High-Velocity Hurricane Zones
This coastal commercial building, Gulf Shores, experienced wall cladding and secondary structure failure following a hurricane event.
Roaring thunder, crackling lightning, incredible gusts of wind, and utter chaos—the severe weather whims of Mother Nature, while awe-inspiring,are unfortunately some of her most destructive. And while scientists have yet to prove that climate change is linked to the frequency of hurricane events, their occurrence is statistically increasing. Indeed, the 2008 Atlantic hurricane season spawned a record number of consecutive storms, according to the National Oceanic and Atmospheric Administration (NOAA, www.noaa.gov).
While protecting human lives is the top priority surrounding hurricane disasters, hurricane-zone buildings that resist failure are also taking center stage. This is reflected in changing building codes and stringent product approval requirements in states like Florida and Texas. In addition, the Federal Emergency Management Agency continues its efforts, using mitigation assessment teams, to study property damage and glean knowledge on best building practices.
What are some key points that the industry has picked up along the way?
According to Jack D. Burleson, AIA, CSI, CBO, a regional manager for government relations with the International Code Council (ICC) Texas field office, the combination of high winds, huge waves, and storm surge in hurricane events makes resistance to wind and water the principal area of concern. Secondarily, a building's structural system must be protected from corrosion, decay, and termites to give it a fighting chance against these forces.
Roof aggregate turned into windborne missiles during Hurricane Ike, damaging glazing in mid- and high-rise buildings. Photo: Courtesy FEMA
However, most damage from hurricane winds and rain occurs when building elements are compromised as a result of improper design, application, material deterioration, or roof system abuse, according to Tom Smith, AIA, TLSmith Consulting, Rockton, Ill.
Dr. Leighton Cochran, CPEng, MASCE, CPP, principal of Fort Collins, Colo.-based CPP, a consulting engineering firm specialized in wind (www.cppwind.com), agrees with that assessment. He explains that the pressure created on a building's external surface increases with the square of the wind speed. As a result, building product specification and design must be able to withstand such pressures—as well as flying debris caused by the winds.
“This is key,” he warns, “because once the building envelope is broken by a flying object during a storm, the large external pressures can move to the inside of the building and generally increase the net load seen across the remaining intact building skin, at which point further failures and water damage will ensue.”
INSURE FIRST, AND BUILD RIGHT
Design and construction techniques are important, but surety is the first step. In addition to more specialized and higher-quality building design, property owners in hurricane zones are being encouraged to invest more in insurance.
Figure 1. Windborne debris areas of Florida, by county. Values are nominal design, three-second gusts, wind speeds in mph at 33 feet (10 m)
above ground for Exposure C Category. Islands and coastal areas outside
the last contour use the last wind speed contour of the coastal area.
According to James Lee Witt, director of FEMA during the Clinton administration and now a disaster-recovery consultant, if a building is sited in a 100-year flood plain, wind and flood insurance are imperative. Addressing the front-end cost, Witt estimates that every dollar spent on prevention could potentially save $3 to $5 in future losses, and perhaps more when business interruptions are taken into account.
Recognizing this, Congress has mandated that regulated and insured lenders require flood insurance on properties located in high-risk flooding zones.
Another trend to note is the migration of the U.S. population toward coastal areas, making more people and properties increasingly vulnerable to hurricane damage. This will ultimately affect the insurance industry as well.
Taking all of this into account, insurance companies are offering lower premiums to owners and property managers who elect to better fortify their facilities against hurricane damage by installing building components such as storm-resistant glazing and roofing systems.
The other big trend has been the evolution of more-stringent building codes, standards, and product approval systems, namely Miami-Dade and Broward Counties' High-Velocity Hurricane Zone (HVHZ) program. Essentially, building products specified in these counties must be tested to HVHZ's high standards of performance, the two main requirements being laboratory-tested resistance for structural wind loads and an ISO-based production quality assurance program, audited by an approved third party.
In addition to setting up and enforcing its product approval program, Miami-Dade County offers a user-friendly database of approved products which can be searched according to such parameters as material category, company name, and impact rating: http://www.miamidade.gov/buildingcode/pc-search_app.asp.
Beyond the reach of Miami-Dade and Broward, a number of other codes and standards are also addressing design requirements for high-wind zones. For example, the American Society of Civil Engineers standard, ASCE 7 – Minimum Design Loads for Buildings and Other Structures, is widely used to determine the wind loads on buildings. The standard takes into account such variables as geographic location; basic wind design and speed; surrounding terrain; building use, size and shape; and the location of components within the building envelope. Similarly, FEMA 361 – Design and Construction Guidance for Community Shelters, delineates key product approval standards for these important community facilities.
The 2007 Florida Building Code also addresses several key areas, such as minimizing damage from windborne debris by requiring missile impact testing for glazing and cladding. According to Nathan C. Gould, DSc, PE, SE, director of St. Louis-based risk-management specialist ABS Consulting (www.absconsulting.com), the small missile impact test, which projects solid steel balls weighing two grams at a speed of 130 feet per second, is required for all applicable envelope components that are to be used above 30 feet in height. The large missile impact test, which projects a specified length and weight of 2x4 wood stud at a speed of 50 feet per second, is required for applicable envelope components that are to be used at or below 30 feet.
“The building code also has specific material, strength, and installation requirements for windows, doors, and other glazing to mitigate potential impact damage,” adds Gould.
DETAILING FOR SAFETY'S SAKE
To design the essential building systems that can stand up to hurricanes, Building Teams must get the details right. This is borne out by damage survey conducted after Hurricane Charley by the Institute of Business & Home Safety and Wyndham Partners Consulting in 2004 (http://www.weatherpredict.com/pdf-downloads/HurricaneCharleyDamageSurvey.pdf), which showed that construction detailing had a significant impact on damage caused by hurricane-force winds.
For example, poor attachment methods—the type of fastener and the correct spacing of fasteners—were found to be the leading cause of envelope failure to metal sheathing. Similarly, poorly detailed vinyl soffits ultimately contributed to progressive roof failures and indoor wall/ceiling damage. Also, poorly detailed roof overhangs played a key part in progressive roof failures and other damage to interior walls and ceilings.
Based on his ICC experience, Burleson points out that forces created from the combination of wind and water place extreme loads on building components, including framing, anchor bolts, connections, fasteners, roofing, sheathing, doors, and windows. Consequently, if proper attention is not given to the connection details, this can create a “weak link in the structural chain,” ultimately leading to partial or total building failure in frequent cases.
Flashing. David Cox, senior staff engineering specialist for FM Global (www.fmglobal.com), reports that 90% of yearly wind losses experienced by the global commercial insurance firm's client base are related to flashing, which is considered to be the single most important component of a roof system.
John Ingargiola, a senior engineer in the building sciences branch of the FEMA mitigation directorate (www.fema.gov/about/divisions/mitigation.shtm), Washington D.C., elaborates on this point. “Flashing around openings is an area of concern. If the flashing is not properly designed and installed, flashing failure around openings and penetrations can cause peeling of the building envelope and lead to water infiltration.”
To address this, Smith recommends enhanced flashing details—in other words, extra-long flanges, the use of sealant and tapes, and the like—for strong wind zones. These details are outlined in his article “Wind Safety of the Building Envelope,” in the online Whole Building Design Guide, http://www.wbdg.org/resources/env_wind.php?r=e.
In addition to flashing, it is important to focus on such details as adhesives, sealants, and gaskets as well, says Chuck Anderson, codes and industry affairs manager for the American Architectural Manufacturers Association (www.aamanet.org), Schaumburg, Ill. “Compatibility between the substrates being joined and the material chosen is critical, as incompatibility could result in adhesive failure or possible discoloration of the substrate or the adhesive, sealant, or gasket,” he says. “Specifiers should also consider the durability and warranty of these products.”
Fasteners. Fasteners must be of substantial size and strength to resist shear loads. “The bearing surface between the window and the fastener must be adequate to transmit the load without deforming the window frame or allowing the frame to pull away from the fastener head,” says AAMA's Anderson. “In addition, bending stress must be checked. The latter becomes more important as the length of the fastener extending from the rough opening is increased.”
Based on post-Hurricane Ike research findings, Ingargiola reports that, in many cases, building cladding was prone to damage due to improper fastening and inadequate fastener spacing. “Fasteners should be corrosion-resistant, especially when building near salt water,” he says.
In terms of an overall strategy against water intrusion, Tim Reinhold, PhD, chief engineer and SVP of research for the Institute of Business & Home Safety (www.disastersafety.org), Tampa, Fla., points out that the use of caulking and sealing must be evaluated based on the building material involved. “For example, brick and cement blocks are porous, so it makes no sense to try to caulk or seal the joint between windows or doors and these elements. On the other hand, cracks and openings around pipes or cables can allow significant water flow into wall cavities or to the interior of the buildings, and all of these gaps should be filled to help minimize the flow of water.”
Air and moisture barriers. Another key strategy is the use of air barriers, moisture barriers, and vapor retarders to protect against water intrusion. “Air barriers are critical to the overall health and performance of air-conditioned buildings to keep moisture from flowing through the walls and around windows and doors,” says Reinhold. “And moisture barriers and drainage planes with venting to the outside of the building are important for many types of walls to keep from accumulating water inside the walls.”
AAMA's Anderson notes that rainscreen systems may require venting capacity to allow for drying of the areas intended to get wet. In addition, weeps and channels should be designed to minimize clogging.
In terms of specifying these types of systems, good resources include FEMA 499 Technical Fact Sheet No. 9: Moisture Barriers Systems, and books and articles put out by the Boston-based Building Science Corporation, www.buildingscience.com.
EXTERIORS: FIRST LINE OF DEFENSE
When choosing a cladding system for hurricane-zone buildings, it's important to consider the pros and cons of various system types. In all cases, however, faulty installation will make any system inadequate in the face of hurricanes. The top concern, says FEMA's Ingargiola, is “selecting a product that is rated for the appropriate loads and impact resistance for that location and installing according to the manufacturer's instructions.”
Cladding. Every envelope system has an Achilles heel in terms of severe weather performance. A few examples are listed here:
• Metal. Although metal cladding generally offers higher impact resistance, Reinhold cautions that these systems can be susceptible to fatigue failure around fasteners. Moreover, wind performance varies depending on the system's attachment assemblies. As Smith explains in his WBDG primer, the performance level is determined by: 1) the strength of the specified panel—which is a function of material, panel profile, panel width, and whether or not the panel is a composite—and 2) the adequacy of the attachment, which can either be by concealed clips or exposed fasteners.
• EIFS. While exterior insulation and finish systems are popular for their insulating properties, flexibility, and affordability, most experts report that they need to be carefully specified for hurricane zones. “Having been on several post-hurricane assessment teams, I have seen EIFS fail frequently in high-wind areas,” says Cochran, chair of ASCE's Structural Wind Engineering Committee and past president of the American Association for Wind Engineering. Reinhold points out that commercial EIFS systems should be designed with a drainage plane and weep holes, so that water that gets into the wall cavity can be directed out. That helps to prevent trapped water in the enclosures, which could lead to corrosion damage to the anchors and delaminating of construction materials, he says.
On the other hand, Dave Johnston, executive director of the EIFS Industry Members Association, Morrow, Ga., points out, “Tests prove that EIFS, when properly designed and installed, can repel windborne debris from a hurricane. It's time to look at the improvements that have been made to these systems and not rely on experiences of well over a decade ago to assess them.” Johnston says that “any type of structure can fail during a hurricane, given the right conditions,” noting that all approved EIFS projects do incorporate a drainage system to allow moisture to escape. In general, EIFS systems that meet the Miami-Dade County product approvals are recommended.
• Stucco. Thick stucco finishes have a good track record of hurricane-zone performance. “We have seen relatively few delaminating failures,” says Reinhold. “Although cracks in the stucco can lead to water intrusion, there have been relatively few problems when they have been painted with two to three coats of paint. Also, the natural hardness of the stucco, when combined with the support structure, provides good debris impact resistance.”
• Concrete and masonry. Although concrete and masonry structures offer strong resistance to windborne debris and high-wind pressures, according to Ingargiola, water intrusion can be an issue due to the way these wall systems are designed. “Concrete and masonry walls are designed to absorb water and release it through evaporation, so under heavy, sustained flooding, this can be an issue.”
One solution, suggests Ingargiola, is to use concrete or masonry and cladding materials in tandem. “By placing siding, panels, or stucco over masonry or concrete, a wall has good capability to prevent water intrusion and resist high-winds and debris.”
Roofing.While good detailing and well-considered cladding systems are essential, even more crucial is the integrity of the roofing system, as roof failure is the leading cause of building performance problems during hurricanes, notes FEMA's Ingargiola. In addition, roofing materials run the risk of becoming windborne debris during a hurricane, thus wreaking further havoc.
There is a wide range of choices when it comes to roofing, so getting a handle on varying performance levels is helpful.
• Single-ply. Although single-ply roofs offer certain benefits, mechanically attached, fully adhered, and air-pressure-equalized membrane systems can be susceptible to progressive failure following missile impact, says consultant Tom Smith. This being the case, they are generally not recommended for office buildings where the wind speed can reach or exceed 120 mph. On the other hand, paver-ballasted and fully adhered single-ply systems work well, as opposed to aggregate ballast, which can be more prone to blow-off, he adds. For more guidelines, Smith recommends the National Research Council of Canada's Institute for Research in Construction, which offers its Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs (http://www.nrc-cnrc.gc.ca/eng/ibp/irc.html). This comprehensive wind design guide includes a discussion of air and vapor retarders, which can be effective in reducing “membrane flutter.”
Whether for single-ply, EPDM (ethylene propylene diene monomer), asphalt, or built-up roof (BUR) systems, Reinhold suggests strengthening the anchorage around the roof's perimeter for the first four to eight feet, depending on the size of the roof, as this is where the uplift loads are greatest. In addition, “Flashing around the perimeter of the roof needs to be well anchored, as a lot of roof failures are initiated when the flashing tears loose.”
• BUR and modified bitumen. In addition to flashing, Smith suggests that the coping and gutter systems be carefully detailed and installed to better prevent failure for BUR and modified-bitumen systems. He also notes that modified-bitumen material adhered to a concrete deck can offer strong resistance to progressive peeling even if the metal edge flashing is blown off.
• Metal. As with metal exterior systems, metal roofing performance can be highly variable. “Metal roofs are best when they are installed over a solid roof deck and when they are mechanically attached at a close-enough spacing,” advises Reinhold. “Again, focus on the attachment around the perimeter and on ensuring that there is an attachment point within about six inches of the eaves for each metal panel.” Smith also recommends calculating uplift loads and determining uplift resistance based upon the ASTM E 1592 test method.
• Tiles and shingles. When comparing metal to tile and shingle systems, the distributive nature of the larger tributary area provided by metal creates a more hurricane-resistant product, according to Cochran. However, metal roof panels are usually designed to resist the full design pressures specified in the building code, whereas tile and shingle roofing is porous in nature and can therefore use pressure equalization between top and bottom surfaces to reduce the design loads, notes Reinhold.
As for best practice design installation methods, Reinhold explains: “Tiles should be mechanically attached or attached using one of the new foam adhesive products, whereas mortar set tile should be avoided, especially along edges and ridges. Instead, ridge boards or metal hat sections should be used on hips and ridges to allow mechanical attachment of these tiles.” He notes, too, that special care should be taken to ensure that eave tiles are anchored more securely than is required for the field tiles. To assist with this, some manufacturers offer clips that can be used to anchor the free edge of the eave tiles.
Further to this point, Ingargiola states that installation is particularly important at eaves, hips, ridges, and rakes, and that proper attachment of roof sheathing before installing the underlayment should be verified, ensuring lapping and fastening of underlayment and roof-edge flashing. Selection of underlayment material is also an important step. For additional guidelines, Ingargiola recommends FEMA 499 Technical Fact Sheet No. 19: Roof Underlayment and Asphalt Shingle Roofs and Technical Fact Sheet No. 20: Asphalt Shingle Roofing for High-Wind Regions.
Even when best practice guidelines are followed, however, some still argue that tiles and shingles are not a good choice for hurricane zones. “When tiles fail, they damage other tiles downwind, creating a cascading effect,” says Cochran. “And shingles simply do not have the general uplift resistance at substantial wind speeds, as their small size means that they are more influenced by the small intense gusts and other flow phenomena.”
FENESTRATION: A HOLE IN THE LINE OF DEFENSE?
“Fenestration is a critical part of the structural and functional design and is often the key element in both the interior and exterior aesthetic design,” says AAMA's Anderson. “The overall height and shape of the building affects its visual appeal and can greatly impact the wind load requirements.”
Because wind loads, windborne debris, and water intrusion are key considerations for structures in hurricane-prone regions, Anderson recommends that all products be tested and certified to rigorous standards such as AAMA/WDMA/CSA 101/I.S.2/A440-08, NAFS- North American Fenestration Standard/Specification (for windows, doors, and skylights), as well as AAMA 506-08, Voluntary Specifications for Impact and Cycle Testing of Fenestration Products.
However, due to the nature of windows and doors, they are actually expected to leak water during a significant hurricane event. For that reason, the current rating standards only require that windows and doors not leak at a maximum of 15% of the design pressure. “Consequently, thought needs to be given to managing the water that does enter through the use of materials and products for floors and walls that are not particularly water-sensitive,” says Reinhold.
With regard to doors, leakage can occur between the door and frame and between the frame and the wall, while water can be driven between the threshold and the door. To help mitigate this, Smith suggests designing a vestibule where both the inner and outer doors are equipped with weather stripping. In addition, the vestibule itself can be coated in water-resistant finishes, and the floor can be equipped with a drain.
Another noted development in door design is component testing required by the latest version of the Florida Building Code. Whereas the previous code allowed testing as a complete assembly, now each component—door, frame, lock, and hinges—must be tested separately. The total windstorm assembly gets the rating of the lowest-rated component.
Concerning door hardware, one important point here is the type of metal used. Whereas cast aluminum and zinc may not be ideal for severe-duty openings, stainless steel and carbon steel can be good choices, although carbon steel can corrode and deteriorate rapidly if exposed to salty conditions. Where corrosion is an issue, Smith recommends anodized aluminum or galvanized doors and frames, and stainless-steel frame anchors and hardware. As for the main swinging entry and exit-door hardware, specifications must ensure that wind suction will not pull the doors open.
Glazing and storm shutters. When glazing systems fail due to pressure or debris impact, significant wind and water intrusion can follow, dramatically increasing loads on the exterior walls, interior partitions, ceilings, and the roof, notes Reinhold. For that reason, says Cochran, “Protecting the building envelope via the use of laminated glass or well-installed and -designed shutters is a must for hurricane zones.”
Fortunately, Miami-Dade and Broward Counties have set performance requirements very high, leading a national trend toward the use of safer materials. “Consequently, choosing systems that have Miami-Dade product approvals and ensuring that they are installed according to the specifications provides the highest measure of protection,” says Reinhold.
One caution, raised by the International Window Film Association, is the possibility of mistaking energy-efficient window films for safety glazing. While some of these products do offer a certain degree of glass breakage protection, it is only a side benefit. Therefore, safety glass should only be selected based upon testing and product approvals.
As for storm shutters, they should be able to fully withstand the impact of a nine-pound piece of debris traveling at 34 miles per hour, per Miami-Dade standards. Miami-Dade also recommends hardwoods as the choice shutter material, although a number of composite and metal products have succeeded in passing code-compliance testing.
In terms of installation, “Over any opening, make sure that the shutter is connected directly to a continuous load path that travels through the building and into the foundation,” says FEMA's Ingargiola. “If the shutter is attached to the window frame or a nonstructural building component, then the loads incurred on the shutter will not be transferred to the foundation, and the shutter will not perform as designed.”
Ingargiola also cautions that a rated shutter does not exempt the window behind it from have the capacity to resist design wind pressure—unless the entire shutter and window system was tested together, which is typically not the case.
DO MEP SYSTEMS MATTER?
Mechanical and plumbing equipment are rarely selected with hurricane-zone performance in mind, but some experts say they should be.
For example, when it comes to rooftop HVAC equipment, one mistake often made is assuming that the machinery is too heavy to be moved by the wind. So the equipment is either not secured, or it is fastened with simple self-drilling, self-tapping TEK screws, which hardly suffice, according to Cochran.
According to Reinhold, “Failure of rooftop equipment has been widespread in hurricanes.” Wind speeds increase with height and architectural features, causing turbulence, vortices, and flow separations near corners and edges. These effects dramatically increase downwind surface pressures, says the ABS's Gould. Lightweight rooftop equipment, “typically out of sight and out of mind,” is often most vulnerable to being picked up by high winds, says Gould. So whether it's exhaust vents, air-handling units, ductwork, transformers, switchgear, or generators, “Equipment anchorage should be designed for wind loads appropriate for a specific location and function, and should also be periodically inspected and maintained,” he adds.
In general, Cochran recommends a bolted-frame connection to the roof structure; in some cases, galvanized steel straps are called for.
Today, there is plenty of useful information available to Building Teams regarding loads to be applied when designing rooftop equipment anchorage. ASCE 7-05 now contains much better guidance on the subject, and the 2010 edition of ASCE 7 is expected to provide even more detailed guidance, particularly for uplift forces, according to Reinhold. Additional resources include FEMA 499 Technical Fact Sheet No. 29: Protecting Utilities and FEMA 548: Summary Report on Building Performance: Hurricane Katrina 2005, which includes guidance on best practices for anchorage of rooftop equipment.
As for electrical equipment, surge protection is an important consideration, especially with the proliferation of sensitive electronic equipment. “In addition to properly protecting and securing this equipment from becoming windborne debris or damaged from debris, proper grounding, corrosion resistance, and other safety measures are important for minimizing damage to electrical systems and attached equipment,” says Ingargiola.
DETAILS, DETAILS, DETAILS
When it comes to severe weather and building safety, the devil is in the details.
It's true that the punchlist for hurricane-zone design is quite long, whether it's detailing, cladding, roofing, fenestration, MEP systems, or even other building components. But the stakes are too high to take shortcuts. And in Florida's Miami-Dade and Broward Counties—as well as other regions of the U.S. that have followed suit—this is not even an option.
For the benefit of the Building Team, the growing body of research, standards, product approval programs, and lessons learned will provide a great benefit to building owners and occupants.