Don’t be blown away by metal building requirements for high-wind areas
The Joplin, Mo., EF5 multiple-vortex tornado and hurricanes Sandy, Ike, Katrina, Charley and Andrew have heightened the focus on wind and its effect on metal buildings. Destructive winds can tear roofs from buildings, strip metal panels from exterior walls and rip buildings off foundations turning them into piles of rubble. Of course, the greater concern is the safety of the occupants, and an intact building protects people.
Structures built to meet or exceed current model building requirements for high-wind regions have a much better chance of building and occupant survival during violent windstorms. Some wind speeds are so great that they are beyond the scope of building codes and engineering standards. While no building can survive a head-on collision with an EF5 twister, contractors and installers knowing and following wind requirements can lessen the chance for disaster. Adhering to wind requirements should provide sufficient resistance to destructive winds provided the metal building is located on the outer edge of the tornado vortex.
Studying flow and pressure
Designing metal buildings to withstand destructive wind is an important aspect of engineering. Wind requirements study the air flow and pressure fields around buildings. Not only will wind requirements ensure a building will resist high winds, they also maintain pleasant conditions in outdoor spaces, assess natural ventilation potential and verify that any exhaust fumes are dispersed adequately. During the 1970s and 1980s many research studies were conducted using wind tunnels to measure the forces of the wind on various types of structures. Modern wind design requirements started with ANSI A58.1972 published by American National Standards Institute (ANSI) in 1972.
This document delineated wind load criteria based on probabilistically determined wind speed and tabulated forms of design load parameters. Since then, the wind load criteria have gone through major changes as revisions of the standard were made in ANSI A58.1-1982, and Reston, Va.- based American Society of Civil Engineeers’ ASCE 7-88, ASCE 7-93, ASCE 7-95, ASCE 7-98, ASCE 7-02 and ASCE 7-05. Major changes occurred in the wind load criteria in ANSI A 58.1-1982 and in ASCE 7-95. The most significant change is the reference to wind speed, which changed from fastest-mile to a 3-second gust. Each revision made changes and adjustments in several different factors including the importance factor, terrain factor, directionality factor, gust effect factor and the pressure/force coefficients.
ASCE/SEI 7-10
ASCE/SEI 7-10 “Minimum Design Loads for Buildings and Other Structures” is a complete revision of ASCE Standard 7-05. ASCE/SEI 7-10 completely updates and reorganizes wind load provisions, expanding them from one chapter into six to make them more understandable and easier to follow. It is the principal reference for determining maximum wind speeds likely to be experienced by metal buildings in the United States. It provides the methodology for determining design wind pressures and forces, the design wind speeds, exposure categories and requirements for wind-borne debris protection. “All of the model building codes like the International Building Code (IBC) reference this standard,” says Brad Fletcher, structural engineer, Atlas Tube, Chicago. “Any structure designed under those model building codes are affected by the provisions of ASCE.”
ASCE/SEI 7-10 provides new ultimate event wind maps with corresponding reductions in load factors, so that the loads are not affected. It updates the seismic loads of ASCE 7-05, offering new risk-targeted seismic maps. Also, the snow load, live load and atmospheric icing provisions of ASCE 7-05 are all updated. “ASCE 7/SEI 7-10 is a primary resource document for building design and is referenced in the structural design chapters of the IBC to provide design information for all wind load areas,” says Scott Kriner, president of Green Metal Consulting Inc., Macungie, Pa., and technical director of the Metal Construction Association (MCA), Glenview, Ill. “Engineering information and guidance provided in ASCE/SEI 7-10 are key to the successful design and performance of any wall structure. MCA is not involved with specific project calculations; however we do reference the IBC for all guidance, and comply with the requirements of the code which govern our members’ metal wall and roofing applications in high-wind areas.”
In addition to wind, ASCE/SEI 7-10 provides requirements for general structural design and includes means for determining dead, live, soil, flood, snow, rain, atmospheric ice and earthquake loads, and their combinations that are suitable for inclusion in building codes and other documents.
Revisions and maps
ASCE/SEI 7-10 revises the method used for establishing basic wind speed, resulting in three different wind speed maps of the U.S. The multiple map approach eliminates inconsistencies between different locations, and between hurricanes and non-hurricane regions.
Wind speed maps provide valuable information for the engineering design of the exterior envelope and the support structure. “Wind speed maps establish two requirements for the building,” says Rick Kincy, director of sales at Dominion Building Products, Houston. “The first way is to establish debris impact requirements. If the wind speed is above the code threshold for impact, then impact resistance is required. The second way the winds speed map is used is to use the wind speed as one of the factors to establish the required design pressure.”
ASCE/SEI 7-10 revises the calculation mechanics used to incorporate building design considerations 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 values 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 predictions for windows, exterior doors and skylights. However, except for few locations where variances created by the new wind maps produce significantly different wind pressures, the products that would be specified 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 clarification to how the design loads from ASCE/SEI 7-10 relate to exterior 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 requirements 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 defined 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 construction 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 office, Florida Department of Business and Professional Regulation, Tallahassee, 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 strength design and the 0.6 factor on wind for allowable stress design, result in net decrease or approximately 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/specification nationally used for designing metal buildings like AISC 360, ASCE 8 and AISI standards. After you define 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 specified in Chapter 22 of the Code.”
What to do
What is the best way to find 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 assistance 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 Assistance Project (BCAP) is a good resource for contractors to learn which version of the various model codes is enforced in local and state jurisdictions 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 windows. 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.
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Roofing wind resistance tests
It was standardized testing measuring wind resistance of various roofing systems that went into current metal building requirements and codes. The uplift pressure on roofing depends on many factors, including wind velocity, structure location, roof slope, roof shape, roof height and others.
The UL 580 standard test for uplift resistance of roof assemblies is appropriate when the roofing product is a structural panel installed over open framing without the need for a solid deck. This test incorporates both pressure beneath the system and a vacuum above in an oscillating manner according to a specific test protocol.
The UL 1897 standard for uplift test for roof covering systems evaluates the attachment of the roof covering systems to the roof deck. It is conducted by either pulling a vacuum above the assembly or by pressurizing an air bag placed loosely between the deck and the roof covering.
The ASTM E 1592 standard test method for structural performance of sheet metal roof and siding systems by uniform static air pressure difference measures the bending capacity and attachment strength when a system is subjected to a uniform static pressure. Air pressure is applied beneath roofing panels and attachments in a laboratory controlled test chamber until failure at varying purlin spacing occurs. For standing seam and through-fastened metal panel systems, the IBC requires test methods UL 580 and ASTM E 1592.
Factory Mutual FM 4471, “Approval Standard for Class 1 Panel Roofs,” provides criteria for evaluating panel-type roof systems, including metal roof systems. Specific criteria evaluated include not only wind-uplift, but also fire, foottraffic, hail and water-leakage resistances.
