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Mitigation Assessment Team Report Hurricane Katrina in the Gulf Coast July 2006 "Severe flood damage occurred to one- and two-family residential buildings throughout the study area. In areas close to the shoreline, the damage was principally a result of waves, velocity flow, and floodborne debris, while in areas distant from the shoreline damage was primarily a result of inundation by storm surge. Since Katrina's flood elevations greatly exceeded mapped BFEs [Base Flood Elevations], flood damage was more extensive and severe than would be expected from a design level flood event." (p. 3-5) "Single-family and other light-frame buildings are generally incapable of resisting coastal flood loads and, therefore, are designed to avoid those flood loads through elevation above the design flood level (including wave effects), and by limiting flood loads to the building foundation. In coastal areas, foundations must be designed to resist wave forces, wave-induced erosion and localized scour, and floodborne debris, all of which can threaten the stability of the foundation (and the building). Thus, the foundation type makes a significant difference in the ability of the structure to resist a variety of flood conditions and flood loads. Where Hurricane Katrina's storm surge level exceeded the lowest floor level and waves were present, virtually all of the buildings were destroyed or heavily damaged, regardless of foundation type. However, some foundation types exhibited clear advantages during Katrina, such as those buildings constructed with foundations that are integral to the structural building frame." (p. 4-4) "Where Katrina's breaking waves rose above the foundation and impacted floor beams and walls, most houses quickly disintegrated into debris. Residential buildings that survived Katrina's worst storm surge and wave conditions typically had heavier than normal open foundations that were part of the building's structural frame. Examples of other surviving buildings that had foundations that are integral to the structural frames include steel-framed buildings in Mississippi that survived storm surge and wave action above the first floor level, wood-framed buildings along Mississippi's Jourdan River, and houses with reinforced concrete frames and walls in Long Beach, Mississippi. These houses, though heavily damaged, survived next to destroyed houses on slab, pier, or pile foundations that had first floor elevations below the wave elevation." (pp. 4-10 and 4-11) "The most notable building envelope issues pertaining to Hurricane Katrina were the wide-spread poor performance of asphalt shingles, vinyl siding, and exterior insulation and finish systems (EIFS) on several mid- and high-rise buildings. Rooftop equipment anchorage and glazing breakage by aggregate from roof surfaces was also prevalent." (p. 5-3) "The wind speeds during Hurricane Katrina were below current design wind speeds in most areas, but the wind pressures still exceeded some of the older building code-level wind pressures. The wind conditions from the storm resulted in limited structural damage to buildings, but widespread damage to building envelopes. Poor envelope performance led to extensive damage to the interiors of residences, businesses, and critical and essential facilities. The wind-related building damage was generally a result of inadequate design, outdated codes, building age, lack of maintenance, and/or poor construction/code enforcement." (p. 10-9) Throughout the Hurricane Katrina damage zone, the limited structural damage caused by wind was most commonly observed in residential wood roof framing and occurred below current code wind pressures design levels. Inadequate nailing of roof sheathing panels, gable end wall failures, and lack of properly installed metal framing connectors were the major factors in these structural failures." (p. 10-10 and 10-11) |
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