WITH A BSCE AND more than 40 years of experience in the light-gauge metal industry, including drafting, engineering, testing, sales, estimating, management, weld inspection, quality control and product development, I have come to be known as something of an expert in the metal industry.
In the upcoming year, I will be sharing insight into the technical aspects of this industry in hopes of providing you a better understanding of a variety of technical topics that are important to everyone in the metal industry today. We'll start at the top with the roof.
ROOF WIND LOADING
There are two types of loading on a roof. The first type consists of positive forces pushing the roof down onto a structure. The second type consists of negative forces pulling the roof off a structure. The negative forces are the greater of the two types. These forces are dynamic in nature because of their erratic distribution patterns and the inconsistency of wind. In an effort to address this phenomenon, building codes were created with a load distribution that applies heavier uplift loads to the perimeter and corner areas of the roof while lowering the uplift loads in the field or middle of the roof.
If you look at a building code, it describes the roof as having three different load zones: Zone 1 is the field, or middle area, of the roof; Zone 2 is the perimeter, or the eave, rake and ridge; and Zone 3 is the corner, or eave/rake and rake/ridge intersections. The ridge and rake/ridge corners are only on roofs with a slope of 2:12 (9 degrees) or greater. All three of these zones have different loading. Generally the positive loads do not change for these areas, but the uplift or negative loads increase from Zones 1 through 3 with Zone 3 having the largest load.
For example, if a building is 50 by 100 feet (15 by 30 m) with a 12-foot (4-m) eave height and a 3:12 (14 degree) roof slope and is enduring a 90-mph (145-kph) wind speed, ASCE 7 from the Reston, Va.-based American Society of Civil Engineers states the loading for Zones 1, 2 and 3 would be -13, -23.2 and -34.3 pounds per square foot (-63.5, -113.3 and -167.5 kg per m2), respectively. All three zones would have a positive load of 8.4 pounds per square foot (41 kg per m2).
The wind speed, importance factor, exposure factor, eave height, roof pitch and enclosure type are items that impact the loading on a building.
The size of these zones is determined by the size of the building and the building code. The code states the zone size will be determined by the following: 10 percent of the least horizontal dimension, 0.4 times the eave height, and not less then 40 percent of the least horizontal dimension or 3 feet (0.9 m). For our example, the zone size would be 4.8 feet (1.5 m). This is controlled by the eave-height portion of the zone size criteria (0.4 times the eave height).
GOT IT COVERED
Now that we know what the loading requirements and zone sizes are for this example, the next thing is to check what type of metal roof covering is to be utilized.
If the project has an exposed-fastener metal roof system, the calculated load tables can be used to determine the support locations. For our example, let's say this roof is a 26-gauge, 36-inch- (914-mm-) wide panel with major ribs located at 12-inch (305-mm) centers, and the rib heights are 1 1/4 inches (32 mm). This is typically known as an "R panel." Looking at the calculated load table for an R panel, it shows that for a 5-foot (1.5-m) span, this panel is good for loads greater than the corner load requirement and therefore can be shown as a typical 5-foot (1.5-m) span in all locations.
If the metal roof covering is a standing-seam system, West Conshohocken, Pa.-based ASTM International's E1592 test results must be used to determine what clip and support spacing is necessary to accommodate the loading requirements. Using the same loading example, let's say this roof is a 24-gauge, 24-inch- (610-mm-) wide, 3-inch- (76-mm-) high, trapezoidal standing-seam roof system snapped together--a common product manufactured by most component suppliers.
Looking at the ASTM E1592 test results, it shows a 5-foot (1.5-m) clip and support spacing will be sufficient for the field of the roof, or Zone 1, but will not work for the perimeter or corner areas of the roof. In those areas the clip and support spacing will have to be decreased to 4 feet (1.2 m). This means the first two support spaces at the eave and ridge will have to be something less than the typical 5 feet (1.5 m). And for an area of 4.8 feet (1.45 m) in from the rakes, the support spacing will have to be modified to something less than the 5-foot (1.5-m) spacing.
There are two types of loading on a roof. The first type consists of positive forces pushing the roof down onto a structure. The second type consists of negative forces pulling the roof off a structure.
Usually we will leave the 5-foot (1.5-m) spacing the same in these areas and add an additional stub support between the standard supports, which will make the spacing in the rake areas 2 1/2 feet (0.8 m). This is much more than necessary but is the easiest way to address the modified spacing needed to meet the uplift loading requirements for this example.
It should be noted that each building has its own specific requirements that are dependent on the building parameters and location. The wind speed, importance factor, exposure factor, eave height, roof pitch and enclosure type are items that impact the loading on the building. When any of these items change, the loading information changes and the application of the roof system is impacted. It is important to give the engineer all the information needed to properly analyze the building roof system.
If you have any questions or comments about this article or want to see something addressed in future articles, please contact me.
David Fulton is the vice president of research and development for a metal building and component manufacturer. He is on the board of directors for the Pittsburgh-based Cool Metal Roofing Coalition and holds several U.S. patents related to the metal industry.
