PRODUCT INSTALLATION FEATURE Wi 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 oc- cupant 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 suffi cient resistance to destructive winds provided the metal building is located on the outer edge of the tornado vortex.
Studying fl ow and pressure Designing metal buildings to withstand destructive wind is an important aspect of engineering. Wind requirements study the air fl ow and pressure fi elds around buildings. Not only will wind requirements ensure a building will resist high winds, they also maintain pleasant conditions in outdoor spaces, as- sess 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 Ameri- can 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 signifi cant change is the reference to wind speed, which changed from fastest-mile to a 3-second gust. Each revision
38 METAL CONSTRUCTION NEWS December 2012
made changes and adjustments in several different factors including the importance factor, terrain fac- tor, directionality factor, gust effect factor and the pressure/force coeffi cients.
ASCE/SEI 7-10 ASCE/SEI 7-10 “Minimum Design Loads for Build- ings and Other Structures” is a complete revision of ASCE Standard 7-05. ASCE/SEI 7-10 completely up- dates and reorganizes wind load provisions, expand- ing 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, Burr Ridge, Ill. “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 docu-
ment 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 specifi c project calculations; how- ever we do reference the IBC for all guidance, and comply with the requirements of the code which govern our members’ metal wall and roofi ng ap- plications 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, fl ood, 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 informa-
tion 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 fi rst 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 estab- lish the required design pressure.”
Roofi ng wind resistance tests It was standardized testing measuring wind resistance of various roofi ng systems that went into current metal build- ing requirements and codes. The uplift pressure on roofi ng 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 roofi ng 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 specifi c 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 sub- jected to a uniform static pressure. Air pressure is applied beneath roofi ng 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. Specifi c criteria evaluated include not only wind-uplift, but also fi re, foot- traffi c, hail and water-leakage resistances.
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