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Are Your Windows Missile Resistant??

By Keith M. Bouchard, P.E.

 

Hurricane season is just getting underway on the Atlantic coast.  That doesn’t exactly alter anyone’s vacation plans here in the northeast, but we are likely to get one or two threats per season to remind us that our shorelines are indeed prone to the mighty storms.  Eventually, a direct hit by a Category 2 or 3 hurricane will cause extensive damage to New England’s coastal infrastructure and our buildings will experience the type of damage typically reserved for the Gulf Coast or Carolinas.  To prepare for that day, it’s helpful to get familiar with some building code terminology that addresses the most vulnerable part of a building envelope – the windows.

Wind-Borne Debris Region:

According to the building code, a “wind-borne debris region” is defined as anywhere within one mile of a coastal high water line where the basic design wind speed is at least 110 mph or anywhere – regardless of the proximity to the shore – with a basic design wind speed of at least 120 mph.  In Massachusetts, all coastal towns have a design wind speed of at least 110 mph with the exception of the cities surrounding Boston Harbor.  All towns on Cape Cod and the Islands have a design wind speed of 120 mph. 

Impact Resistant Glazing:

If you’re constructing a new building or replacing the windows of an existing building within a wind-borne debris region, the Massachusetts State Building Code (MSBC) requires that the new windows have an impact-resistant covering or have impact-resistant glazing.  ASTM standards E1886 and E1996 define the levels of impact resistance for different wind zones.  The ASTM tests involve firing different “missiles” at the glazing (i.e. a 2×4 at 50 feet/second) and then subjecting the glazing to cyclical pressure differentials.  Windows above 60 feet from grade do not need to meet these criteria.

Partially Enclosed Building:

The purpose of the impact resistant glazing is not simply to prevent broken windows and leaks.  The larger concern is preventing internal pressurization of the building.  If enough windows on a building are shattered by flying debris during a hurricane, the openings in the building envelope create what the code classifies as a “partially enclosed building.”  The pressure differentials resulting from the gusting wind cause the internal pressurization of the building; however, the limited number of openings may not be large enough to accommodate the dynamic changes from the wind.  As a result, the pressure differential from the interior to the exterior of the building becomes too great and the remaining windows are blown out.  This could lead to the building being a complete loss.  Preventing this outcome is the main reason for the code requirements for impact resistant glazing.