August 11, 2010

High-Velocity Hurricane Zones (Continued from p. 40 of the September 2009 issue of BD+C)
                    
                      

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.”

                           
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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.
      
     
About the authors

C.C. Sullivan is a communications consultant and author specializing in architecture and construction. Barbara Horwitz-Bennett is a writer and contributor to construction industry publications.



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       This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.


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