By Mark Robins, Senior Editor ASCE/SEI 7-10 revises the calculation mechan-
ics used to incorporate building design consider- ations and convert wind speed into appropriate load requirements for fenestration (windows, door and skylights) based on the wind speeds shown in the updated maps. Furthermore, ASCE/SEI 7-10 calculations now provide design wind pressure val- ues based on strength design/load and resistance factor design in place of previously used allowable stress design. ASCE/SEI 7-10 allows for conversion from strength design to allowable stress design by applying a factor of 0.6. This conversion is important in correlating the correct design load to fenestration product ratings. Because of the way these revisions relate to
each other, editions of the ASCE/SEI 7-10 standards cannot be intermixed. Doing so could result in excessively high or inappropriately low-load predic- tions for windows, exterior doors and skylights. However, except for few locations where variances created by the new wind maps produce signifi cantly different wind pressures, the products that would be specifi ed by correctly using either version of the ASCS/SEI 7-10 standard are essentially the same. Ken Brenden, technical director at American
Architectural Manufacturers Association (AAMA), Schaumburg, Ill., says AAMA has jointly sponsored a technical bulletin that adds clarifi cation to how the design loads from ASCE/SEI 7-10 relate to ex- terior fenestration product ratings and performance grades. The technical bulletin is available as a free download and can be accessed through a link at
www.aamanet.org.
Florida is different The Florida Building Code has minimum require- ments to ensure buildings in high-wind hurricane areas can withstand the impact of wind-borne debris. In addition to those wind zones prescribed by ASCE/SEI 7-10, Florida has specially designated wind zones and they are defi ned as high velocity hurricane zones (HVHZs). Florida’s most well- known HVHZ is Broward and Dade County. In these highly vulnerable counties, stricter design and con- struction measures have been adopted in addition to those provided by ASCE/SEI 7-10 most notably, the requirement to protect windows, walls and roofs from wind-born debris. Mo Madani, technical manager for the building
codes and standards offi ce, Florida Department of Business and Professional Regulation, Tallahas- see, Fla., says: “ASCE 7/SEI-10 is the state-of-the national wind protection standard we use as the minimum design loads for buildings and structures for the state of Florida, which encompasses all the loads the building is required to be designed for. In terms of wind loads, the new maps, when used in combination with the 1.0 load factor on wind for
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This was the Turner Agri-Civic Center in Arcadia, Fla., a two-year-old, 3,600-square-foot steel frame building, designed for 140 mph winds that was supposed to serve as a shelter for 1,400 people. The building was a twisted ruin that collapsed in estimated 100- to 110-mph Hurricane Charley winds. The windward corner of the roof separated fi rst and the building partially collapsed progressively along its length. (Photo courtesy of Englert Inc.)
strength design and the 0.6 factor on wind for allow- able stress design, result in net decrease or approxi- mately the same design wind loads when compared to previous editions of the maps/standard ASCE 7-05. We don’t see that much impact compared to previous editions of the standards with regard to the wind loads. The contour wind speed lines for Florida have changed because of improved science like computer simulations and the change in wind speed calculation philosophy. “Per the 2010 Florida Building Code (the Code),
the standard that governs metal buildings is spelled out in Chapter 22, Steel. This chapter of the Code governs the quality, design, fabrication and erection of steel used structurally in buildings or structures. Chapter 22 of the Florida Building Code provides design standards/specifi cation nationally used for designing metal buildings like AISC 360, ASCE 8 and AISI standards. After you defi ne the loads, based on ASCE 7, you have to take the loads and design the structures, and you design the structure using the applicable reference design standard(s)/manual(s) as specifi ed in Chapter 22 of the Code.”
What to do What is the best way to fi nd out and interpret what the most relevant and current metal building requirements and local codes for high-winds are? Kincy believes a strong working relationship with local authorities in advance can reduce the risk of delays and extra expense. Also, “the contractor should seek the assis-
tance of a local design professional to ensure the project is designed to withstand the anticipated wind loads,” says Andy Williams, director of codes
and standards at MCA. “The Building Codes As- sistance Project (BCAP) is a good resource for contractors to learn which version of the various model codes is enforced in local and state jurisdic- tions around the country.” More information can also be found at
www.bcap-energy.com. Associations can assist with information on
how to comply with high-wind requirements. Cleveland-based Metal Builders Manufacturers Association (MBMA) has been working to better educate the industry. “The metal building manufacturer will take
care of designing the building for the appropriate wind loads,” says MBMA chairman Fred Koetting. “It is important that the contractor follows the installation procedures established by the metal building manufacturer. In addition, the contractor has the important responsibility of making sure the components that are not supplied by the metal building manufacturer are selected, designed and rated to the proper wind load. This can include large vehicular access doors, man doors and win- dows. These are important parts of the envelope, and if one or more are breeched, the building can be subjected to internal pressures for which it was not designed. Therefore, a clear understanding of the required rating is necessary, and the change in the wind speed maps might confuse this important decision. If there are any questions at all, consult the metal building manufacturer.” MBMA’s 2012 Metal Building Systems Manual
has step-by-step examples for calculating wind, snow, and seismic design loads and requirements for metal buildings per the IBC, and it references the ASCE standard.
December 2012 METAL CONSTRUCTION NEWS 39
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