Depending on the weather, windows sometimes have to hold up under considerable stress. High winds from winter storms or hurricane gales may not be prevalent where you live, but you still want peace of mind that your bedroom window cannot blow in some night during a freak storm, bringing shards of glass and rain with it.
How is window strength determined?
Design pressure (DP) is a performance specification for how strong a window is, expressed in how many pounds per square foot (psf) of pressure the window can withstand. This spec may be targeted for performance in a particular geographic area–for example, one with strong winds–or for a particular type of building design.
Testing is done in a laboratory: a window is attached to a wooden frame and clamped to a specially-designed test board. Air pressure is gradually reduced on the inside of the window, which increases the pressure on the outside until the window fails – actually breaks apart – and the pressure needed to achieve failure becomes the window’s psf.