Average Rating:

(0)

Rate this:

Severe Weather Solutions: New Threats, New Technologies

Earn 1 AIA/CEU Learning Unit by reading this feature and passing the test!

-- Building Design & Construction, 2/29/2008 11:23:00 AM

Severe Weather Solutions: New Threats, New Technologies

By C.C. Sullivan and Barbara Horwitz-Bennett

Hurricane-prone regions have taken quite a beating in recent years—and so have Building Teams working in those areas.
 
According to the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center, Washington, D.C., an average weather season consists of 10 to 11 named storms and eight hurricanes, two of which are considered major. Yet, the 2007 Atlantic hurricane season spawned 14 named storms, along with six hurricanes and considerable inland flooding. In 2005, 26 named storms and 14 hurricanes, seven of which were severe, wreaked havoc in the continental United States. That year also brought Hurricane Katrina, which joined the annals of the country’s most notorious weather events, now inscribed as the nation’s greatest natural disaster on record.

Aside from the tragic loss of human life and community displacement that Katrina left in its wake, insurance companies have paid out a staggering $40.6 billion in claims in six different states. On a similar note, the National Association of Home Builders (NAHB), Washington, D.C., says that seven of the 10 most costly natural disasters in the United States have occurred since August of 2004.

“While 2005 was by far the worst year ever for insured catastrophe losses in the U.S., future storms could prove

even costlier, reaching upwards of $100 billion,” predicts Dr. Robert Hartwig, president of the Insurance Information Institute, New York City. Furthermore, risk levels continue to increase as a result of rising property values and development in coastal areas.

So what does this all mean for Building Teams?
“When a major hurricane comes ashore in a populated area, some property loss is inevitable. But sturdy construction of buildings and the addition of protective devices, such as storm shutters, can reduce losses,” states a recent report released by ISO, a Jersey City, N.J.-based firm that supplies research and data to the insurance industry. [See A Half Century of Hurricane Experience.]  With this in mind, many insurers have begun rewarding developers and owners with credits that reduce premiums based on steps taken to mitigate hurricane property losses. Lower insurance costs may come from installing protective devices, such as hurricane-resistant glazing or roll-down shutters on doors and windows. Some insurers are even going so far as to reward property owners for installing devices capable of withstanding the impact of wind-borne debris.

DEFINING SEVERE WEATHER
Whether it’s hurricanes, tornadoes, flooding, hail, or storms, the wonders of Mother Nature pose a serious threat to property integrity. The dangers are not shared equally: U.S. regions most vulnerable to extreme weather include all Atlantic and Gulf of Mexico coastal areas, which run the risk of hurricanes or tropical storms from June to November. Elsewhere, parts of the Southwest and the Pacific Coast are plagued by heavy rains and floods each year.

Essentially, hurricanes are capable of not only causing catastrophic damage to coastlines, but several hundred miles inland as well, with winds sometimes exceeding 155 mph. Hurricanes and tropical storms can also create tornadoes and storm surges leading to heavy rainfall. Although hurricanes tend primarily to impact coastal regions, no state is immune to the threat of tornadoes, capable of producing winds up to 300 mph, with damage paths exceeding one mile in width and 50 miles in length.

Wind-induced effects. The means by which high winds and projectiles actually cause damage to buildings is important to consider in building design and detailing. First, winds produce a combination of pressure and suction forces that can lead to uplift, sliding, racking, and, in severe cases, overturning of structures, according to the Construction Estimating Institute (CEI), Sarasota, Fla. [www.estimating.org/mysafefloridahome]. 

“The suction effect of wind flowing over a roof creates uplift forces that can strip the roof coverings and sheathing or, in extreme cases, overcome the fasteners that connect the trusses or rafters to the top of the supporting walls and destroy the entire roof assembly. These uplift forces increase dramatically when there are

large openings in the walls of the structure,” CEI states in an online training module.

Sliding can occur when the positive wind pressure on walls facing the wind combine with the negative pressure on the walls parallel to or away from the wind. The counteracting forces can rip off fasteners anchoring the floor assembly to the foundation. Racking can be caused by the same combination of pressures, leading to horizontal forces applied to the structure. If the forces are strong enough, diagonal wall bracing, shear walls, and wind bracing can fail, compromising the structure and possibly leading to catastrophic failure.

Projectile damage. For natural projectiles such as hail, the building components most prone to damage are on the roof plane and other horizontal elements. “Hail is the single greatest cause for shingle replacement.” says Joseph Lstiburek, PhD, PEng, a principal with Building Science Corp., Westford. Mass. 

Wind-borne projectiles, however, cause more damage annually than hail. Such projectiles include tree limbs, building materials, and debris—any object not anchored securely to the ground. Shot through the air by heavy winds, these objects instantly “become air-borne missiles capable of breaking through windows, doors, garage doors, or even wall assemblies,” says the Construction Estimating Institute. “Once these missiles have created holes in the building envelope, wind can enter the structure, threatening the roof, and rain can damage the building components and contents, even if the structure remains intact.”

Water infiltration. The Federal Emergency Management Agency and the Florida Home Builders Association have found that most of the damage and insurance claims that resulted from the storms of 2004 were caused not by wind-induced building failure, but by water infiltration.

To understand this better, consider a 1/16-inch crack surrounding a 36-inch window perimeter. Although the crack may seem small, it exposes an opening of nine square inches Consequently, resulting wind-induced pressure differentials created by this hole could contribute to the catastrophic failure of windows, doors, skylights, and even the roof. Suddenly, large portions of the envelope are compromised, and the amount of water that can then enter occupied spaces could be immense.

“A major problem with storm-induced water infiltration is that anything that gets wet is likely to stay wet for a long time,” says the CEI. For example, rain-soaked insulation in ceilings or cavity walls is difficult to remove and, with the extended power outages that occur after major storms, unlikely to be dried out by air-conditioning systems. CEI adds that damp, concealed building components such as insulation, drywall, and carpeting—combined with high ambient air temperatures—will promote mold growth, too.

Fortunately, one important step being taken to better understand and counter wind-related damage is a joint effort between the American Architectural Manufacturers Association (AAMA), Schaumburg, Ill., and the University of Florida to analyze the effects of wind-driven rain on a building mock-up inside a simulator. “This is part of a pioneering effort to measure wind-driven rain at structural height during hurricane landfall,” says Ken Brenden, AAMA’s manager of codes and industry affairs. Ultimately, AAMA plans to share the data with product developers to improve the performance of windows, doors, and wall assemblies in extreme weather.

CODE CHECK: SEEKING STRICTER BUILDING PRACTICES
AAMA’s effort is being reinforced by code groups, consultant associations, and standard-setting organizations. With record numbers of devastating weather events and related property damage in recent years, the codes-and-standards community has been especially vocal in calling for stricter building practices.

According to building scientist Lstiburek, two significant changes related to severe weather have been introduced to the International Building Code (IBC) and the International Residential Code:
1. a new allowance for unvented, conditioned attics for wall assemblies to promote better drying, principally to the interior.

		All Atlantic and Gulf of Mexico coastal areas are subject to hurricanes or tropical storms. Parts of the southwestern United States and the Pacific Coast experience heavy rains and floods each year from hurricanes spawned off Mexico, according to FEMA. The Atlantic hurricane season lasts from June to November, with the peak season from mid-August to late October.
		A radar composite shows Hurricane Andrew making landfall on August 24, 1992, in Dade County, Fla. Image: wea00522, NOAA's National Weather Service Collection

2. a change in vapor barrier requirements, allowing for wall and roof alternatives, also to enable better drying.

(In addition, two new disaster-resistant standards for severe weather are expected for release this year: the International Code Council’s 500 Standard on Design and Construction of Storm Shelters, and ICC 600 Standard on Residential Construction in High Wind Regions.)

Much building damage can be avoided simply designing to code, says John Ingargiola, senior engineer, FEMA Mitigation Directorate Building Sciences Branch, Washington, D.C. “The IBC and the Florida Building Code have advanced and workable design parameters incorporated in them that we know help reduce damage from hurricane events,” he says. “Buildings built after the adoption of these newer codes and then struck by a hurricane have performed much better than those buildings not built to those newer codes.”

In fact, a study commissioned by the Institute for Business & Home Safety measured and compared the performance of homes conforming to newer, wind-resistant building codes in Charlotte County, Fla., during the 2004 hurricane season (The Benefits of Modern Wind Resistant Building Codes on Hurricane Claim Frequency and Severity). What the IBHS discovered was a 44% reduction in total roof covering replacements and a 38% reduction in window glass or frame damage (or both) for structures built after 1996. On the other hand, although the percentage of total soffit failures for more recently built structures was also reduced, as revealed by this study, partial soffit failures were still prominent. Consequently, the Florida Building Code responded, in 2006, by requiring soffits to be designed for the adjacent wall pressures and installed in accordance with engineer and manufacturer  specifications. 

Other recent changes to Florida’s state code include:
• Increased design loads for screen enclosures.
• A requirement for ridge boards to provide a mechanical attachment of edge and ridge tiles on tile roofs, as opposed to mortar.
• Two layers of required waterproofing membrane materials for some systems, to assure drainage planes for water that gets into walls.

Another up-to-date standard is the American Society of Civil Engineers’ ASCE 7-05, Minimum Design Loads for Buildings and Other Structures [www.pubs.asce.org/books/standards/], which has been revised recently to incorporate better methods for determining wind loads on parapets and open structures such as carports and gas station canopies.

But even with all these guidelines and guidance, “Adoption of the current model code does not ensure good wind performance. Rather, the code is a minimum tool that should be used by knowledgeable design professionals in conjunction with their training, skills, and professional judgment,” says Tom Smith, AIA, of TLSmith Consulting, Rockton, Ill., in the Whole Building Design Guide’s section on wind safety for the building envelope.

BUILDING STRUCTURE AND DESIGN
With that in mind, Building Teams can adopt a number of best practices meant to help protect structures and their occupants from the effects of severe weather. Beginning with the building’s structure, FEMA engineer Ingargiola notes that most structural systems, including steel and cast concrete and concrete block, can work well under certain conditions. One common element of all these successes is the foundation, which he describes as “robust in design, well anchored to the supporting soil, and sufficiently redundant to resist collapse, even if impacted with heavy flood-borne debris.” In addition, numerous types of structural framing systems have also stood up well to extreme weather, when they are properly designed and constructed according to relatively recent codes and standards.

	Weathering the storm: FEMA resources for the Building Team The Federal Emergency Management Agency (FEMA) offers the following resources on severe weather and building performance:  A. The FEMA Mitigation Assessment Team’s extensive documentation of successful and unsuccessful building systems in hurricanes, floods and tornadoes, http://www.fema.gov/rebuild/mat/mat.reprts.shtm. B. FEMA data on specific building systems that standing up to tornadoes and extreme winds, accessible at http://www.fema.gov/library/index.jsp: • FEMA 361 Design and Construction Guidance for Community Shelters • FEMA 431  Tornado Protection: Selecting Refuge Area in Buildings  C. FEMA guidance on building envelope and rooftop equipment wind performance: • FEMA 488 Mitigation Assessment Team Report: Hurricane Charley in Florida • FEMA 489 Hurricane Ivan in Alabama and Florida: Observations, Recommendations and Technical Guidance • FEMA 549 Hurricane Katrina in the Gulf Coast: Mitigation Assessment Team Report, Building Performance Observations, Recommendations, and Technical Guidance
	According to FEMA, hurricanes are classified into five categories based on their wind speed, central pressure, and damage potential. Category Three and higher hurricanes are considered major hurricanes, though Categories One and Two are still extremely dangerous. Hurricane winds can exceed 155 miles per hour. Hurricanes and tropical storms can also spawn tornadoes and microbursts, create storm surges along the coast, and cause extensive damage from heavy rainfall, says FEMA.

With regard to siting and orientation of coastal buildings, Ingargiola recommends building far enough away from the water line that any large waves, as well as the scour and soil erosion that comes along with tidal surge, have sufficient distance to lose energy before striking the facility. Ideally, the building should be elevated, carefully taking into consideration door and window heights. Ingargiola also notes that, “under rainfall conditions, doors that are located above deck or walking surfaces by a few inches will not usually experience as much wind-driven rain under door sills and around door jambs.”

To protect coastal property against storm surge, “It is critical that the building be elevated above the waves that may be generated as part of that surge action and to make sure that the building is on an open foundation where the water can flow as freely as possible under the building,” says Timothy A. Reinhold, PhD, PE, director of engineering and vice president with the Institute for Business & Home Safety. “It is also important that the foundations have deep enough piles to keep the building from settling if there is some erosion around the foundations. If the walls of the building are hit by waves, most light-frame systems will be demolished if the waves get higher than about one-and-one-half feet against the bottom of the wall.”  

When it comes to strong winds, one effective mitigating approach is strategic landscaping. “Architectural design has a big impact on whether the building catches the wind and can have a big impact on wind damage,” says Lstiburek. According to a construction trade group based in the “tornado alley” of Topeka, Kan., properly selected and placed landscaping can provide excellent wind protection. For example, evergreen trees and shrubs planted to the north and northwest of a building are common wind protection, and can reduce wind speed for a distance of as much as 30 times the windbreak’s height.

In order to maximize protection, the Topeka group recommends planting windbreaks at a distance from the structure of two to five times the height of the trees at maturity. Also, to prevent landscaping damage during fierce winds, large broken branches should be cut all the way back to the main branch.

At the same time, consultant Tom Smith cautions against trees greater than six inches in diameter anywhere near the structure. “Blow-down of large trees, poles, and towers can severely damage a building and injure occupants,” he says.

As for the facility itself, design decisions such as roof shape are also important. For instance, Lstiburek observes that in flat areas devoid of trees, hip roofs shaped like pyramids are common, as high winds are more likely to blow over the top of the building, creating less of a structural threat. However, specifying such elements as roof overhangs is not quite so straightforward.

“Overhangs are good for sheltering the structure from rainwater, but they’re bad because they catch the wind. Consequently, they have to be very strong,” says Lstiburek. “It’s like Goldilocks—not too big, not too small, but just right. A good rule of thumb for residential construction is 18 inches of overhang per story in height.”

Similarly, the IBHS’s Reinhold points out that along with overhang’s benefits come vulnerabilities. In cold climates, he says, “They can create a colder region near the bottom of the roof where water from snow melt can re-freeze if the roof over the interior of the house is warmer than that along the overhang.”

FEMA’s Ingargiola also suggests physically protecting openings in the building envelope with products like impact-resistant shutters or glazing. He notes, however, that “locating large, unobstructed-view glazing in windows away from a primary wind direction could reduce potential glass breakage from wind or wind-borne debris. However, hurricane-force winds can come from any direction, and thus there may not be a ‘primary direction.’”

SPECIFYING BUILDING SYSTEMS
The building systems and components most vulnerable to severe weather damage, and consequently the most important to design and specify correctly, are the roof, windows, and doors.

“Because roof covering damage has historically been the most frequent and costly type of wind damage, special attention needs to be given to roof system design,” says consultant Tom Smith. The IBHS’s Reinhold advises that “a high wind-rated roof cover is essential in hurricane prone regions. 

Further, as the roof cover ages, it may become more vulnerable to damage when a hurricane strikes. Consequently, we recommend a backup plan that provides a barrier that is protected from aging by the roof cover and will keep water out if the roof cover is damaged.”

For additional design guidance, the Florida Division of Emergency Management’s Bureau of Mitigation’s Hurricane Retrofit Guide delineates four key areas for strengthening roofs:
1. Improve the condition of the roof structure, especially bracing gable ends and reinforcing the connection between the trusses or rafters and the supporting wall.
2. Ensure that roof deck sheathing is sound and nailed (or otherwise fastened) according to the appropriate standard for the basic wind-speed zone where the building is located.
3. Check that all underlayment beneath the roof covering is sound, fastened appropriately, and can function as a secondary water barrier if the roof covering fails.
4. Make sure the roof covering is appropriate for the basic wind-speed zone, is sound, and is appropriately fastened to the roof deck.

Lstiburek also stresses the importance of installing air barriers in the roofing system to keep the wind out. On this point, he says, adhered systems—such as spray polyurethane foam and other spray-applied membranes—often provide more reliability as barriers during high winds, as opposed to mechanically adhered membranes.

As for protecting soffits against wind-driven rain, in cases where they extend more than 18 inches from the wall, IBHS recommends installing sharp-pointed stainless-steel screws or other comparable fasteners every 12 inches through the soffit material and into its supports. The contractor should then apply a bead of polyurethane sealant or a comparable material along the joint between the edge of the channel and the wall, and apply sealant in the grooves where any fascia material butts up against the fascia and wall channel.

“If the soffit is blown away and the wall does not extend to the roof deck, wind-driven water can be blown into attic spaces and lead to collapsed ceilings and interior water damage,” cautions Smith.

Windows. According to the Construction Estimating Institute, one of the best ways to protect against wind-borne debris is to use impact-resistant glazing. The reason such protection is so critical is that when windows fail, the wind that penetrates inside the structure creates a buildup of internal pressure, which can eventually lead to different levels of structural failure. When this occurs, the building becomes vulnerable to water intrusion, possibly resulting in considerable economic loss.

“Windows are a first line of defense against windstorm damage,” says the Institute for Business & Home Safety. “When windows are lost, the full force of the wind and rain enters, destroying interiors and contributing to the loss of the roof and walls.” While roll-down and swinging storm shutters may also provide a level of defense—and are considered one of the easiest, most effective methods of protection—they have not yet been tested for water penetration in the way that impact-resistant windows have. Consequently, they should not be relied upon as the sole safeguard.

While still under development, the ICC’s new prescriptive hurricane standard for low-rise construction in high wind regions is expected to be an excellent guide for specifying glazing products, says AAMA technical director John Lewis. He also notes that while the standard is residential in scope, there is a strong possibility that the ICC consensus committee currently working on the standard will eventually address commercial buildings as well.

In the meantime, IBC 2006 does reference SBCCI SSTD 10-99, Standard for Hurricane Resistant Construction, which is the most current, widely adopted, and enforced document for hurricane-resistant construction of nonresidential buildings.

Lewis also recommends two documents from AAMA that address glazing and may also be of use to specifiers: AAMA GAG-1-97, Glass and Glazing, which is a guide to architectural glass types and applications as well as structural, acoustical, and thermal performance characteristics; and AAMA GDSG-1-87, Glass Designed for Sloped Glazing, which outlines design considerations necessary for choosing the proper glass for skylight and other angled glazing applications and describes the minimum requirements for sloped glazing as specified in the major model building codes.

Doors. Yet another potential weak spot in the building envelope is the door. Consequently, all aspects surrounding door specification—the frame, anchorage, material choice, and sealing—must be carefully evaluated. To assist the specifier, Florida’s Division of Emergency Management offers the following tips:
• Consider the strength of the door itself with regard to its ratings of impact resistance against wind-borne debris.
• Determine if the anchorage of the door is adequate. This includes the hinges, the lockset, and the deadbolt.
• Evaluate the door frame itself. It should be sufficiently anchored to the frame so that hurricane force winds will not easily separate the two.
• Consider the door’s ability to seal against potential water intrusion.
• Avoid doors that are not rated for wind pressure or impact resistance. For example, wood doors with raised panels mounted in wood frames are rarely rated for such effects. Similarly, just specifying a metal door may not provide adequate design strength. Many metal doors are mounted in wood frames and have hollow form cores.
• Some shutters on exterior doors often allow enough leakage around the opening to cause the doors to burst open.

Some other design recommendations put forth by the Washington, D.C.-based Partnership for Advancing Technology in Housing include:
• Installing a deadbolt that penetrates well into the stud framing of the structure.
• Improving hinge strength by replacing hinge screws with longer screws that penetrate well into the stud frame.
• Installing additional head and foot bolt locks at the top and bottom of the door.

“Another solution, and probably the best one, is to replace a non-rated or weak door,” says the Construction Estimating Institute. “This may not be the most economically viable option, but it will yield the best results in the event of a strong storm. Installing new wind- and impact-resistant doors will also offer the additional advantage of leak resistance, as most new doors are designed with better seals.”

AAMA’s John Lewis recommends that buildings in high-velocity hurricane zones specify doors and windows that comply with AAMA 506, Voluntary Specifications for Hurricane Impact and Cycle Testing of Fenestration Products. “In other hurricane areas where impact glazing is not required, ensure that the product is certified and rated to resist the wind loads it will encounter during a hurricane,” he adds.

Consultant Smith further recommends keeping rain-screen principles in mind when designing non-load-bearing walls, doors, and window frames. “Although prevention of building collapse and major building damage is the primary goal of wind-resistant design, consideration should also be given to minimizing water damage and subsequent development of mold from penetration of wind-driven rain.”

KEY DETAILS: FLASHING, SEALANTS, AND GASKETS
“If you want to save cash, flash,” says Building Science Corp.’s Lstiburek. His comment is a reminder of the importance of carefully applying flashing, sealants, and gaskets to prevent damage from storms and severe weather.

“It’s important to understand that windows and doors will most likely leak water during a hurricane,” cautions the IBHS’s Reinhold. “Consequently, it is important to have a water-management plan that will minimize the damage that the water intrusion may create. This means providing good flashing details around windows and doors, a drainage plane that will direct any water that does enter the wall to the outside of the building, and to make sure that any water that leaks into the interior can be cleaned up without extensive damage to interior finishes.”

In general, Lewis recommends installing fenestration products in strict accordance with the manufacturer’s installation instructions, which often requires flashing. He notes that AAMA 711-07, Voluntary Specification for Self-Adhering Flashing Used for Installation of Exterior Wall Fenestration Products, establishes the test methods and minimum performance requirements for self-adhering flashing products that are used around the perimeter of openings. “It also provides a method to determine the minimum width of the flashing products and to evaluate the influence of the environmental factors on the installation of self-adhering flashing products applied under typical field conditions,” says Lewis.

As for sealants and gaskets, Lewis recommends that specifiers refer to AAMA 800-07, Voluntary Specifications and Test Methods for Sealants, a compilation of standards and test methods for determining the performance of both compounds and tapes used in the manufacture and installation of windows, sliding glass doors, and curtain walls. The guideline specification covers back bedding compounds and mastic tapes, glazing tapes, narrow joint seam sealers, exterior perimeter sealing compounds, non-drying sealants, and expanded cellular glazing tapes.

CONCLUSION: REDUCING PROPERTY LOSS
Although factoring severe weather into the design, new construction, and retrofitting of facilities can entail time, effort, and expense, the benefits are significant. “Buildings that don’t blow away, rot, or rust, and that perform better from an energy perspective, can earn lower insurance rates,” says building scientist Joe Lstiburek.

Moreover, the reduction in property loss from well-designed and constructed buildings can provide an exponential benefit to the neighborhood and adjacent properties. “There is less debris that requires disposal and a faster recovery for the community,” says FEMA’s Ingargiola. “Citizens are back to work faster, generating revenue for the community in the form of taxes paid and services rendered—and children are back to school faster.”

But it’s not just a matter of good upfront design and construction. Rather, sound facility maintenance and operations must also be key components in the overall plan. “The benefits of doing so are that well-designed, properly installed, and well-maintained buildings are less susceptible to damage during [severe weather] events,” says the FEMA engineer.

While praising the building and construction community for many recent advances in the arena of weather-resistant design—design guides, improved model codes, new test methods, new materials and systems, and more research and education—Smith cautions that “Although these developments have been significant, the need for further development in all of these areas is immense.”

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.

Take the AIA Exam

This BD+C continuing education program qualifies for 1 AIA HSW learning unit.

Reed Business Information is a Registered Provider with the American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members are available on request.
       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.

Average Rating:

(0)

Rate this:

Talkback

We would love your feedback!

Post a comment

» View All Talkback Threads

Related Content

• Topics
• Author
• Top Rated

No related content found.

Also by this author

• Blue ribbon panel unveils recommendations for NYC's green building codes

• Tufts University puts bite into new dental school addition

• A few bright spots for AEC firms in 2010

• Sustainable features central to independent-living building

• Roof panels make solar easy

»MORE

• 40 Under 40

• Housing America's Heroes 7 Trends in the Design of Homes for the Military

• The Enablers

• New data shows low construction prices may soon be coming to an end

• ABI drops more than three points in November

Sponsored Links