Volume 41, Issue 7 - July 2006

theFarnadyFiles

Getting Loaded
Conscious Awareness of Glazing Loads
by Dez Farnady

In the run-of-the-mill glass application, load issues seem to be either taken for granted or forgotten all together. In residential applications itís usually both. The monumental, the high-end and the exotic glass jobs that make the cover of this magazine and architectural and metal magazines are engineered to withstand Katrina forces or worse. In residential applications, the glass frequently will have trouble when a door is slammed. Windloads are an essential on commercial and storefront and curtainwall jobs, particularly on mid- and high-rise buildings. Commercial specification requires point loads and windloads on glazing systems but, even on some commercial jobs, designers often fail to deal with the loading on the glass. We do skylight business in the high Sierra, where snowfalls at altitudes of more than 10,000 feet can drive dead loads up to nearly 300 pounds per square foot. As a result, we are more conscious of loading on the glazing system and on the glass than those who do primarily vertical glazing or those who are down in the flatlands. 

Glass and Windloads

One major issue in the residential area has to do with the windload on insulating glass. This is not necessarily a flatland matter. Wind can be a major contributor to insulating unit failure. IG failure happens often when windows are not designed for extreme wind and weather conditions and actually move under wind pressures. 

A large picture window, for example, made up with 1/8-inch insulating glass, may be perfectly fine in the suburbs and will not necessarily break under excessive windload, but may deflect under the pressure of 30- or 40-mile-an-hour wind gusts. The gusting wind will cause a continuous pumping action that eventually will break down the perimeter seal contributing to premature insulating unit failure. I have seen units replaced in the same window three or four times before someone figured out that they better change the glass make-up from 1/8- over 1/8-inch to heavier glass to withstand the pressure. 

Higher Up

Vertical glass often has problems in heavy glass storefronts where butt-glazed applications and glass doors are used to dress up high-end retail entrances. The problem is that while the half-inch tempered glass usually is strong enough to meet the load requirements, it frequently exceeds the allowable deflection criteria. This is particularly important in applications without vertical structural support since silicone joints contribute no support to prevent deflection. Similar problems apply to skylights; while the glass may meet the load requirements from a strength standpoint, the dead load and the weight of the glass can cause deflection that may cause the laminated glass to crack.

Tempering increases glass strength four-fold and makes it easy in most applications to meet strength requirements, but it will often fail in deflection because laminating and tempering, while increasing glass strength, will not help reduce deflection. Single-glazed, half-inch glass in a butt-glazed application may load safely when the analysis indicates deflection enough to pull the glass clear out of the glazing pocket under the specified load. Half- to three-quarter-inch coverage, top and bottom, with four inches of deflection on floor to ceiling glass makes for bad math. 

Load requirements are specific as determined by location and elevation. Deflection criteria, however, are seldom specified for glass. The industry standard (and donít ask me who set it) limits deflection maximum to three quarters of an inch. The enforcers of the deflection limit seem to be the manufacturers who are protecting themselves from liability exposure. And I donít blame them. Glass strength and deflection are not the same, and glass is a funny materialówhile it mostly does what we want it to do, sometimes it doesnít.

USG
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