Volume 47, Issue 6 - June 2012
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Defining Design Pressure
Design pressure is a complex issue, no doubt, but one important to glaziers and specifiers. This month, USGlass went to several experts on the subject to get their insight on this issue and how it differs from another crucial topic in hurricane and tornado glazing: wind speed.
“I’ve been through this discussion with so many clients over the years,” says Lucas Turner, president of Turner Engineering & Consulting Inc. in North Port, Fla. “There are so many factors that determine how wind affects a building.”
Despite confusion among some, Turner defines the two terms simply. “The wind speed is the American Society of Civil Engineers’ [ASCE] estimate for a given area—how fast is the wind going to be blowing there?” says Turner. “But the design pressure—the effect the wind has on a building as it blows over and around and possibly through the building is very different. Depending on the size of the building, where it’s located, the size of the opening—all of those things affect what kind of reaction that wind has on the structure.”
This is particularly important to a building’s glass and windows. “Designing for the expected windloads is crucial for the entire building, and especially the glass and windows, to protect people’s lives and their property,” says Turner. “ If windows fail during a storm, at a minimum it can lead to water damage, which means flooring repair, lost belongings and mold/mildew issues. Worst case is that window failure causes the building to pressurize, leading to roof failure or even complete structural collapse of the building.”
“In a high-wind event, a building that wasn’t designed per the building codes or didn’t have the proper rated products installed could result in broken glass, which creates an opening in the building,” says Urmilla Jokhu-Sowell, technical director for the Glass Association of North America (GANA). “Once there is an opening in the building, this allows for pressurization of the building and if that is severe enough there is a possibility of the roof blowing off causing major damage and destruction. It is very important that buildings are designed to withstand high-wind event pressures.”
Ken Brendan, technical services director for the American Architectural Manufacturers Association (AAMA), points to the North American Fenestration Standard/Specification for Windows, Doors, and Skylights, created by both AAMA and the Window and Door Manufacturers Association (WDMA), for a simple definition.
“NAFS defines … design pressure [as] a rating that identifies the load, induced by window and/or static snow, that a product is rated to withstand in its end-use application,” says Brendan. “Design windload [is] the window load pressure a product is required by the specifier to withstand in its end-use application.”
Additionally, ASCE/SEI 7-10, Minimum Design Loads of Buildings and Other Structures, is an often-referenced standard on this topic. “Windborne debris protection is required for exterior glazed openings within hurricane-prone regions identified with in the International Residential Code and the International Building Code (IBC) [according to the standard],” says Brendan.
However, some say recent discussions and the latest version of the standard, released in 2010, have made current requirements less clear. “Significant questions (and possibly confusion) exist concerning the use of terminology such as ultimate design load, allowable stress design load and design wind pressure,” says Jokhu-Sowell. “I witnessed this occurring at ASTM, ICC and other industry meetings. The questions stem from the recent changes within ASCE 7-10.”
What Does ASCE 7-10 Say?
“ASCE 7-10 provides wind speed charts for all of the U.S. including hurricane-prone areas like Florida, Texas and Louisiana,” says Jokhu-Sowell. “Based on the ASCE 7-10 wind speeds, states are able to adopt the standards and codes for impact resistant products at a stringency aligned with the needs of hurricane-prone areas.”
Many standards reference the term “allowable stress design” but ASCE 7-10 addresses “ultimate design.”
“ASCE 7-10, as referenced in the I-codes, uses ‘ultimate design load’ while ASTM E1300, ASTM E1886/1996, existing testing data and product approvals references ‘allowable stress design load,’” says Jokhu-Sowell. “The latest 2012 ICC codes reference ASCE 7-10 and thus reference ‘ultimate design load.’ ASCE 7 and ICC employs a conversion factor between ultimate and allowable loads, thus allowing a user of both the standards and the codes to speak the same language.”
“Load resistance factor design is the same as ultimate design,” adds Turner. “It applies factors of safety in a different way … It’s basically an engineering pathway, if you will, determine how a structure will perform under various types of loads such as wind, seismic, dead loads, etc., using multiple factors of safety.”
And how does that ultimately affect a project’s design pressure? “With the ultimate design you end up with a much higher design pressure in these combinations of loads,” says Turner.
Allowable stress, however, “is a different engineering path where you’re basically calculating what you expect the stress to be on all those different parts of the building, and comparing that to the allowable stresses using a single factor of safety,” according to Turner.
It’s crucial to focus on allowable stress when viewing any type of test report for product approval. “It’s apples and oranges to try and compare [ultimate design] to a test report,” he says. “It is critical to only use allowable stress design pressures when comparing to test reports, product approvals, or ASTM E 1300 calculations.”
On a related note, certain regions no longer are required to have impact protection. “In some coastal regions, like Virginia and north on the Eastern Seaboard, the requirement for impact protection for normal buildings is going away,” says Turner. “It’s not clear yet whether those jurisdictions are going to say, ‘wait, we didn’t want it anyway,’ or whether they will adopt local impact requirements to maintain stringency of the code.”
Despite the changes, Turner says the latest version of ASCE 7 certainly is a user-friendly one. “As far as usability of the standards, I think they definitely made some enhancements this time around,” he says. “I think they clearly broke it out into the different paths you can follow.”
Aside from the recent changes, though, Turner offers a word of advice for those utilizing the standard. “Don’t necessarily rely strictly on the ASCE 7 map,” he says. “You definitely need to check with the local building authority for their interpretation since local landmarks such as roads are often used to set the exact wind speed lines.”
What’s in the Future?
And what a difference the codes have made. “The hurricane protection codes and standards started with Hurricane Andrew and the subsequent destruction in Homestead, Fla.,” says Jokhu-Sowell. “Because of that destruction, industry worked on windborne debris ASTM standards. Because of the standards that have been put in place and the use of impact-resistant products, we are not seeing the destruction that we did before in those areas.”
Future adaptations to ASCE 7-10 also will play a role, according to Brendan. “Much will depend on the evolving basic wind speed maps and whether or not they continue to increase as they have been,” he says.
Turner says the standard is continually reviewed but there’s no set schedule for updates.
“It’s a live committee, so they’re actively working on the next stages of the standard now,” he adds.
Design Pressure Discussed at Recent ICC
S175 - 12, submitted by the Glazing Industry Code Committee (GICC), and S339-12 by the American Architectural Manufacturers Association (AAMA), both addressed this topic.
According to the proposals, which concerned exterior doors
and windows, “products in Risk Category I and II buildings tested and
labeled as conforming to AAMA/WDMA/CSA 101/I.S.2/A440 shall not be subject
to the requirements of Sections 2403.2 and 2403.3 provided one of the
following is met:
The GICC proposal pointed out that both Chapter 24 and ASTM E1300 require glazing to be firmly supported to prevent breakage under the design load by establishing maximum framing deflection limits. “The glass strength calculations in ASTM E1300 use this as a basis to establish a probability of glass breakage less than 8 in 1000. However, Section 1710.5.1 currently exempts certain residential and light commercial products from this requirement if they are labeled to the AAMA/WDMA/CSA 101/I.S.2/A440 standard,” read the proposal. “While this may be appropriate when these products are used in applications with lower design loads and/or lower risk building types, allowing this exception for all product types in all occupancies is far too broad.”
As the proposal stated, it sought to “correct this overbreadth by ensuring that products used in higher risk situations be firmly supported and meet the frame deflection limit to restore an appropriate safety margin consistent with ASTM E1300.” Specifically, it proposed limiting the exception to risk category I and II buildings, and added that products used in higher risk category buildings would meet the Chapter 24 requirement for firmly supported glazing. It also maintained the exception for lower design pressures less than 60 psf, and also where tempered or laminated glass is used as an alternative method.
Ultimately, both proposals were disapproved.