Hurricane Andrew—10 years later

• Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.
• Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.
• Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.

On Monday morning, Aug. 24, 1992, Hurricane Andrew struck south Florida. With peak wind gust speeds up to about 175 mph (78 m/s), it was the third-strongest hurricane to hit the U.S. mainland in recorded history. The storm's center made landfall near Homestead, a community in Dade County about 25 miles (40 km) south of Miami. Fortunately, though it was strong, the storm's diameter was relatively small. Hence, Miami's downtown did not experience exceptionally high winds, and damage within the downtown area and areas to the north was minimal.

The storm had a forward (translational) speed of 18 mph to 20 mph (8 m/s to 9 m/s), which is relatively fast for a hurricane. It crossed the Florida peninsula in about four hours and still was a major hurricane when it entered the Gulf of Mexico. On the morning of Aug. 28, 1992, the hurricane made landfall in a relatively unpopulated area about 40 miles (60 km) from Lafayette, La., which is about 100 miles (160 km) from New Orleans, thus sparing New Orleans from the threat of catastrophic flooding.

Because of the hurricane's small diameter and fast translational speed, flooding was minor. With limited flooding, advanced warning of the approaching storm and a lot of luck, the death toll was surprisingly low for a storm that delivered extremely high wind speeds; there were 15 deaths in Florida, eight in Louisiana and three in the Bahamas. Although Hurricane Andrew spared lives, it did not spare property; it was the costliest hurricane and most expensive natural disaster in U.S. history. Property damage was estimated to be $40 billion (in 2000 dollars). The number of homes destroyed totaled 25,524, and 101,241 homes were damaged. As a result of Hurricane Andrew, several insurance companies were unable to cover their insured losses and declared bankruptcy.

Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.

A precast concrete twin-tee deck panel was lifted and thrown back toward the center of this roof. Three large roll-up doors collapsed, which increased the internal air pressure in the building and uplift load on the deck. Although precast deck panels are heavy, they need to be anchored to resist large uplift loads.

The effects on people in the damaged areas only can be partially measured by the cost of damaged property. Emotional costs experienced were associated with lost memorabilia, temporary living arrangements, disruption of schooling, and disruption or loss of jobs because businesses were shut down because of damage. Many businesses never reopened.

The response

The damage's magnitude and large number of people affected overwhelmed the local disaster response entities' resources. The U.S. Federal Emergency Management Agency (FEMA) was harshly criticized for its slow response.

In Dade County, eventually more than 20,000 military personnel set up tent camps for emergency housing, distributed food, assisted with debris removal and guarded businesses to prevent looting. A positive legacy of Hurricane Andrew is that FEMA recognized its shortcomings and has responded more quickly to subsequent disasters.

As the hurricane left south Florida, an enormous number of roof systems were in need of replacement or repair. In most instances, water was leaking into buildings and causing further damage and disruption. Roofing contractors were inundated by telephone calls for assistance. During the next several months, a large number of roofing crews diligently worked long hours.

In addition to long hours, roofing contractors were faced with interim changes to the local building code. Amendments frequently were issued in the weeks and months after Hurricane Andrew to try to address deficiencies in the South Florida Building Code (SFBC) that existed before the storm.

Building owners who obtained professional contractors' services were grateful to have new or repaired roof systems. But many unscrupulous people representing themselves as roofing contractors flocked to south Florida. There were numerous instances of homeowners who paid a substantial amount of contract prices before work commenced and then never saw the contractors again.

The findings

The magnitude of damage also resulted in extensive evaluation of buildings' wind performances. Hurricane Andrew was the most extensively investigated storm in U.S. history, and NRCA was a member of one research team. The results of NRCA's research were published in several conference papers and Professional Roofing articles (click here for a complete list of papers and articles related to Hurricane Andrew).

Photo courtesy of TLSmith Consulting Inc., Rockton, Ill.

Hurricane Andrew severely damaged this structural standing-seam metal panel roof system.

Buildings' structural systems (beams or columns) generally performed well though there were a number of structural failures in residential construction; manufactured housing; and marginally engineered buildings, such as strip malls and warehouses.

Damage investigations revealed that most building damage was caused by building envelope failure. The leading cause of envelope damage was blow-off of roof coverings and puncture of roof coverings by wind-borne debris followed by glass breakage (typically caused by wind-borne debris) and collapse of large doors (such as garage doors and roll-up doors). With the widespread building damage, even a small breach in a building envelope typically led to significant interior water damage because of the great amount of time it took to perform emergency repairs on most buildings.

With a storm as strong as Hurricane Andrew, a lot of damage is expected. But the magnitude of damage surprised many because Dade County had been renowned for having a strong building code in terms of wind-resistance requirements. But in the scrutiny that followed the storm, it was discovered SFBC's wind-load requirements for building envelopes were grossly inadequate. The building department's inspections also were found to be inadequate. Soon after the storm, SFBC adopted ASCE 7-88, "Minimum Design Loads for Buildings and Other Structures," for wind loads. Eventually, SFBC extensively was revised. Some provisions of the new code should help improve wind performance of buildings designed and constructed in accordance with it.

For example, Dade County became the first area in the United States to require exterior windows and glazing in doors to resist debris impact. The importance of glazing resistance to wind-borne debris had long been known and advocated, but building officials were not receptive to adding protection requirements in the model building codes (The BOCA National Building Code [BNBC], Standard Building Code [SBC] and Uniform Building Code). Many provisions in the revised code were controversial and placed substantial burdens on manufacturers, designers and contractors.

Although SFBC had significant shortcomings, poor design and construction practices played key roles in buildings' poor wind performances. Inadequate maintenance also was a contributing factor in some cases.

Three years before Hurricane Andrew, Hurricane Hugo caused significant damage in South Carolina and several Caribbean islands, including the U.S. Virgin Islands and Puerto Rico. A comprehensive report about roof system performance was published by Texas Tech University, Lubbock, in cooperation with NRCA. The report alerted the roofing industry to the need for improved wind performances of roof systems. If Hurricane Andrew had not struck, perhaps the Hurricane Hugo experience would not have been sufficient enough to cause much change in terms of roof system wind performance. But the punch delivered by Hurricanes Hugo and Andrew within a three-year period had profound effects—within Florida and the rest of the United States.

Developments

With respect to wind performances of roof systems, a lot has been accomplished during the past 10 years. Not all accomplishments are directly attributable to Hurricanes Hugo or Andrew (such as some work related to asphalt shingles and mechanically attached single-ply membranes, which already was under way). But these two powerful hurricanes provided a great impetus for improved wind performances of roof systems. Notable accomplishments include the following:

• The roofing chapter in the International Building Code (IBC) now refers to the structural design chapter for wind loads on a roof system. At the time of Hurricane Andrew, none of the model building codes provided a link between the roofing chapters and wind-load sections of the structural design chapters. As a result, many people thought the wind-load provisions in the structural chapter did not pertain to roof coverings, and, therefore, did not calculate wind loads on roof coverings. The roofing chapter in IBC also prescribes certain test methods for determining the wind resistances of several roof system types.
  
  
• After Hurricane Andrew, BNBC and SBC included amendments that linked their roofing chapters to their structural chapters and prescribed certain test methods (these provisions became the basis for IBC requirements previously mentioned).
  
  
• IBC adopted ASCE 7 for determining wind loads on buildings. The model building codes previously allowed the use of ASCE 7 for low-rise buildings and required ASCE 7 for tall buildings. Ten years ago, few designers (including structural engineers) were familiar with ASCE 7, but it has become the de facto standard for determining wind loads in the United States. Dade County's quick adoption of ASCE 7 after Hurricane Andrew likely facilitated widespread acceptance of ASCE 7.
  
  
• The 1995 edition of ASCE 7 specifically addressed roof coverings. For the first time, the 2002 edition will provide criteria for determining wind loads on rooftop equipment. In 2002, FM Global began using ASCE 7 as its basis for wind loads in its loss-prevention data sheets.
  
  
  

  These clay tiles were mortar-set. Three failure modes are illustrated: Separation of the mortar from the cap sheet, separation of the tile from the mortar and debris impact damage. On the opposite side of this roof, a fourth failure mode occurred where the built-up membrane and tiles adhered to the membrane blew off.

• SPRI developed ANSI/SPRI ES-1, "Wind Design Standard for Edge Systems Used with Low Slope Roofing Systems." In addition to providing a methodology for determining wind loads, ANSI/SPRI ES-1 contains test methods for determining wind resistances of metal edge flashings and copings. ANSI/SPRI ES-1 was adopted by IBC in 2001.
  
  
• NRCA modified some metal edge flashing and coping details in The NRCA Roofing and Waterproofing Manual, Fourth Edition, to enhance wind resistance. Some details have been tested by FM Approvals and Intertek Testing Services/Warnock Hersey, Middleton, Wis. More information about NRCA and ITS testing can be found in "NRCA receives an approval listing," February 2001 issue, page 100, and "FM Research approves NRCA details," March issue, page 64.
  
  

In addition, several important developments have occurred with respect to test methods. For example, before Hurricane Andrew, FM Approvals only rated roof systems up to 90 pounds per square foot (4.3 kPa); testing no longer is limited to that amount. FM Approvals tested mechanically attached single-ply roof systems on a 5- by 9-foot (1.5- by 3-m) test frame. With large row spacings, much of the test load actually was resisted by the test frame; this frame size can overestimate roof system resistance. A 12- by 24-foot (4- by 7-m) test frame now is used for most mechanically attached roof systems. The larger frame also is used for all roof systems when a rating in excess of 1-90 is sought, metal roof systems, roof systems installed over lightweight insulating concrete decks and all roof systems that incorporate air retarders.

Other organizations realized changes needed to be made to test methods. For example, the National Research Council Canada, as part of a consortium-sponsored multiyear research project by sponsors from Canada and the United States, developed a dynamic test method for mechanically attached modified bitumen and single-ply roof systems. A dynamic test method also is being developed by FM Approvals. In addition, the Metal Building Manufacturers Association now advocates ASTM E1592, "Standard Test Method for Structural Performance of Sheet Metal Roof and Siding Systems by Uniform Static Air Pressure Difference."

A new load calculation procedure and test methods for tile were incorporated into SBC and IBC. As a direct result of how poorly mortar-set tile roof systems performed during Hurricane Andrew, a new method for attaching tiles using specially formulated spray polyurethane foam adhesive was developed and has become widely used in south Florida.

In addition, a new test method, ASTM D6381, "Standard Test Method for Measurement of Asphalt Shingle Tab Mechanical Uplift Resistance," was developed to determine the bond strength of asphalt shingles (bond strength is a key factor in wind resistance). A new load calculation procedure is nearing completion, and it will be published by Underwriters Laboratories Inc.

Since Hurricane Andrew, the available bond strength greatly has increased for many shingles because of enhancements in the strength of self-seal adhesive. Other enhancements include more optimized sealant location (moving the sealant closer to a shingle's leading edge) and the use of a double row of adhesive.

Finally, the 2000 edition of FEMA's Coastal Construction Manual: Principles and Practices of Planning, Siting, Designing, Construction, and Maintaining Residential Buildings in Coastal Areas provides a number of design recommendations regarding roof systems in high-wind regions. (For more information about the manual, see "Constructing for the coast," March 2001 issue, page 24.)

Although there still are many needs with respect to wind performance of roof systems, much has been accomplished during the past 10 years. If the new information and tools are put to use, more reliable wind performance should be experienced.

Hurricane Andrew's legacy

Hurricane Andrew's power and destructiveness assured it a prominent place in the history of hurricanes. For those who experienced the terror of its ferocious winds and those who repaired and rebuilt the damaged and destroyed buildings, Hurricane Andrew will not be forgotten. However, its greatest legacy may be that it taught us to more carefully design, construct and maintain roof systems to resist design wind loads in hurricane-prone and nonhurricane-prone areas. If we use the knowledge and tools developed since Hurricane Andrew and continue to pursue the subject of wind performance of roof systems, eventually the communitywide wind resistances of roof systems should increase. If so, future strong wind events will cause less damage, less injury to occupants and less emotional trauma.

As in the past, major hurricanes will again strike along the coasts of the Atlantic Ocean and Gulf of Mexico. But in the years ahead, many more people will be living in these areas. As people look back years from now, it is hoped they will see Hurricane Andrew's legacy resulted in practices that minimized damage to roof systems. The roofing industry's response, whether through the actions of contractors, manufacturers, designers or building owners, can shape the legacy of Hurricane Andrew. If we learn, implement and improve on the lessons and developments from Hurricane Andrew, the legacy can include some positive attributes.

Thomas L. Smith is president of TLSmith Consulting Inc., Rockton, Ill. Smith performed extensive research in south Florida during the aftermath of Hurricane Andrew and wrote several papers and articles about wind performance of roof systems.

The coverage

Professional Roofing magazine extensively has covered Hurricane Andrew and wind performances of roof systems. Following are some articles written by Thomas L. Smith, president of TLSmith Consulting Inc., Rockton, Ill., related to the topics:

"Performance of mechanically attached single plies—Part II," April 1996 issue, page 16

"High-wind performance of mechanically attached single-ply roof systems," February 1996 issue, page 18

"Insights on metal roof performance in high-wind regions," February 1995 issue, page 12

"Tile performance in hurricane regions—Part II," December 1994 issue, page R4

"Insights on tile performance in hurricane regions," September 1994 issue, page R6

"Hurricane Andrew provides insights on roof damage," March 1994 issue, page 36

"Design guidelines for wind-resistant roofs on essential facilities," October 1993 issue, page 24

"How did PUF roofs perform during Hurricane Andrew?" January 1993 issue, page 20

Additional information also is available by reading the following:

"Aggregate Blow-Off from BUR and SPF Roofs: Recognizing the Potential Hazards and Avoiding Problems," proceedings of the Eight U.S. Conference on Wind Engineering, June 1997

"Improving Wind Performance of Asphalt Shingles: Lessons from Hurricane Andrew," proceedings of the 11th Conference on Roofing Technology, September 1995, page 39

"Improving Wind Performance of Mechanically Attached Single-Ply Membrane Roof Systems: Lessons from Hurricane Andrew," proceedings of the IX Congress of the International Waterproofing Association, April 1995, page 321

"Causes of Roof Covering Damage and Failure Modes: Insights Provided by Hurricane Andrew," proceedings of the Hurricanes of 1992, American Society of Civil Engineers, 1994, page 303

"Improving Tile Wind Resistance: Lessons from Hurricane Andrew," proceedings of the Second International Congress of Roof Technology in Argentina, August 1994

"Preliminary Design Guidelines for Wind-Resistant Roofs on Essential Facilities," proceedings of the Seventh U.S. National Conference on Wind Engineering, 1993, page 709