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TECHNICAL NOTE 1476 Performance of Physical Structures in Hurricane Katrina and Hurricane Rita: A Reconnaissance Report (June 2006) "In coastal areas and in New Orleans, storm surge was the dominant cause of damage. Storm surge heights, in general, exceeded the levels defined by existing flood hazard maps as well as historical records. While design provisions exist to address storm surge and flooding, existing flood hazard maps which provide the basis for design of structures are outdated and not consistent with the risks posed by storm surge in these coastal areas. Better definition of the storm surge hazard is required to appropriately apply existing design provisions and elevation levels to mitigate the effects of storm surge on buildings and residences." (Executive Summary, p. xxxi) "In many areas, there were very distinct patterns of damage. In the coastal areas exposed to the direct impact of the storm surge, damage was extreme. Extending about one-quarter mile inland, nearly all of the single-family residential structures were completely destroyed by the impact of the moving water, leaving only foundations and other rigid masonry components in place. Even much of the debris had been washed away. The only exception was when storm surge height did not exceed foundation elevations, the performance of elevated residential structures was noticeably better. Farther inland, surge flooding caused massive damage, pushing homes off their foundations and moving them some considerable distances. This level of damage was generally found in the downtown areas of larger coastal communities such as Biloxi and Gulfport, Mississippi." (Chapter 5, p. 143) "Residential structures subjected only to wind sustained less structural damage than those subjected to storm surge and flooding. Structural damage, such as gable end roof damage and loss of roof sheathing, was observed infrequently. Overall, among the main exterior components of residential construction, roof coverings experienced considerable damage; in contrast, claddings, windows, soffits, porches, doors, and garage doors generally sustained relatively minor damage compared to roofing. For residential structures that suffered modest or no visible damage to the weather-resistant envelope due to wind, no assessment of wind-driven rain penetration could be made in most cases. This type of damage often results in significant economic losses." (Chapter 5, p. 144) "Overall, with few notable exceptions, residential structures exposed to impact forces of the storm surge did not survive, while structures that were somewhat protected from the force of the moving water survived but in many cases sustained major damage. For the most part, destruction of single-family dwellings in coastal regions was limited to areas affected by the impact of storm surge flow and wave action. The nature and scope of the damage was similar in communities along coastal Alabama, Mississippi, Louisiana, and Texas. With few exceptions, non-elevated structures (i.e., houses built at grade level immediately adjacent to the coast) were completely destroyed (see Figs 5-1, 5-2, and 5-3). The surviving structures were, invariably, severely damaged with little prospect for restoration. Some had enough residual strength to remain standing, but most did not. In some cases, the upper levels of multi-story residential structures appeared to have sustained little damage, even though the ground level and the first floor above ground level were heavily damaged by the surge (Figure 5-4)." (Chapter 5, p. 144) "In the coastal areas just beyond the direct surge impact zone, the water generally rose above the roofs of most residential structures, causing extensive damage to interiors even when major structural damage was not immediately evident. In those areas, floodwater was predominantly clean seawater that had not picked up large amounts of silt or other contaminants, and many buildings showed few outward signs of having been flooded. In such coastal areas, observations by the NIST reconnaissance team indicated that the clean floodwater and the relatively short duration of flooding apparently15 helped to keep damage levels lower than those in and around New Orleans where floodwaters remained for much longer periods of time. Nevertheless, contents, interior finish, appliances, and mechanical/electrical equipment damage due to flooding was commonplace and extensive for residential structures experiencing predominantly low velocity flooding." (Chapter 5, p. 151) "Wind damage observed throughout the hurricane impact zone was far less severe than damage caused by storm surge and flooding. With few exceptions, severe wind damage was not observed. Even immediately adjacent to the coast, relatively little residential structural damage appeared to be attributable to loads imposed by wind. Data indicate that the wind speeds in both hurricanes were, in general, less than the design wind speeds specified by the International Residential Code [1] and ASCE 7 [2], and field observations support that conclusion (see Chapter 2). Residential structures subjected only to wind loads sustained relatively minor damage to roofing materials, siding, windows, soffits, porches, doors, and garage doors. Structural damage, such as gable end roof damage and loss of roof sheathing, was observed infrequently. An example is seen in Figure 5-15, which shows an apartment building in Waveland, Mississippi. In the few cases where wind loads caused major damage, the buildings tended to be older and somewhat deteriorated. A few older buildings in relatively low wind speed zones (e.g., Pascagoula, Mississippi, see Chapter 2) had their roofs blown off; however, such cases were rare. For the most part, the scope of the wind damage steadily decreased with distance from the coastline. Isolated exceptions were noted where damage was substantial when residential structures were situated adjacent to large open areas. "There appears to be an imbalance between the policies and practices used in evaluating coastal flood hazards and those used for wind hazards. Field observations indicate that the magnitude of the forces associated with storm surge and flooding significantly exceeded the potential wind-induced loads addressed in modern building codes and engineering standards. Buildings, especially conventional residential structures, designed and constructed to resist wind damage in coastal environments sustained little damage due to wind loads. Those buildings were much more likely15 to have been severely damaged or destroyed by wave impacts and flooding. Currently, buildings are required to be placed on foundations raised to the 100-year flood elevation and the foundation to resist forces from a 100- year flood event." (Chapter 5, p. 171) "Damage due to wind loading was not unusual or nearly as severe as damage caused by storm surge, waves, and flooding. Damage due to hurricane winds was observed to roofing materials, siding, windows, soffits, porches, doors, and garage doors. Windborne debris did not contribute significantly to overall damage, though various forms of debris damage were observed." (Chapter 6, p. 181) |
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