American Society of Civil Engineers 2010 Wind-Load Design Standard is Under Fire
The American Society of Civil Engineers has decided that emergency changes to wind-load provisions in ASCE's 2010 building design standard are not needed. ASCE recently reviewed the provisions, prompted by a red flag raised by structural engineer-researcher Emil Simiu, who says the wind standard is flawed and needlessly complex.
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| SIMIU |
To aid in its decision, ASCE canvassed structural and wind experts regarding Simiu's concerns. “We are of the collective opinion that none of the issues warrant immediate action,” says Ronald A. Cook, the ASCE/SEI 7-10 wind subcommittee chair and a professor of civil engineering at the University of Florida, Gainesville.
Simiu is not the only one who takes issue with the 130-page wind section, which ASCE describes as an easier-to-use version of its 60-page, 2005 predecessor. Wind provisions are “way too complicated for practicing engineers,” says Clifford W. Schwinger, a vice president of structural consultant Harman Group Inc., King of Prussia, Pa. “I argue that, in some respects, the public may be less safe with these ever-increasingly complex codes.”
Timothy A. Reinhold, a subcommittee member and chief engineer for the Insurance Institute for Business & Home Safety, Tampa, says the standard is longer, in part, because it contains first-time guidance on wind loads for special conditions, such as rooftop solar panels. “For fairly simple buildings, there are about six [relevant] pages,” says Reinhold. But it may take users “some time, at first, to figure out which six pages to use,” he adds.
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| COOK |
Specifically, Simiu objects to insufficiently differentiated wind-speed maps for non-hurricane zones and to wind-pressure coefficients that are too low for some building types. Underestimated wind speeds or coefficients can result in unconservative design wind effects, he says.
James R. Harris, founder of J.R. Harris & Co., Denver, and chair of the ASCE/SEI 7-10 main committee, “strongly disagrees” with several of Simiu's concerns, including the wind-speed maps. “The spatial averaging procedure is valid,” Harris says. “It is a useful tool for removing the effects of some uncontrollable natural and [human] errors in data sampling.”
But Harris concedes that some provisions are “less clear, less user-friendly and more complex than would be desirable, in spite of our collective best efforts.”
Simiu also takes issue with new, simpler design methods. The methods can yield a 30% difference in design results for the same structure, he says.
In a trade-off, the simpler methods were deliberately developed to render more conservative results than the more complex procedures, says Ronald O. Hamburger, a senior principal in the San Francisco office of structural consultant Simpson Gumpertz & Heger and chairman of the ASCE/SEI 7-16 main committee. “The different answers are not an indication of a lack of safety,” he says.
Simiu also is concerned about inconsistencies in the standard's specification of wind pressures on rooftop equipment. “There is a large difference between design pressures” for roofs at 59 ft and 61 ft, when there isn't in reality, he says.
The engineer further objects to the provisions for wind-tunnel test procedures. “[They] lack sufficient specificity and can lead to different estimates of design wind effects,” says Simiu.
Not only that, he is concerned with an “unprecedented profusion of tables [when] convenient, user-friendly computational tools are universally available.” The tables may be unnecessary and may lead to errors in interpretation, he says.
During development of the 2010 edition, there were no actionable proposals that would have allowed the committee to adopt new or alter existing provisions in the areas addressed by Simiu, says Reinhold. This might have been because the standard was rushed to meet a deadline for inclusion as a reference standard in the 2012 version of the model International Building Code. Without the rush, “valuable changes” would not have been adopted until the next IBC, Reinhold says. A bigger problem is a “woeful” lack of federal funding for wind research to support change, he adds.
Simiu and four NIST colleagues have co-authored a critique on the wind-loads design standard. The paper, currently in draft form, will be posted for public comment, likely by Oct. 7, on www.nist.gov. Once finalized, it will become an official NIST document, says Simiu.
For the 2016 cycle, the engineer plans to submit a change proposal on the maps and possibly on reducing the number of design methods. Work should begin on the 2016 standard “immediately” to avoid a rush at the end, says Simiu.
Hamburger says time is the true test of ASCE/SEI 7: “Structures designed in accordance with the standard have generally performed well. That is the best way to judge whether it is adequate.”
