Performance in the Making

The basic process of manufacturing very high quality float glass begins with melting about 70 percent silica sand, 13 percent dolomite and limestone, 12 percent soda ash and small amounts of other materials. Often some percentage of the batch is in the form of cullet, or cleaned and crushed glass recovered from previous glassmaking, which lowers the melting temperature required. About 50 other elements are available to add in precise formulas to affect performance, depending on the application. The melted mixture produces a continuously rolling 12-foot wide glass ribbon. The molten glass flows from the furnace and "floats" over a bed of molten tin. It is then "annealed", a carefully controlled cooling process to minimize internal stresses and maximize potential mechanical resistance. For structural glazing systems, the glass is also "tempered", to make it four times stronger than annealed glass. (The term "heat strengthened" actually refers to a slightly different process. Heatstrengthened glass, while twice as strong as annealed, will break into large jagged fragments, unlike tempered glass, which breaks into small, much less dangerous fragments.)

Transparency and performance are both enhanced with an ionoplast interlayer in the glass roof of Yorkdale Mall, Yorkdale, Ontario, Canada. Architect: MMM International Architects Photo courtesy of W&W Glass, LLC

 

Tempering involves reheating the glass to the point where it starts to soften followed by rapid controlled cooling or "quenching." The outer surface of the glass cools faster than the inner layer. As the inner layer cools it contracts and compresses the outer layer, increasing the flexural strength of the glass by up to four times.

All fabrication of the glass is completed before tempering. All holes are drilled, and the other carefully controlled polishing, edging, notching and finishing processes completed, before tempering, because nothing can be altered afterwards.

Not all tempering is created equal. One measure is compressive strength. Typical furnaces average approximately 11,500 psi (pounds per square inch, a unit of pressure), and some of the most advanced modern furnaces achieve minimum compressive strength of 16,000 psi. The added strength can be especially important in structural glazing for the added safety at countersunk hole locations where maximum stress occurs.

Another measure is the occurrence of edge dip, roller wave and bow. These may sound like garage bands, but they are actually visual distortions that, although inherent to the tempering process, can be minimized by stringently controlled manufacturing processes. Edge dip and roller wave are caused when the ribbon of semi-molten glass sags even a small amount on the continuous casting roller during the tempering process. Bowing is caused when the two sides of the glass are cooled at even slightly different rates. These distortions are difficult to control for and are quite visible in the wavy trees and runny clouds reflected in many glass facades. The peak-to-valley "waves" in glass can be measured, and the published norm is 0.05".

However, it is possible to specify a minimum 0.0008" peak to valley wave, which virtually eliminates visual waves and results in high clarity and the perfect reflection of the surrounding environment.

The following are some of the most important additional analyses, tests and measurements that should be specified to ensure safety and performance in bolted structural glazing systems:

Heat soak. Many experts consider the heat soak to be one of the most important safety tests for tempered glass, to be specified and performed on all structural glazing systems before they are shipped. The heat soak process is a destructive test developed to find and eliminate the tiny, invisible impurity nickel sulfide, naturally present in the silica in float glass. Even a single inclusion of nickel sulfide can cause spontaneous breakage in a piece of glass.

In the most stringent heat soak tests, glass is exposed to a temperature of 290°C (+/-) for a period of 8 hours. In these tests 99.9 percent of the nickel sulfide impurities will be destroyed. Only the fully heat soaked glass that has survived this process should be specified for bolted structural glazing systems.

Strain gauge. Drilling the countersunk holes into glass creates areas of additional stress. These effects have to be factored into the system's overall performance parameters. The strain gauge is a mechanical test to assess a variety of loads applied to glass panels in horizontal or vertical orientations. Sensors translate pressure forces into measurable electrical resistance.

Uniform load test. Tempered laminated glass is loaded past the breaking point, and the laminate layer must support the weight of the broken glass.

Finite element analysis. a numerical analysis to define how a structure or material will react to loading conditions depending on the anticipated stress levels at various points and under various conditions. In the early 1970s finite element analysis was limited to the most expensive mainframe computers such as those belonging to aviation, defense and the nuclear industry. In our age, of course, increased computer power makes it possible for 3-D computer models to predict accurate results for all kinds of parameters and variables, such as mass, volume, temperature, strain energy, force, displacement and many others.

Tests for specific applications. Glass and assemblies destined for bolted structural glazing systems typically undergo many additional tests for specific conditions, including wind load resistance, hurricane performance, air and water penetration, seismic performance, impact resistance and bomb blast loading.

Layer Forward

One of the newest technological advances in glazing for structural systems is between the panes: a new interlayer for laminated glass. Laminated, or safety glass, was one of the key breakthroughs in glass technology for widespread use. Although it had been invented as early as 1910, the roots of the thin interlayer used in today's laminated glass go back to the development of windshields for automobiles. Clear polyvinyl butyral (PVB) was sandwiched between two panes of glass. If the glass broke, most of the glass fragments adhered to the plastic layer. PVB in various formulas is still widely used today.

Most laminated glass is used within frames, like your windshield, and safety glass is very good at keeping broken glass within the frame. But in bolted structural glazing systems, the framelessness is the point. Lamination, however, is still essential in large glazed areas, sometimes at considerable heights and in horizontal roofs or canopies. For these applications, PVB has limitations. It doesn't react well to moisture, so it can discolor or break down if edges are exposed. It also doesn't react well to the silicone sealants used for weatherproofing at the butt joints between panes in structural glazing. And there is a safety concern. With PVB, in a frameless panel, if both pieces of glass are broken, the glass will fall out.

The new ionoplast interlayers were originally developed for hurricane prone areas but are now increasingly specified for structural glazing systems. Not only is the substance not reactive with silicone or with moisture, but it also bonds better to the glass, is more rigid, and allows the glass to act as a composite monolithic unit. The interlayer's structural strength allows the use of much thinner, lighter panes, and is clearer even than clear PVBs. Sealing the deal is its innate structural strength and stiffness. If the glass breaks, the stiff interlayer tends to stay in place, making it much safer when considering post-breakage performance. The strength and stiffness also allows architects to design large, relatively thin glass panels with minimal support systems that can handle massive loads -whether impact from hurricane or blast, or snow and ice loads. Loads like that normally would require extremely thick laminated tempered glass.

The ionoplastic interlayers are being used in assemblies that look exceptionally transparent and light, particularly when used with low-iron ultra-clear glass, but are actually composites that solve multiple problems. For instance, in the Yorkdale Shopping Centre in Yorkdale, Ontario, Canada, each 7-foot by 4 -1/2 foot panel of glass in the huge barrel-vaulted glass roof consisted of fully tempered heat soaked clear insulated glass, a high-performance ceramic frit for solar control, an ionoplast interlayer for structural strength and argon gas in the air gap for insulation.