The third of the ABC Ratings describes the window’s resistance to wind loading. The C rating is determined in accordance with the test procedure described in ASTM E330.
The window is subjected to very high wind pressures, and the amount of deflection (lateral bending of sash or frame elements) is measured. The deflection of any sash components cannot exceed 1/125th of the length of that component, and the deflection of a mullion cannot exceed 1/175th of the length of that element at a given applied pressure, for the window to receive the appropriate C rating. The window is then tested to “blowout” (that is, the window breaks, or the operable sash is dislodged from the frame). The C rating is also quoted for blowout.
A window must meet the C1 level to meet the minimum requirements of A440 (and, therefore, the Building Code requirements). The A440 Standard includes a User’s Guide, CSA A440.1, which gives some design assistance in selecting an appropriate C rating according to the location and building height. The specifics of the design process are best obtained by reviewing the A440.1 Standard: for tips on what to do with the design value once it is obtained, see the section entitled “Specifying ABC Ratings.”
Reviewed by Guiseppe Strazzeri