Wind Pressure

The specifications of the strength of tempered glass under wind pressure insure that the tempered glass will be four times stronger than the same thickness of annealed glass (ordinary glass).

Surface Compression

The strength of tempered glass is directly proportional to its surface compression. According to Federal Specification DD-G-1403B, fully tempered glass must have a surface compression of 10,000 psi (pounds per square inch) or more. This compares to annealed glass that has a rating of 3,500 psi, and heated glass that has ratings of 3,500 to 7,500 psi.

Edge Compression

Specifications for edge compression of tempered glass require that the glass resists breakage up to compression of 9,700 psi or more for a thickness of 6mm or .23 inches. This allows tempered glass to be used in spider and point fixed glazings. This component of tempered glass also allows it to resist thermal breakage which is due to the uneven heating of glass surface when sunlight hits one area of a window and not another. Tempered glass resists temperature differences of 200 to 300 degrees Fahrenheit that would crack annealed glass. This is about three times superior to regular glass.

Mechanical

Tempered glass has a typical breaking stress of 24,000 psi when using a large light 60-second load. This unit of measure means that the Typical Impact Velocity Causing the Fracture of tempered glass is done using a 1/4-inch light 5-gram or .176-ounce missile with normal impact to tempered glass surface at 60 feet per second, which is twice the speed of a similar missile that would crack annealed glass. This translates to impact strength five times higher than regular glass.

Tensile Strength

Tensile strength, or the greatest amount of longitudinal stress that a glass can bear before breaking, or the bending strength for tempered glass is 120 to 200 N/sq.mm or newtons per square millimeter. A newton is a metric unit of force where when applied for one second to a 1-kilogram mass at rest causes the mass to reach a speed of one meter per second. Tempered glass tensile strength is about four to five times higher than that for annealed glass.