Volume 36, Issue 6,  June 2001

Deflection Debate

by Tara Taffera

“We probably have as many opinions concerning what we should do about deflection as we have members.” 
That was a statement made by Carl Wagus, technical director for the American Architectural Manufacturers Association (AAMA). With approximately 270 manufacturing members and 130 professional members, that’s a lot of opinions. It’s no wonder why the issue of deflection is causing such a stir in the industry. 

Why the Debate?
Before delving into the multitude of issues involved in this debate, let’s take a look at deflection standards. According to ANSI/AAMA/NWWDA 101/I.S.2, for the architectural and heavy commercial hung categories no member shall deflect more than 1/175 of its span, which is commonly known as L/175. However, AAMA’s remaining window categories—residential, light commercial, commercial and heavy commercial (except hung windows)—do not have any limits regarding deflection. This is where the debate begins. 

Bill Deuschle serves as director of engineering quality/operations for TRACO and is also chairman of the AAMA Deflection Limits Task Group. According to Deuschle, commercial window manufacturers had requested that L/175 be included in the new North American Fenestration Standard (NAFS). “When we took this to AAMA they said we can’t do this,” said Deuschle. “They said we need documentation.”

So Deuschle set out to find the necessary information. He referenced the American Society for Testing Materials (ASTM) E 1300 disclaimer which references L/175 and contacted the American Institute of Architects. Both organizations offered support for L/175 or referenced the deflection requirement. Deuschle then approached the primary glass manufacturers, each of whom wrote a letter to Deuschle saying they support a deflection limit of L/175 for commercial categories. “AAMA came back and said this is not conclusive,” said Deuschle. “They needed documentation as to why L/175 is the number to follow.”

Why L/175?
Although L/175 had been around the architectural community for years, some in the industry began to question where this number originated. According to Wagus, current deflection standards are based on how it’s been done in the past. “It works over here [with these conditions], but it doesn’t work over there. There’s a whole gray area to explore,” he said. Rather than go with arbitrary data we want to develop data based on a variety of performance needs and materials requirements.” 

Wagus says materials need to be tested in a variety of situations. For example, AAMA wanted to study different types of glass, such as single, annealed and tempered, used in different products, such as hung and casement windows, with different framing materials. They also wanted to consider whether or not the material is supported on all sides, as well as subjecting the materials to different stresses. 

“Rather than look at this from a marketing or emotional perspective, we decided to do some research,” said Wagus. So AAMA commissioned a research project with the goal of developing technical data on this issue using mathematical analysis. Bill Lingnell of Lingnell Consultants is heading up the research efforts. “We are looking at 120 glass configurations and load conditions,” said Wagus. 

According to Wagus, glass differs from materials, such as steel, which offers straight-forward results. “Glass is different because it behaves in a statistical manner,” said Wagus. “Until recently, the glass companies couldn’t agree on acceptable glass stress loads. Each had its own chart … but they finally were able to come together.” The result was the development of the ASTM E 1300 standard in 1989. Although this standard was developed, Wagus says it is based on one support condition. “If you choose a different support condition, you will get a whole new series of results.”

This is why AAMA commissioned a mathematical analysis using a variety of factors, such as size, load and glass edge support conditions to gather additional data. 

A Look at ASTM
According to Wagus, ASTM E 1300 has limitations for evaluating window and door performance because it takes only the glass into consideration. “We want to know how the glass works with the framing materials,” he said. “Determination of glass strength requires a statistical analysis and can’t rely on a single glass test.”

The deflection standards that exist currently are for single lites of glass. “ASTM E 1300 looks at each lite of glass but doesn’t put it together,” said Wagus. “The testing doesn’t pertain to the frames.” Wagus said a variety of questions still need to be answered such as, “How do you evaluate the glass strength in framing assemblies for conditions other than those outlined in E 1300?” 

As said earlier, only the architectural and heavy commercial categories have to meet deflection limits (L/175). “For the remaining classifications, there is no recommendation,” said Wagus. “So, architects are beginning to come to us and ask if deflection limits would be appropriate for the other window categories.” 

A Look at ANSI/AAMA/NWWDA 101/I.S.2
According to Wagus, the AAMA/WDMA Guide Specification really became popular in 1997 when AAMA harmonized all specifications in the country. “The architects loved it,” said Wagus. They thought it should be in the codes.”

Although all AAMA standards are voluntary, the various code organizations have taken note of what AAMA was doing. The AAMA/WDMA specification 101/I.S.2 was incorporated into the International Building and Residential Codes. According to Wagus, this is when the deflection debate started to intensify. “This brought to the forefront an issue that’s been going on for decades.”

AAMA Research
“We determined as an association that we must have a sound technical basis for any standard we develop,” said Wagus. “So, we put the resources of the AAMA Research Foundation on the line for this research project.”

In fact, that research was the topic of discussion at AAMA’s summer meeting held June 10-13 in Denver. A Glass and Frame Deflection Limits Symposium was held in which experts discussed this controversial topic. A summary of preliminary findings were also presented and discussed by the panelists. The panel consisted of Lingnell, who discussed the preliminary research; Scott Norville of Texas Tech University, who discussed the current deflection standard and its limitations; and Brad Boone of PPG, who represented the view of the glass suppliers. (At press time, this event had not yet occurred. Look to the August issue of USGlass for a detailed report of that meeting). 

The panel was not the only forum in which deflection was discussed. There were also three other sessions devoted to deflection. 

“You’ll also hear this topic discussed in the halls,” Wagus said. “This issue is not only topical but can be 

Controversial indeed. While some in the industry may spend time arguing over this issue, Wagus says the debate will not be resolved in this way. “Putting people in a room to see who shouts the loudest is not the best way to solve this debate,” he said. “For a potential critical revision to our standards, we decided to start with the existing science and improve upon it with additional research.”

Wind Loads
According to Wagus, another component of the deflection debate has to do with the method of construction, including how wind loads are calculated and distributed. “The specifier must be allowed flexibility while at the same time be provided with enough direction to safely and responsibly address project considerations and requirements. There is a lack of firm guidance which addresses all facets of this issue,” he said. 

AAMA is commissioning research in this area as well. Jim Krahn of Marvin Windows and Doors and chairperson of the performance, testing and certification committee for the Window and Door Manufacturers Association, says he believes current testing pressures are more than adequate. “We structurally test at 50 percent over design pressure,” Krahn said. “Where the deflection measurement is currently taken, we test beyond design as a safety factor to make sure the glass and other components don’t break. I recommend we take a deflection reading at a lower pressure.”

As do many in the industry, Krahn questions whether L/175 at new higher design pressures are appropriate, thus he said he supports the AAMA testing. “A rule was established 40 years ago when pressures were low and carried forward,” he said. “This needs total review to make sure it still applies.

“Wind speed map velocity pressures and resulting design pressures at which deflection is taken have changed,” he added. “Products that once passed deflection testing and performance standards now can’t meet the criteria.”
Krahn said there is one important question that needs to be asked. “Is the pressure that deflection is taken at the appropriate pressure? At this point I don’t believe it is,” he said.

So, what is Krahn’s solution to the problem? “Leave L/175 but test it at an appropriate pressure,” he said. “If L/175 is to be used it should be at a reduced pressure that is more representative of real-life situations.”
“I would prefer that there be some limitations that would rate the structural deflection strength of the products,” Krahn said, “but not to the extreme that is proposed.” 

Deuschle’s proposal may be one of those extremes to which Krahn is referring. According to Deuschle, TRACO wants to see L/175 be the standard for commercial, heavy- commercial and architectural structures. So, why exclude the residential and light commercial categories? “Do I think it [L/175] should be applied across the board?

Absolutely,” Deuschle said. “But I’ll settle for commercial on up because this has the largest liability.” 

Krahn did say he is not opposed to an across-the board-approach to L/175. “I would support that as long as it was at a reduced pressure,” he said. 

Materials Debate
Although some people don’t want to talk about it and some tiptoe around the issue, many are concerned that if the lack of standards continue to exist, tragedy may be the end result. For example, Deuschle believes that materials not suited for particular situations, such as vinyl (for example, in multi-story structures), will still be used and accidents will occur. 

“We make vinyl windows and we make aluminum windows,” Deuschle said. “We know that if vinyl is used in various situations it is very dangerous.” 

Who is liable? “God forbid there is breakage,” he said. “The glass manufacturers will say it was the vinyl manufacturers. The vinyl manufacturers will say it was the glass manufacturers.”

Deuschle said some in the industry may respond by saying, “Deflection really doesn’t matter, nothing has happened yet.” He responds to this argument with an analogy. “If you were going over a bridge that deflects 8 to 10 feet, but doesn’t collapse, how safe would you feel?” Deuschle asked. 

The aluminum industry weighs in on the deflection debate as well. In the summer issue of USGlass’ sister publication, Door & Window Maker, Rand Baldwin, president of the Aluminum Extruders Council, wrote about deflection. He warned, “There are very few framing materials that can withstand the performance requirements in buildings of more than three stories for large openings in building envelopes.”

“Only the architectural and heavy-commercial categories have deflection limit testing and design load testing. The designation of commercial in this instance does not signify long-term structural performance. The requirements in this window class are not stringent enough to ensure that the fenestration product withstand the expansion and contraction and the resultant air infiltration that is essential in a multi-story structure,” he said. Baldwin added later, “There needs to be deflection standards.”

Where Do We Go From Here?

So, is the industry close to such a standard? According to Wagus, once AAMA completes the research and comes to a conclusion based on a scientific approach, it will put this in a guide specification. “The codes can then adopt this if they wish,” Wagus said.

When will this conclusion be reached? It might not be anytime soon. “This [preliminary research] is really just an engineering paper,” Wagus said. There won’t be any conclusions presented because we didn’t ask for them. More research and discussion will be required.” 

So, while AAMA says it is waiting to see what the research says before it comes to any conclusions, others in the industry are left to speculate on what that end result might be. 

“At the present time, they [AAMA] don’t think the commercial category should be required to have a deflection limit of L/175,” Deuschle said. 

“I don’t see consensus [a solution] happening that fast,” Krahn said. 

It doesn’t look as if there is a cut-and-dry solution. “There are many solutions to the problem,” Wagus said. “It’s not a matter of pass/fail. It’s about how we technically assess glass strength for use in window and door assemblies.” 

Only time will tell how, if ever, this issue gets resolved. Look to the August issue of USGlass for complete coverage of the AAMA deflection panel and the latest news on this ongoing debate. 

Tara Taffera is the editor of USGlass magazine.


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