Assessing damage to engineered buildings in the wake of Hurricane Katrina

Hurricane Katrina made landfall on Aug. 29, 2005, at approximately 7:10 a.m. eastern daylight time in southern Plaquemines Parish, La., as a Category 4 hurricane, and proceeded inland along the Louisiana-Mississippi state line.With sustained surface winds before landfall exceeding 175 mph, unprecedented storm surges approaching 30 feet, and hurricane force winds extending 125 miles from its center, the hurricane caused major flooding and damage that spanned more than 200 miles along the U.S. Gulf Coast.

Damage extended beyond residential communities, with significant damage to engineered infrastructure, including buildings, roads and bridges, utility distribution systems for electric power and water, wastewater collection facilities, and vital communication networks.

Funded by the National Science Foundation, investigators from the Multidisciplinary Center for Earthquake Engineering Research (MCEER) headquartered at the University at Buffalo, conducted post-disaster field reconnaissance to examine the impact of Hurricane Katrina on the built environment.

Teams of investigators were deployed within a week of the hurricane (Sept. 6-11) along the Mississippi coastline, with a second deployment a month later (Oct. 3-9 and Oct. 17-22) focusing on the New Orleans area.

The objectives of MCEER reconnaissance missions were to collect perishable data and to examine damage from a multi-hazard perspective. Implications of lessons learned from Hurricane Katrina are being applied to mitigate damage not only from future hurricanes, but also from other extreme events such as earthquakes or terrorist attacks. One specific, multi-hazard objective of this mission was to identify similarities between damage typically observed after earthquakes and damage caused by Hurricane Katrina, with the goal of recommending seismic design principles that could mitigate structural damage caused by wind and storm-surge forces. Ultimately, MCEER is seeking to develop design strategies that will make communities more resilient against any extreme event.

This article presents a brief summary of the observations related to damage to engineered buildings. More detailed reports of MCEER’s reconnaissance efforts can be found online at http://mceer.buffalo.edu/.

Architectural damage to buildings

The majority of multistory commercial buildings constructed of steel or reinforced concrete framing performed well structurally during Hurricane Katrina.

However, extensive losses were incurred from nonstructural damage to cladding, windows, and roof-mounted equipment.

In the New Orleans Business District, for example, the 28-story, reinforced concrete Hyatt Regency Hotel building suffered extensive window damage, as well as some damage to its single-ply membrane roof, loss of some rooftop ventilation equipment, and damage to the exterior soffit of the rooftop bar structure. Three fourths of the glazing on the north façade curtain wall broke during the storm. The curtain wall comprises 1/4-inch tempered single-pane vision glass, and 1/4-inch non-tempered single-pane black spandrel glass.Wind, water, and wind-born debris, including pea gravel, infiltrated several of the rooms on the north side. As a result, furniture, carpets, and drywall became infested with mold. The building has no basement and was located in an elevated region that was spared from flooding.

Buildings adjacent to the Hyatt suffered significant window damage also, while buildings just blocks away had only a few broken windows. During the investigations in early October, The Hyatt Regency was in limited operation and was hosting the city of New Orleans Emergency Operations Center, although it had no potable tap water.

The downtown area of Gulfport, Miss., located about one-half mile north of the shoreline, was subjected to storm surge on the order of 1-2 feet. Restoration efforts were under way to repair water damage at the ground level and also the upper stories of many buildings with fractured windows. Closer to the Gulfport shoreline, damage to infrastructure was more extensive, as demonstrated by the beachfront, 17-story Grand Casino Hotel, constructed of reinforced concrete frame with shears walls. The approximately 20-foot-tall first story was partially enclosed, probably with break-away walls, to serve as a welcome lobby with access to stairs and elevators. Only the bare structural concrete columns and shear walls remained at this first level, with all else swept away by the storm surge. The second and third stories, with heights of about 15 feet each, housed the reception desk, shops and banquet rooms, among others. A light steel frame extended these two levels over the drive-thru valet parking area. The exterior cladding of these two stories had been removed completely in some areas. The layers of gypsum board, Styrofoam, and fabric lathe with thin coat of colored plaster were stripped from the nonstructural steel stud framing.

As a result, high winds and rain entered into the building and destroyed the interior space. The cladding was also stripped at several locations above the third story level, exposing the steel stud framing.

These and many other buildings performed well structurally through the storm; however, more attention needs to be given to protecting the building envelope from hurricane forces. Damage to the building exterior also resulted in costly damage to the interior, thus investment in hurricane-resistant cladding and windows appears beneficial.

Structural damage to buildings from impact

Buildings that did suffer structural damage along the Gulf Coast were exposed to extreme loads. Partial collapse of a five-story, reinforced concrete building with unreinforced masonry infill, located on Highway U.S.90 in Biloxi, Miss., was caused by the impact of a three-story casino barge that floated ashore with the rising storm surge. One corner column on the front southeast corner of the building was impacted, resulting in the collapse of one bay at the bottom four stories of the building. The beams at the fourth level were sufficiently strong to form a redundant load path for the load carried by the remaining portion of the column at the upper story. Other than the region of impact, the south façade of the structure appeared in good shape with only a few missing window panes. Thus it is probable that this building would have faired well through the storm, with the exception of flood damage to the lower levels.

In a similar scenario, a five-story (sixlevel) parking structure constructed of cast-in-place reinforced concrete partially collapsed. The parking structure formed part of a hotel and casino complex in Biloxi, with adjacent barges housing the gambling halls. Four bays on the west façade collapsed, apparently because of impact from the adjacent casino barge that impacted four columns as it was raised by the storm surge.

Structural damage to parking structures from storm surge Several parking structures in Biloxi and Gulfport were examined after Hurricane Katrina. Five parking structures were constructed of precast concrete with pretensioned, double-tee beams for decks; the other three were of cast-inplace, reinforced concrete. All were either on the shore of the Gulf of Mexico or facing the Gulf across the coastal U.S. Highway 90; all were subjected to storm surge estimated to be 20 to 30 feet in height. While none of the cast-in-place structures suffered any structural damage (with the exception of one that partially collapsed from impact by a casino barge), all of the precast concrete structures suffered partial collapse of the secondfloor deck. Storm surge seems to have reached the level of this deck, which typically was 10 to 15 feet above grade (grade being roughly 3 feet above normal sea level), but not the third-level deck, which was generally 20 to 25 feet high. In cases where only part of the deck collapsed, it was typically on the side from which the waves approached.

Preliminary conclusions

The concentration of collapsed decks on the seaward side, where wave action would be strongest, implies that wave action played a role in the collapse, either vertically, through uplift of the deck beams, or horizontally via the spandrel beam. Evidence of horizontal pounding by waves was observed in spandrel beams where the shear key at the end of the deck-beam appears to have punched holes. But this horizontal action may not have been the principal cause of the failure. In some cases, the collapsed deck beams had been oriented parallel to the seashore while in others the collapsed deck beams were perpendicular to the seashore. In some cases, the spandrel beam rested on the outside of the column and could only have moved inward at mid-span in flexure.

Vertical loading was evident by shear failure in deck-beam shear keys and in supporting girders. Such loading could have been caused by pounding resulting from deck beams being lifted and then dropped, or by the weight of water accumulated on the top of the deck before it could pour down a ramp. However, water would probably flow quickly enough to distribute around the entire second-floor deck and cause a wider distribution of damage. This loading would not justify the concentration of damage on the seaward side of the deck.

Also, vertical loading could have resulted from uplift and dropping of the deck beams. While wave action alone might conceivably have lifted the deck beams, buoyant forces from the rising storm surge would have contributed strongly. The deck beams and their concrete topping weigh approximately 150 pounds per cubic foot (pcf ); seawater weighs approximately 66 pcf.

According to Archimedes’ Principle, buoyancy alone would have reduced the downward vertical load on the deck beams by almost half, a loading condition that these pretensioned members were probably not designed to withstand. In addition to buoyancy of the deck beams and topping, the shape of the doubletees, whose ends were enclosed by their supporting girders, would have allowed for the creation of air pockets. As the storm surge rose to the soffit of the second-level deck, the concrete topping over the deck would have limited greatly the ability of air to escape, and additional buoyant force equal to the weight of water displaced by the air pocket would have been applied upward to the deck beams, potentially reaching or exceeding the self-weight of the deck beams and their topping.A similar air pocket condition is believed to have occurred between the girders of many bridges that were damaged by Hurricane Katrina.

Under such a condition—buoyancy caused by air pockets and the volume of the deck concrete itself neutralizing the self-weight gravity loading—the negative bending moment induced by the pretensioning would have been unopposed by gravity-induced positive bending in these simply supported beams. The negative bending induced by pretensioning is greatest in the region of the midspan, so if uplift were the cause of the collapse, one would expect to see evidence of negative-bending flexural damage at the midspan, such as concrete spalling at the bottom fiber at midspan and diagonal cracks radiating outward and downward from the top fiber. If on the other hand it were the weight of water on top of the deck that caused failure, one would expect to see the opposite: concrete spalling at the top fiber at midspan and diagonal cracks radiating outward and upward from the bottom fiber. The damage observed here supports the negative bending hypothesis.

It should be noted that the structural failures reported here are preliminary findings of field reconnaissance based on limited information—brief one- to two-hour visual examination of only five structures.No material testing, examination of structural drawings, mathematical modeling of the structures, nor analysis of the imposed hydrodynamic forces has been carried out to date.

Therefore, no firm conclusions are offered regarding the safety of other, similar structures that could be exposed to storm surge.

Future studies

MCEER researchers have collected the necessary data from Hurricane Katrina field investigations to expand on current multi-hazard studies that will extend the lessons learned to other extreme events such as earthquakes and terrorist attacks. Based on the data collected, design details to mitigate hurricane-induced damage on precast concrete construction will be studied. A literature review also will be conducted to check whether this phenomenon has already been observed elsewhere.

Gilberto Mosqueda, Ph.D., is an assistant professor in the Department of Civil, Structural and Environmental Engineering at the University at Buffalo, in Buffalo, N.Y., where he teaches courses and conducts research in structural and earthquake engineering. He can be reached via e-mail at mosqueda@eng.buffalo.edu. Keith A. Porter, Ph.D., P.E., is GW Housner Senior Researcher at the California Institute of Technology, in Pasadena, Calif., where he conducts research on seismic vulnerability, performance-based earthquake engineering, and multi-hazard risk management. He can be reached via e-mail at keithp@caltech.edu.