waterville snow2  [1]
Building: Waterville Junior High School
Location: Waterville, ME
Year of Construction: 1978
Year of Failure: 1978
Type of Failure: Roof collapse

On the first day the Waterville Junior High was opened, February 9, 1978, the roof collapsed in a classroom soon after a teacher noticed the ceiling was deflected.   Waterville, Maine had been experiencing one of the most severe snowstorms it had ever seen for two days prior to the collapse.  [a]

The collapse was determined to occur due to a singular cause which was failure of a joist in an open web joist system.  The architect accounted for a uniform snow load, however he did not account for snowdrift load which was the cause of the failure of the joist.  The school was divided into six sections, each a different elevation.  The collapse was in a section with a relatively low roof elevation.  Thus, during the snowstorm, snow was blown off of higher roof sections and drifted onto the roof section of the collapse, which also happened to be downwind. [a]

Computer analysis of the joists involved in the collapse revealed that the joist with the greatest load during the time of the collapse was Joist 3, as shown in the diagram below.  However, the dead load was calculated assuming a more realistic load distribution amongst Joists 2, 3, and 4.  Ultimately, the total snow load on these three joists was calculated to be 422 lb/ft while the allowable load was calculated to be 190 lb/ft.  The specific open web joists used in the roof were determined to be able to hold a load of up to 2.2 times the allowable load; however, this buffer was not wide enough to contain the difference between the actual and allowable loads.  The official and “most probable” cause of failure was said to be that the ultimate strength of either Joist 2, 3, or 4 was overcome by the snow load, and one of these joist failed.  This caused the forfeited load to rest on adjacent joists that were unfit to carry the added load, and several said joists failed. [a]

In conclusion, snow drifting largely contributed to the snow load on the portion of the roof that collapsed.  The structural designer failed to consider snow drift loads because their nature was not recognized by the industry at the time.  Had he done so, the roof would probably not have collapsed, although it would be overloaded. [b]