Abstract: A sliding window assembly includes two rectangular sash frames each enclosing an insulated glass panel unit and supported by a surrounding main support frame. At least one of the sash frames is supported for horizontal movement, and horizontally extending interlocking members are formed on the adjacent upper and lower horizontal frame members of the sash frame and main support frame. The interlocking members have interfitting horizontal hook portions positioned to permit each movable sash frame to move freely between a closed position and an open position. The interfitting hook portions are effective to engage and interlock each other in response to a substantial positive or negative windload or such a windload after an impact which breaks the glass panel unit, to limit deflection and prevent permanent deformation or release of the sash frame.