Here’s how metal helps buildings weather storms
Hurricanes, thunderstorms, blizzards and tornadoes have been in the news a lot over the past decade. In many different ways their harmful effects can damage or destroy buildings, including metal buildings. However, with proper design and storm-resistant construction, the damage to buildings by Mother Nature’s tantrums can be greatly reduced. A complete structural load path Extreme winds can cause buildings to tilt, and even slide off or completely flip off their foundation.
According to the Uniform Building Code/1997 Section 1615-General, “Every building or structure and every portion thereof shall be designed and constructed to resist wind effects.” According to the 2009 International Building Code Section 1604.4, “Any system or method of construction to be used shall be based on a rational analysis in accordance with well-established principles of mechanics. Such analysis shall result in a system that provides a complete load path capable of transferring loads from their point of origin to the load-resisting elements.”
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What exactly is a complete structural load path and how does it apply to storm-resistant construction? According to the International Building Code, there are two types of load paths: vertical and lateral. The forces must be physically transferred from the point of origin through the various structural elements to the foundation and finally to the supporting soil below. The vertical load path includes beams, joists, girders, trusses, posts, columns, walls, foundation elements and the soil that supports the structure. The lateral load path includes diaphragms, walls, bracing, moment frames, boundary elements, retaining walls, foundation systems and the soil that supports the structure.
To ensure complete load path continuity, the connections of the above structural members should be adequately designed and fastened together by direct mechanical attachment or anchorage. Line up metal bracing and tie-downs at critical load points to maintain the integrity of the load path. Use light-gauge steel straps to anchor the building’s first story to its foundation.
Design and strength
The devices that reinforce a building’s framing connections include tie-downs, embedded connectors, metal plates, straps and anchor bolts. They should be made of stainless or double hot-dipped galvanized steel to prevent corrosion. They come in many shapes and styles, allowing them to be used in different situations.
Generally speaking, any reinforcing of the metal roof, wall and structural framing systems are addressed at the design stage. “Engineers have several ways to account for forces due to storms,” says Mark Henry, senior research engineer at Butler Manufacturing, Kansas City, Mo. “The strategy is to design the building for storm loads, rather than strengthening or reinforcing the metal roof systems during installation. For instance, during the design stage, the engineer may increase the thickness of the roof panel or increase the number of roof purlins to account for storm forces. Butler Manufacturing offers proprietary roof-panel clips and seam-reinforcing clamps that can be specified by the engineer to enhance the wind uplift resistance of the metal roof system. Engineers may choose to increase the panel thickness and/ or increase the number of wall girts of exterior wall panels to resist the storm wind forces.”
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“In order to strengthen a metal building so that it will be able to survive severe winds, you would need to beef up purlins and girts, that is adding more girts along the walls and more purlins on the roof,” says Ian Strompf, president, MBMI Steel Buildings, Delray Beach, Fla. “Adding more screws to all the sheeting will also strengthen the building for heavy winds. Reinforcing roofs, extra caulking, and more gutter straps and clips will all help to add strength to a metal building, but if the rest of the building is not properly prepared, they will not be very useful.”
“The roofing contractor must furnish and supply a metal roof, and should be very concerned about storm-resistant construction,” says Ken Buchinger, vice president of business development and R&D, MBCI, Houston. “The roofing contractors are free to order components and it is incumbent on them to understand the engineering behind the panels, choose the proper accessories, and use the right fasteners and screws to attach the structure. There is nothing stopping them from ordering one of our standing seam roofs and not ordering the right clips. They could order no clips and simply screw it down like a through-fastened panel instead of clipping it down like it was designed to be.”
Water warning
Wind damage accounts for only a fraction of a building’s storm-related destruction. More frequently, it’s water infiltration, not wind, that causes most storm-related damage. “Water penetration through or around otherwise intact openings from wind-driven rain can do a significant amount of physical damage, cause occupant displacement and business interruptions, and lead to extensive restoration expenses,” cautions Dean Lewis, educational and technical information manager, American Architectural Manufacturers Association, Schaumburg, Ill. “It can also create a mold-friendly environment in unseen wall cavities and in spaces between exterior sheathing and cladding.”
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Fortunately, according to Henry, wind-driven rain is usually not an issue for metal roof systems. “They are designed to be weathertight,” he says. “The long interlocking roof panels do not provide a gap through which wind could drive rain into the building interior. Metal roof systems that have been approved by Miami-Dade County-such as Butler Manufacturing’s MR-24 Roof System-must pass a static water head test to demonstrate their resistance to rain forces.” With metal roofing the weak spot could be a poor weld, a rusted panel or a screw that’s backed out.
“The metal standing seam roofing material you choose is always an issue,” says Joe Tripod, director of technical services, Englert Inc., Perth Amboy, N.J. “Some, not all, metal roofing products meet industry requirements, such as the Florida Building Code wind-uplift requirement, Miami Dade uplift testing, FM uplift testing, UL Uplift Standard 580 and ASTM 1592 uplift standards. These products, like Englert’s Series 1300 and Series 2000 standing seam panels, have gone through rigorous, controlled performance examinations that make them more qualified than others to withstand the onslaught of Mother Nature. But a sturdy product makes up only one aspect of a hurricane resistant roof.”
Tripod argues the soundest defense against the fury of Mother Nature should also consider the following:
• an American Society of Consulting Engineers (ASCE) analysis to specify the most storm proof construction plan to withstand hurricane force winds for a specific building,
• a weather-tightness warranty from the roofing material manufacturer with on-site installation inspections,
• and the services of a certified installation contractor who will properly install the roof following the instructions of the ASCE analysis.
Gutters, louvers, curbs
Gutters and downspouts protect buildings in storms by quickly channeling excess water away from the building and the foundation. Gutters should be designed so that they overflow outwards for the full length. If they overflow only at a low point, the volume of falling waterfall can wash away the soil. The spacing of gutters can be decreased to increase to load that gutters can resist, a common practice in regions with high snow loads. Steve Bradley Sr., national operations manager of specialty products at Englert, recommends hidden hanger spacing 24 inches on-center for a gutter. “But in high winds and heavy snow load areas where a building may face stress, it is recommended that the hangers be spaced every 16 inches to prevent problems with gutters,” he says.
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For best drainage, gutters and downspouts must be kept clean. Modern reinforced wall louvers, built to meet local building codes, can help minimize storm effects. “Adjustable louvers can be closed in the event of heavy rain, allowing zero water entrance from rain,” says Blake Edgeworth, vice president of sales and marketing,
Moffitt Corp., Jacksonville Beach, Fla. “Alternatively, fixed louvers can be placed in locations with different rain and wind loads. While they don’t close completely, they are designed for maximum water diversion.”
The importance of properly installed roof curbs cannot be understated. “The best roofing job can be made worthless if the openings for roof ventilators and exhaust fans are not properly installed,” Edgeworth says. “Furthermore, having the right curb caps and/or dampers is a must for any project. Getting input from a professional with experience in ventilation is crucial to having a watertight, weatherproof building.”
Windows and doors
In addition to straight-forward load paths and impenetrable roofs, it’s also important to protect buildings by installing storm shutters or impact-resistant windows. Some building codes require them in areas where wind speeds are 120 miles per hour or higher. Violent windstorms can hurl objects and flying debris that act like bullets and missiles that can break windows and damage building envelopes.
Permanent or removable shutters reduce forces on buildings and minimize water- and wind-related damages to the interior by protecting windows. Although the negative pressure caused by high-velocity wind flowing over a building roof can cause the roof to fail with the building envelope intact, broken windows allow the air pressure to rise inside a building, creating an even greater pressure difference, and increasing the likelihood of roof failure. When struck by objects, impact-resistant glass may crack or shatter, but the fragments stay in place.
Shutters are usually constructed from steel, aluminum and polycarbonate. They can be affixed to the outside of a building with screws, hurricane clips or a track system. Advanced shutters can be motorized and may even fold away when not in use. West Palm Beach, Fla.-based Armor Screen’s Wind, Rain & Impact Abatement System is a flexible, transparent membrane designed to withstand strong hurricane winds without rupture while deflecting large and small projectiles. Impact-resistant doors can also withstand wind-born projectiles. Outward-swinging doors reduce the likelihood of system failure in high wind. Garage doors in metal commercial buildings often fail in storms due to their large size and relatively weak material. In the past, wind-resistant garage doors required vertical bracing, which had to be manually installed prior to a wind event. Most door manufacturers now offer impact-resistant doors that do not require vertical bracing, which makes them much more convenient, and can even stand up to workplace accidents like forklift abuse.
Roofs and snow
The steeper the pitch of the roof, the more easily a steel building sheds snow and rainy downpours from storms. “When it comes to severe weather affecting metal buildings, the greatest concern is that the building was designed for the proper loads,” says Strompf. “When it comes to snow, however, the concern is that if the building cannot carry the weight of the snow, it could come crashing down. Adding more purlins on the roof is the best way to deal with this scenario. For snow, adding more gutter support straps and using a heavier gauge steel will help, but unfortunately, heavy snow will always bring down gutters.”
“There is a why when you go to a ski resort, you see a lot of metal roofs,” Buchinger says. “They are good at shedding the ice and snow without damaging the roof in many cases. Sometimes people use a southern-style gutter, that’s when the outside face of the gutter sticks out high than the pan of the roof panel. This hides the end of the panel seam. It’s great down south, it makes the roof look better a lot of times. Up north, that gutter will stop ice and snow for a little while, but then that ice and snow will tear that gutter off.”
Many roof collapses are due to snow collecting on a flat or low-pitched roof. A more steeply pitched roof is easier to clear because gravity does most of the work. A roof slope between 12 and 14 degrees is desirable. However, the lower the building, the less the wind intensity and the less the area on which it acts, so the extra height of a high-sloped roof must be taken into account. While lighter roofs are more efficient in warm regions, dark roofs are the better choice in areas where there is a high likelihood of heavy snowstorms. Dark roof coatings absorb heat and help melt the snow more quickly and efficiently.
Overall, one of the best approaches for storm readiness and its elements is to design the metal building system according to the local building codes and then install the systems correctly. “It is both expensive and time-consuming to go back and make a building storm-ready during installation or after the building is completed,” Henry says. “Every project should be built to the existing codes at the time of installation, which takes into consideration extreme weather conditions. Following building codes is the best defense against the worst-case scenario as far as weather is concerned.”
