Snow loads and school safety
Calls to the school district by concerned parents were prompted by the recent snow storms. The amounts of snow left on roofs in varying depths from wind driven snow ranged from one foot at Kings Beach Elementary to six feet at Donner Trail Elementary. Due to the wind, many schools sites received differing amounts depending on the elevation and configuration of the building.To help allay fears of possible collapse, district officials explained how roof snow measurements and removal processes are set into motion during all storms of significance. The overriding message to all parents is that the school district takes roof snow loads very seriously, and the safety of students and other occupants is of primary importance.To emphasize that statement one has only to look at the reason schools were closed on Monday and Tuesday following the latest storm when the weather was clear and roads open. All schools were closed to provide district maintenance staff time to take snow load measurements and perform calculations on buildings throughout the district. This takes quite a bit of time, as access to each roof is different, and often requires digging through overhanging snow cornices to eventually snow-shoe across the roof to take measurements in the deepest areas. With each school building having up to six or seven different live roof loads there can be several measurements to take at each school. The differing live roof loads are attributed to different parts of buildings being built over a period of decades. A case in point is Truckee High School, where different sections of the school were built in the 1950s, 1970s, 1980s, and current building this year. Each building comes with a different and upgraded snow load due to newer codes and regulations from the Division of State Architects (DSA).Many concerns revolved around the modular classrooms, which are often incorrectly suspected of having a lower snow load than standard permanent buildings. In fact, modular classrooms have a greater snow-load rating than many of the main building roofs in the district. Modular classrooms are rated at between 140 and 188 lbs. per square foot, depending on the age and location of the building. Each school site roof snow loading is measured and calculated with regard to its specific live roof load. District policy requires that when a roof reaches 80 percent of rated live engineered roof load, snow removal will commence as soon as possible.This was the case for three schools in the district: Tahoe Lake Elementary, Truckee High, and Donner Trail Elementary. These schools had main building sections that reached or exceeded the 80-percent load. The higher percentage load roofs were cleared of snow prior to school opening on Wednesday, while the others were worked on during school hours, as they were not dangerous. None of the modulars at these sites were near eighty percent.The measurements are taken with a special hollow tube marked in increments of inches. The tube is pushed into the snow when it hits the roof. When the tube is removed, the compacted snow depth reading is noted. The tube is then weighed. This weight along with the compacted depth is then called in for calculation of the actual load and percentage based on each particular roof live load. Calculations produce moisture content, water ratios, and actual snow weight.The actual depth of the snow can be deceptive in relation to the actual weight. In some instances four feet of snow can produce less weight than one foot, depending on the moisture content. One of the stories that seems to surface each time snow loads come into question is the collapse ten years ago of an unoccupied trailer at Glenshire Elementary. This unfortunate incident seems to bring out concerns about the modular classrooms. Both being relatively portable in nature, the comparison ends there, as the modulars are load rated for our area and constructed of steel beams and girders. The overriding factor in all decisions for snow loads is safety. No school building will be opened unless calculations prove out. The 80 percent rule was revised seven years ago from 90 percent to provide additional time to react to removal, especially when additional snow is forecast and may possibly add to the weight. This additional time not only allows for increased protection for occupants, but also reduced chance of damage to structures.Christopher Cooper is the director of Risk Management for the Truckee Unified School District. contact him at 582-2575, or firstname.lastname@example.org.
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