Technical Point – Let The Wind Blow
What a windy spring! As usual, the middle portion of our country has experienced tornado-strength winds, resulting in devastation and loss of lives. Additionally, a series of tornadoes in eastern North Carolina wreaked havoc. In late May, a deadly tornado also destroyed property in Massachusetts, a location that isn’t used to twisters.
Can we prepare a design for a roof system to withstand the forces of such an event? Although we cannot completely design for the magnitude of a tornado’s force or the swirling nature of a tornado’s wind, we can prepare a comprehensive design for normal wind conditions expected in the various regions of the country. Using this accepted design will allow a roof system to not only withstand wind pressures up to the established design wind loads, but also maximize the possibility of a roof to withstand a tornado event.
WIND DESIGN BACKGROUND
From the mid-1980s through the mid-’90s, research and testing was done to attempt to determine how physical conditions of a roof (slope, height, material, etc.) and demographic conditions of a roof’s location (expected maximum wind load, building occupancy, etc.) aff ected the actual wind pressures that could be expected. These studies culminated in an industry- and code-accepted standard that should be used to determine the expected maximum wind pressure for all roofs. That standard is published in the American Society of Civil Engineers Standard titled “Minimum Design Loads for Buildings and Other Structures.”
Chapter 6 of the standard, “Wind Loads,” is the basis for the International Building Code, which is the standard that most state building-code authorities follow. The 2009 North Carolina State Building Code states in section 1609.1.1, “Wind loads on every building or structure shall be determined in accordance with Chapter 6 of ASCE 7.”
ASCE 7 BASICS
It should be noted that in the development of the ASCE 7 standards, it was determined that a certain wind produces different wind pressures around a roof area. There are three distinct roof areas identified: Zone 1 is defined as the interior area of the roof that is not affected by forces associated with wind traveling vertically up the building’s walls; Zone 2 is identified by the perimeter of the roof that is affected by the wind coming vertically up the walls of the building and releasing at the roof level; Zone 3 is the zone commonly referred to as the corner zone where any Zones 2 intersect. The size of Zone 2, considered the edge, is determined within ASCE 7 based on the building parameters. Additionally, a Zone 2 will exist for ridges, hips and parapets under certain slopes.
1. WIND SPEED: The wind speed can be determined for the Carolinas from Figure 6-1B of ASCE 7 (see page 45) or an equivalent wind-speed map located in a particular state’s building code.
2. ROOF SLOPE: The wind creates a more severe pressure for a low-slope roof than it does for a steep-slope roof.
3. BUILDING OCCUPANCY: ASCE 7 uses a different Occupancy Category for buildings according to their intended use, which results in varying safety factors for the design wind loads to be used. Category I is predominantly for agricultural buildings. Category II is predominantly for buildings not in Categories I, III and IV. Category III is predominantly for schools and municipal buildings. Category IV is predominantly for hospitals and emergency related buildings. For example, a hospital roof requires a higher design pressure than an offi ce building with all other conditions constant.
4. TOPOGRAPHY: ASCE 7 defi nes three separate exposure conditions based on topography. Exposure B is for an urban area with many obstructions for the wind. Exposure C is for open terrain with scattered obstructions for the wind and water surfaces in hurricane prone areas. Exposure D is for flat, unobstructed areas and water surfaces not in hurricane-prone areas. As an example, an urban area with obstructions affecting how the wind reacts with the walls and roof of a building (Exposure B) yields a lower wind-design pressure than a coastal region with no obstructions affecting the wind’s force (Exposure D). Let’s consider how wind pressures vary according to the location of a building. A 4:12 school roof (Category III) in an urban area (Exposure B) of Winston-Salem would be designed for a maximum corner (Zone 3) load of about 40 psf while the same-shaped school roof located in Wilmington and exposed directly to the open coast (Exposure C) produces design load pressures of about 100 psf, an increase of approximately 250 percent.
Now that we understand how to determine the correct wind-pressure design, we need a roof system that will adequately resist the design pressure. Within all building codes a description exists of most roof systems available and how the code allows the designer to rate these systems with respect to load resistance.
For example, according to IBC, a structural metal roof system permits a designer to use a UL 580 test or ASTM E 1592 test. Because a UL 580 test is a pass/fail test with the magnitude of the test variable(UL-15, UL-30, UL-60 and UL-90), no actual design data can be extrapolated to perform the necessary structural comparisons with the design pressures derived from the ASCE 7 evaluation. Therefore, the use of an accredited laboratory’s ASTM E 1592 testing results for a particular metal panel system is the only way to determine proper engineering data for use in the structural calculations while meeting the requirements of the IBC code wording.
The same critical analysis for other roofing materials is necessary to determine what testing, calculations or charts can be used to determine a certain roof system’s capacity to resist wind pressure and still satisfy the requirements of the IBC and any particular local code requirements.
METAL ROOF EXAMPLES
In 1996, Hurricane Fran hit land at Topsail Beach, N.C., with winds between 110 and 120 mph. Two metal roofs stood in its path. Topsail High School (Duplin County Schools) had just received a metal roof retrofit over a flat built-up roof and was designed to withstand a 110-mph wind, which was the current design wind speed per code at that time. The only damage the facility received was one section of rake trim was blown away. Topsail High School housed the Red Cross rescue mission during the aftermath of this storm.
Less than 100 yards from Topsail High School was a kindergarten facility with a metal roof. It was designed properly but not installed as designed. The hurricane pulled the roof up from the eaves toward the ridge, completely destroying the roof and all the building’s contents. A new metal roof was installed on the same structure—this time per the structural design—and the roof has withstood several wind events close to the magnitude of Fran since.
Recent tornadoes in eastern North Carolina produced structural devastation never before witnessed in that area. However, after a personal survey of all school districts in the area turned up no indications of metal roofs that had been structurally damaged, I contacted several of the major metal roof suppliers in the area. As my survey indicated, these suppliers did not have any indication of any particular metal roofs on school structures that had been structurally damaged by the winds. Although this is not a statistically accurate analysis, it certainly indicates that metal roofs on schools, which require a licensed design professional and a building permit, can withstand wind pressures that exceed the building code.
A design professional should prepare a structural design for all roofs—not some, but all roofs. Then a roof system should be selected that can resist the design pressures. Finally, the selected roof system should be structurally installed per the engineered design requirements. It really is that simple.
Chuck Howard is president of Metal Roof Consultants Inc., Cary, N.C., a member of Carolinas Roofing’s editorial advisory board. Since 1973, he has been involved with the design and/or installation of approximately 30 million square feet of structural metal roofs throughout North America. Licensed as a professional engineer in 12 states, he has provided his services to design, construct, consult or defend metal roofs in the field or in the courtroom. He can be reached at email@example.com or at (919)465-1782.