associates, Inc., Herndon, Virginia. He can be reached at gwb@ gainc.com. This is his first article for Facilities Manager.

LANHAM, Maryland (CNN) – Search-and-rescue crews were sorting through the unstable wreckage of a Toys “R” Us store Saturday, looking for customers who might have been inside when the store’s roof collapsed under the weight of rain and melting snow. Authorities are “cautiously optimistic” that no one is trapped inside.

The preceding news story is not what you want to see in the newspaper or on television concerning your institution’s facilities. Snow-related roof failures of buildings that vary from hen houses to classroom buildings to shopping centers have cost hundreds of millions of dollars in damage in the last few years. These catastrophic events can cause serious damage to property, and more importantly to occupants, resulting in a debilitating effect on your operations.

Code and Theoretical Implications
The causes of snow load collapses are typically understood by structural engineers and roofing professionals because model building codes contain specific reference to snow load design. The more recently adopted codes are significantly more stringent than previous codes in the requirement for snow load analysis. Prior to 1968-1970, almost no U.S. building codes required structural engineers to consider the increased loads from snow drifting that can occur on roofs as a result of their geometry and proximity to other structures. Depending on the time when the specific codes were adopted, a building that was properly designed prior to 1975 was not likely designed to safely withstand the type of snowdrifts that can accumulate on its roof, according to current codes.

The 2000 International Building Code (IBC), which is in effect throughout much of the United States, requires that design snow loads be determined in accordance with Section 7 of the American Society of Civil Engineers (ASCE) 7 Minimum Design Loads for Buildings and Other Structures. Therefore, to bring your building up to current code requirements, you must perform a separate analysis to check the structure and possibly design reinforcements to the structure.

The IBC and the ASCE-7 standard base their requirements on a “50-year” storm. The East Coast of the United States had a storm over President’s Day weekend in 2003 that resembled this severity. There were more than half a dozen buildings in the region that sustained newsworthy damage, including at least one fatality. A portion of the B&O Railroad Museum, constructed in 1884 and housing the oldest and most comprehensive railroad artifacts in the country, collapsed under the weight of the snow and the heavy winds. Is this to say that there will not be another storm of this magnitude until 2053? Nobody really knows for sure. Furthermore, the failures of the roofs in the region are currently under forensic investigation as to what mechanisms caused the catastrophic collapses. The issues may be related more to the heavy rain on top of the snow than the high drifts.

The flat roof snow load is calculated to yield the likely conditions for the life of the structure. The engineering judgment of the design professional is critical in deciding how conservative this load is based on adjustment factors. The current roof snow load determination factors described by ASCE include the following:

Based on the ground snow load and certain geometric characteristics of the building, the minimum flat roof snow load may be increased beyond the calculated ground snow load. Additionally, roofs with slopes greater than five degrees are adjusted with a slope factor that is primarily due to wind action. The roof pitch, the type of roof covering, and the thermal conditions of the roof all affect the roof slope factor. After all is said and done, the calculated flat roof snow load is checked against a minimum value depending on whether the ground snow load is above or below 20 psf.

After the roof snow load is determined, calculations for unbalanced loading (due to partial removal or melting) and drifting (due to roof projections or adjacent buildings) must be performed.

Steep roofs that shed their snow onto lower roofs do so at a rate of 40 percent of their total surface area load. The snow is expected to extend out from the end of the eaves to a distance of 15 feet; therefore, a sloped roof that is only 40 feet from eave to ridge can be expected to discard at least 320 pounds of snow per foot of edge. This will add over 21 psf of snow to the structure below. You can see how significant this is considering that the flat snow load was previously calculated to be about 20 psf. In addition to this extra load for sloped roofs shedding their snow load, you then need to add in the snow drifting conditions.

Windward and leeward drift each need to be considered. The drift height can be determined by a graph or an equation stipulated by ASCE-7. The variables that must be considered for the determinations are the ground snow load in the region and the lengths of the roof area that are causing the drift. The length of the upper roof governs the leeward drift. The length of the lower roof governs the windward drift. Roof projections that form inside corners may even have drifts in more than one direction.

After the heights of the drift(s) are determined, the weight of the snow is calculated by multiplying the height by the density of the snow. This will ultimately be the information that is needed to analyze the capacity of the existing structural supports to withstand the snow load.

Lastly, a surcharge from rain-on-snow should be considered on roofs with relatively low ground snow load values. A 5-psf load must be added in certain circumstances where there is a possibility of rain over the snow or snowmelt is expected, especially in very low-slope configurations.

Protecting Your Building From Harmful Snow Loads
To prevent the sudden collapse of roof structures, building owners and managers need to perform the following steps to prepare for future weather events.

 A Facility Manager’s Checklist
To prevent a future roof collapse due to snow or ice accumulations, you should commence preparations and precautions as soon as possible. The actions may be as simple as good housekeeping or as complex as structural analysis and augmentation. Even if your structure has already withstood the onslaught of blizzards, there are warning signs to be aware of for the future.
 Conclusion
The model building codes prescribe the performance of a building under extreme conditions that are only expected to occur at infrequent intervals. Typically, the loads due to snow, rain, and wind work in combination of varying degrees with each other and result in circumstances that may even be counterintuitive to ordinary observations. Even though the circumstances for failure are rare, they do happen.

A building owner or facility manager can mitigate the

effects of these rare occurrences by taking proper precautions. At a minimum, the building should be designed, constructed, and maintained to meet the code requirements. The maintenance of structures includes careful inspection of structural members to ensure that the strengths of the materials anticipated during the original design and construction activities are still being achieved. Additional preparations can be made when a significant snow event is forecasted in the same manner that building owners protect against forecasted hurricanes or other predicted events. The best time to prevent building failures due to snow is during the spring, summer, and fall seasons.