When they strike, hurricanes can subject buildings to winds over 150 mph (241 km/h), imposing large lateral and vertical uplift forces. The impact of high winds on a particular building depends on a number of factors, including:
- The wind speed and duration
- Building height (wind loads increase with height above grade)
- How exposed the building is to the prevailing winds (a building on an open plain is subjected to higher loads than one surrounded by buildings or other obstructions)
- The strength of the building elements and connections
- The building's shape, and
- Sizes and locations of openings such as windows and doors.
In the fall of 2005, residents of New Orleans and the entire Gulf Coast region of the United States were subjected not only to high wind events from Hurricanes Katrina and Rita, but also the severe and highly destructive effects of storm surges that wiped buildings completely off their foundations and tore gaping holes in the sides of others.
A team from The Masonry Society arrived in the Gulf States shortly after Hurricane Katrina to investigate the effects of both high winds and storm surges. The four-man team included trained volunteers from the engineering, construction, materials, and forensic communities. They focused on modern building codes and recommended construction practices to protect structures and their inhabitants from events such as hurricanes. Beginning in Mobile, Ala., the team moved westward across the Mississippi coast through the hurricane-ravaged areas of Pascagoula, Biloxi, Long Beach, and Gulfport, and into parts of Louisiana.
As with other hurricanes, the team noted that building failure during high winds often begins with a relatively minor failure. For example, if one roof fastener pulls out or breaks, the load on the surrounding fasteners increases, often leading to the progressive failure of the system. A portion of the roof can be pulled away from the walls in this way, leaving unsupported walls that were designed to be supported by the roof. In fact, the most common source of failure in high-wind events is wind uplift on the roof.
To combat this threat of progressive failure, the building must be designed and constructed to maintain structural integrity during high winds. This requires that the buildings incorporate continuous paths that allow structural forces to be transferred from the roof to the underlying soil, where the forces are dissipated. These continuous load paths allow wind forces on the roof to be properly distributed to the walls and then the foundation. This structural continuity relies heavily on properly sized, located, and installed connectors between the building's various structural elements.
How are improvements in building practices identified? When catastrophic events, such as hurricanes, damage structures, an investigation should be conducted before clean-up efforts begin, to collect forensic data and determine how well the structures performed. Quick-response field studies following major disasters are vital to identify knowledge gaps where further research and building code revisions are most needed. Such investigations also offer opportunities to make recommendations on mitigation measures, disaster preparedness, and emergency responses to better prepare communities for future disasters.
The team noted a vast improvement over structures built as recently as ten years ago. Clearly, the knowledge gained from prior investigations of other high wind and storm surge events provided experts the basis for recommending construction improvements. Each event of this type, studied thoroughly, adds to an ever-growing body of knowledge that forms the basis for recommending new code provisions and building practices to protect occupants and property.
Along the Gulf Coast, examples of masonry homes left standing were noted while adjacent homes were washed away. In other cases, the storm surge wiped out all structures in its path. A concrete masonry K-Mart in Gulfport, Miss., was left intact with the exception of its glass entrance, while the steel-stud structure of the attached adjacent shops was stripped clean of its siding material. Those modern masonry structures that did not perform as desired often appeared to be not constructed to code requirements-highlighting the need for both education and inspection.