Case Study: Lessons From Hurricane Katrina Storm Surge On Bridges And Buildings
Introduction
Hurricane Katrina struck along the gulf coast of US in 2005. In this case study objective is to get a good understanding of the devastating effect of this hurricane storm surge, damages to coastal structures, bridges and buildings surveyed along Louisiana, Mississippi and Alabama. During flooding, uplift force (hydrostatic) due to buoyancy and uplift forces (hydrodynamic) due to wave action were observed for the structures. Because of flooding floating objects affects structures by impact action and also by water damming, because of water damming high hydrostatic force built up by the side of structure because of this force structure may fail. Foundation system experience shear force and because of liquefaction soil losses its strength and foundation system is at its greater risk, if foundation of structure fails then complete structure will collapse no matter how strong was the structure.
Hurricane Katrina was the category-5 storm in the Gulf of Mexico on Saffir–Simpson scale. Katrina storm becomes category 4 as it made landfall on the Mississippi river delta and category 3 at the Louisiana – Mississippi border. The Saffir–Simpson hurricane wind scale (SSHWS), formerly the Saffir–Simpson hurricane scale (SSHS), describe hurricanes forming in the Atlantic Ocean and northern Pacific Ocean east of the International Date Line into five categories distinguished by the intensities of maximum sustained wind for 1 min.
Strom surge of 7m. above sea level was observed in the pass Christian and Gulfport due to the shallow bathymetry and shape of coastline. Because of the onshore wind setup water level increases due to force of wind on the water, shallow coastline bed friction slowdowns their motion leading to pile up of water. Because of storm of this category and height considerable damage was observed due to the flooding and wave action. Structural response for different types of the force conditions:
Residential structures: Light – framed wood and masonry structures experienced devastating effect of this storm. Reinforced and steel frame structures having open wall type foundation and first floor at elevation withstand the flooding and wave action to the mid height of the second level without any damage to the structure.
Structural loading: Factors causing damage to the engineered structures were uplift pressure (hydrodynamic and hydrostatic both), lateral force due to water damming, scouring of the material near to the foundation piles or columns. Hence total force acting on the structure can be taken as summation of following forces:
Fb = vertical buoyancy force;
Fs = horizontal slamming force (due to impact of wave).
Fd = horizontal hydrodynamic drag force.
Fl = vertical hydrodynamic uplift force.
Fi = inertia force.
Dougless et al. (2006) stated that Fd and Fl were the primary load which causes the structures failure. Since many bridge decks and elevated floors of the building were completely submerged hence force Fb should also be in consideration.
Bridge structures: Horizontal hydrodynamic drag force estimated by Dougless et al. on 15. 8m long deck segment of US 90 highway bridge was of the order of 65. 9 KN/m. along the length of the bridge deck. If this lateral load exceeds the restraint provided by the support bearing and shear key, then structure will fail. But being a low seismic zone the shear key was not provided hence friction induced by gravity load and thick steel angels restrain the structure against lateral loading. Hydrodynamic uplift estimated by Dougless et. al. exceeds the self-weight of bridge deck by 30% because of this condition bridge spans were displayed from its position, but for Biloxi bay bridge self-weight was 64% greater than the estimated uplift pressure and this bridge suffered no span failure during the hurricane. This analysis is valid for the elements which are under partial submergence means surge level is below the bridge deck. If structure is under complete submergence then effect of hydrostatic uplift or buoyancy comes in to play. Because of the less volume of air entrapped between bridge deck and water surface the uplift pressure reduced and the prestressed concrete bridge girder and deck remain intact. The reduction in uplift pressure was due to the small width of bridge deck. If sufficient no. of high shear strength shear key provided on either side of restraint, then the damage due to the effect of lateral hydrodynamic load can be minimized.
Double –Tee Floor system: double tee floor system used in parking garage collapse due to the buoyancy effect as the significant air trapped between the girder webs below the floor deck. Because of the spandrel beam at either end of the span this air could not escape. this air induced uplift in excess of the buoyancy. The combined effect of air uplift and buoyancy results in the hogging moment i. e. Negative bending moment.
Double tees are not designed for the hogging moment their design is only done to resist sagging moment over the simply supported span. Some floor system was supported at end by the corbels which allows the section to rise freely under uplift force effect, and some were restrained by the socket which do not allow any free movement. As these double tees system were supported in different ways at ends, the ends support system was not helpful to prevent the collapse, because negative bending moment still occurs at mid span due to the effect of buoyancy.
Floor System Failures: Flat slab fails due to upward punching of the slab at slab column interface. Upward punching is induced by the uplift hydrodynamic force. Because of the lack of reinforcement provided on the bottom face of the slab punching shear capacity of the slab decreases which leads to the failure of the slab.
Maximum wave height which could occur is within the range of wave height required to cause the punching shear failure of the slab. If any column slab interface collapse, then the unbraced length of the above column becomes double and column buckling occurs which results in failure of column and this process continues which finally leads to the progressive structural failure. Another building designed adjacent to this did not experienced any punching shear failure because of the thick flat slab and drop panels at each column slab interface. Posttensioned one-way slab floor system fails in shear failure at the supporting beam.
Floating structures restraints: the hard rock casino was enclosed in fixed shell which was designed to resist wind load. but due to the storm effect the exterior shell collapse the reason of this collapse may be, excessive vertical motion of the barge resulting in the reduction of clearance between the casino and the fixed roof, which finally results in collapse, surge and wave pushes the barge inland causing the exterior column to fail and at the time of receding wave the exterior columns were pushed outward leading to complete structural failure.
Effects of Scour: Scouring means the movement of soil near to the bridge piers, foundation columns, etc. Scouring in case of the storm surges occurs due to two types of mechanism.
- Shear induced scour caused by the water and debris as they move the material near by the structure and transport it to another place.
- Liquefaction induced scour, because of the liquefaction soil losses its shear strength and soil behaves like a viscous fluid which can be easily transported by the flowing fluid. Once the supporting soil moves then pavement supported by soil will collapse and eventually complete structure collapse.
Effects of Debris: Debris means the floating material like vehicles, structures, shipping containers, boats etc. debris created impact on the structures and debris also block the water flow and cause water damming.
Conclusion of case study
Biloxi to pass Cristian storm surge of approximately 7m causes substantial damage to the structures. In case of bridge under partial submergence lift force cause the bridge to lift up, these dynamic loads repeated no. of times which then moved by the lateral forces. Double tee floor system because of the uplift force experienced the hogging moment which causes the failure of the system. In case of floating structures shell was not designed for the storm surge which leads to the complete failure of the hard rock casino. scouring of the soil also causes the structure failure. Floating debris due to impact force and hydrostatic force due to water damming also leads to damage of the structure.
Recommendations
Dynamic loading should also be consider in the structural design in order to provide sufficient toughness and ductility to the structures and shear key restraint should also be provided in order to minimize the damage by the uplift pressure and lateral loads.
Floor system should also be designed to resist hogging moment created by uplift pressure. Flat slabs should have a provision of drop panel in order to resist punching shear at the slab column interface.
Floating structures restraints should be designed for the changes in water elevation occurred during the design life. Force due to impact action of floating debris should also be consider in design in order to protect structure from debris impact force.
When lower floor column fails then column above that floor also fails and it leads to complete collapse of the structure. in order to prevent this type of failure columns should be designed for progressive structural failure.
Scouring effect should also be taken in to consideration because no matter how stable structure is if soil resisting that structure moves then complete structure fill collapse.
Soil stabilization should be done in order to prevent scouring induced by both mechanisms.
