THERMAL STRESSES

Thermally induced edge stresses are usually the result of the warmer center portion of a glass lite exposed to solar energy wanting to expand more than the cooler edges. The rate at which the glass absorbs energy is dependent on the type, size, thickness, and shape and how it is isolated from the framing system. other factors are building orientation, interior shading devices, exterior shading patterns, heating register location, etc. Tempering or heat strengthening increases glass edge strength and decreases the chances for thermal breakage. The following conditions must be taken into account when considering the effects of thermal stresses.

INTERIOR HEAT TRAPS

These situations occur when there is inadequate air circulation to properly remove heat from behind the glass. Spandrel areas are a good example of glass that must be tempered or heat strengthened to offset the rise in temperature. In vision areas, air movement must not be restricted. Suspended ceilings must be well to the room side to allow natural convection. Or the head area should include vents that provide a minimum of one square inch of ventilation for each inch of glass width. Or the glass should be heat strengthened or tempered.

INTERIOR SHADING

Draperies, venetian blinds or other interior shading devices must be hung with space to permit natural air movement over the room side of the glass. The following criteria must be met to avoid formation of a heat trap:

1. Minimum 1 1/2" clearance required top and bottom or one side and bottom between shading device and surrounding construction.
2. Minimum 2" clearance between glass and shading device.
3. Heat/cooling outlets must be to room side of the shading device with airflow directed away from the glass.
4. Use mechanical stops to prevent complete closure of blinds to 60% of closed position.

Heat strengthening or tempering of the glass may be necessary to offset the effects of a lack of adequate ventilation.

EXTERIOR SHADING

Shadows cast by overhangs, surrounding structures, trees and shrubbery can create shading patterns on the glass creating thermal edge stress. Maximum stress occurs when 25% or less area of a lite is shaded and the shade includes more than 25% of the perimeter. Generally, horizontal, vertical, and diagonal shading patterns are not as critical as shading that combines several patterns. Double diagonal shading is generally the most critical pattern. See the sketches of typical shading patterns that are labeled "Acceptable", "Marginal", and "Harmful". It is recommended that HGP Technical Services review designs to determine whether heat treating is required.