A bridge on the Mississippi River in Elvis' hometown of Memphis gets a seismic retrofit.

Overview of the Hernando DeSoto Bridge over the Mississippi River looking north. The skyline of Memphis, TN, is on the right. Photo: Imbsen & Associates, Inc.

On the road to fulfilling the Federal Highway Administration's (FHWA) strategic goals of safety, environmental stewardship, and congestion mitigation, some milestones are noticeable to the motoring public-such as new traffic lights, native landscaping, or new highways. Some projects are not so visible to the naked eye, yet they demonstrate the commitment of Federal and State transportation agencies to enhance the long-term safety of the public.

In Memphis, TN, the Hernando DeSoto Bridge carries Interstate 40 over the Mississippi River. The bridge also sits on the southeast edge of the New Madrid Seismic Zone, which is considered to be the highest earthquake risk in the United States apart from the West Coast. The fault itself runs approximately 193 kilometers (120 miles) from Illinois to Arkansas, and the full seismic zone covers a much broader area.

Recognizing the potential seismic danger, FHWA, the Arkansas State Highway and Transportation Department (AHTD), and the Tennessee Department of Transportation (TDOT) joined forces to provide a seismic retrofit for the well-traveled bridge. The objective was to minimize the chances of potential closures that could affect Interstate 40 in the event of an earthquake.

A Vital Link

For 5 months in 1811-1812, the Great New Madrid Earthquake sent a series of shocks that rang church bells as far away as the eastern seaboard. The quake was several times larger in magnitude than the San Francisco earthquake of 1905. Even today, the risk for a damaging earthquake is high and increases with time as the stresses in the seismic zone build.

The Hernando DeSoto Bridge is one of only two Mississippi River crossings in the Memphis area and is a vital link for transportation, commerce, and defense. The bridge was designed and built in the late 1960s with little seismic protection. The likelihood that it would not continue serviceably if a damaging earthquake occurred is high.

"The closest alternate river crossing is in Helena, AR, approximately 70 miles [112 kilometers] south," says Ed Wasserman, director of the Structures Division at TDOT, "and the closest interstate river crossing is on Interstate 155, approximately 95 miles [152 kilometers] north. Based on these facts, if a sizable earthquake were to occur, TDOT and AHTD estimate that [the costs from] the resulting detour would pay for the initial investment in the seismic retrofit on the Hernando Desoto Bridge in only 40 days."

After considering the regional and national impacts that could result from closure of I-40 over the Mississippi River, FHWA and the two State transportation agencies identified the bridge as a high priority and took action. In June 1992, the team conducted a seismic evaluation, prepared a retrofit design, and oversaw construction of the retrofit.

Description of Bridge

The bridge is 5.3 kilometers (3.3 miles) long and contains 164 spans, 160 piers, and 10 abutments. The main spans over the channel consist of two tied arch truss spans and five steel box girder spans. Each tied arch truss span is 274 meters (900 feet) long and is composed primarily of built-up steel box sections. Piers support the spans, which are made up of 38-meter (126-feet)-tall tapered concrete columns connected by a 1.8-meter (6-foot)-thick webwall, which is a structural wall that adjoins adjacent pier columns to create a single pier unit. The columns taper from a 5.5-meter (18-foot) diameter at the bottom to a 4.3-meter (14-foot) diameter at the top. The piers sit on distribution blocks and are supported by concrete footings on rectangular concrete-filled steel caissons.

The steel box girder spans are continuous and located west of the tied arch spans. Of the five spans, two are 100 meters (330 feet) long, and the remaining three are 134 meters (440 feet) in length. The spans are supported by piers composed of tapered concrete columns varying in height and connected by a 1.2-meter (4-foot) concrete webwall, which originally extended approximately 75 percent of the height of the column. The piers sit on distribution blocks and are supported by concrete footings on top of a concrete seal and steel H-piles.

The original bridge incorporated three steel-plate finger expansion joints. Each joint allowed a maximum of 0.33 meter (13 inches) of longitudinal movement but did not permit any transverse movement.

The approaches and connecting ramps to the west consist of prestressed concrete I-beams and steelplate girders. To the east, the approaches and connecting ramps consist entirely of steel-plate girders. These spans are supported primarily by multipost bents. The bents are supported by footings and concrete piles.

Deficiencies

The Hernando DeSoto Bridge is vital to the region and must remain serviceable after the maximum probable "contingency-level earthquake" in a 2,500-year return period (or a 2 percent probability of exceedance in 50 years).

Assuming a contingency-level earthquake, the bridge's original design had a number of deficiencies. Using a three-dimensional modeling by Rebecca Jaramilla program, the contractor analyzed the original structure and specified the deficiencies. The results showed overstressed truss members and connections, insufficient resistance in the deck in both transverse and longitudinal directions, and excessive plastic hinges with poor confinement at the base of pier columns and webwalls (which could cause the bridge to collapse).

The original structure had an inadequate amount of steel reinforcements in the footings to resist rocking on top of the caissons or longitudinal seismic overturning. In addition, the contractor determined that the joints and bearings were inadequate. The existing expansion finger joints would not withstand the expected seismic displacement. Finally, the existing bearings were tall and poorly braced and had the potential to tumble over or displace laterally, which could cause the bridge spans to drop.

The friction pendulum bearing is shown offsite before installation.

Performance Goals

Because of the bridge's importance to mobility in the region, Tennessee and Arkansas jointly decided that the bridge must remain operational and serviceable after the maximum probable contingency-level earthquake mentioned earlier.

The agencies recognized that the bridge inevitably would need to be closed and inspected after a major event; however it was decided that closure of the bridge would be limited to 2-3 days. Lastly, any damage found during inspection of the bridge would need to be minimal and repairable without closing the bridge to traffic.

Workers install a friction pendulum bearing on a column.

Retrofit Design Strategies

The team considered two retrofit strategies. The first was the traditional strength and ductility strategy. "Ductility" is the property of a material enabling it to undergo large permanent deformation without failure. The strategy involves adding strength to bridge components to transfer all loads through the entire system. This approach required extensive strengthening or complete replacement of numerous components, including the bearings, truss members, and connections; bottom lateral bracing and connections; and the entire deck system, pier columns, webwalls, footings, and distribution blocks. The estimated cost of the retrofit for just the main channel spans using the strength and ductility strategy quickly added up to more than $45 million.

The second strategy combined strength, ductility, and isolation. Isolation bearing technologies enable engineers to limit the structural stresses on the bridge components during an event by increasing the amount of displacement the structure can withstand. The two types of isolation bearings considered for the Hernando DeSoto Bridge were the friction pendulum bearing and the lead-core rubber bearing. The contractor analyzed the bridge model using isolation bearings in place of the existing bearings, and the results showed a significant reduction of stress levels in both the superstructure and substructure. The estimated cost for the seismic retrofit using the isolation strategy for the main spans totaled $27 million. Because this approach offered nearly a 40 percent reduction in construction costs without compromising structural safety or serviceability, FHWA and the two State agencies selected the isolation strategy for the project.

Overview of Design Features

Using the isolation strategy, the contractor completed the final design, plans, specifications, and estimates for the retrofit. Major design features included replacing the existing bearings with isolation bearings, strengthening footings and columns, enlarging column caps (to accommodate the new isolation bearings), and tying the tops of the webwalls to the columns.

Other enhancements included replacing or strengthening the bottom lateral bracings, strengthening the steel cross-frames, replacing the existing finger joints with modular swivel-expansion joints, and retrofitting the trusses (adding members to brace the portal frame posts).

This column cap retrofit shows workers tying steel on the rebar cage around an existing column before placing the concrete.

Isolation Bearings

According to Fred Stephenson, P.E., the resident engineer on site, "Friction PendulumTM bearings are on the leading edge of innovative seismic design." Typical applications include construction of buildings, industrial facilities, and bridges. The Hernando DeSoto Bridge incorporates the largest vertically loaded friction pendulum bearings used to date anywhere in the world. The bearings consist of three major components: (1) a top guide plate mounted to the superstructure, (2) a concave bottom plate mounted to the substructure, and (3) an articulated slider fitted to the top plate and resting on the bottom plate. During an earthquake, the articulated slider will move along the concave surface of the bottom plate, which will guide the top plate (and connected superstructure and deck) in small pendulum motions. The pendulum relies on friction and gravity to help resist and dampen earthquake motions (absorb damaging earthquake energy). This absorption reduces lateral loads and shaking movements throughout the structure. The main channel spans will require a total of 18 friction pendulum bearings. The contractor specified four sizes of pendulum bearings for the bridge. Types 1 and 2 are the larger bearings and will support the tied arch spans. The Type 1 bearings (the largest) have a maximum vertical load capacity of 5.7 million kilograms (12.6 million pounds), which easily qualifies as the largest vertical load capacity of any friction pendulum bearing in the world. They have an inside diameter of 20-25 centimeters (8-10 inches) and a bearing height of 2-25 centimeters (1-10 inches), and they can withstand a lateral seismic force of more than 590,000 kilograms (1.3 million pounds). The smaller Type 3 and 4 bearings that support the steel box girder spans will allow up to 73-76 centimeters (29-30 inches) in lateral displacement.

The team also is using 12 leadrubber bearings in the retrofit project. Lead-rubber bearings consist of a core cylinder of pure lead closely surrounded by layers of rubber and steel bonded together. Under normal loads, the rubber allows lateral and longitudinal displacements while the steel plates strengthen the bearing vertically. The lead core strengthens the system laterally against wind and other nonseismic loads. During an earthquake, seismic loads cause the rubber and steel layers to push and deflect the lead core laterally, dampening and dissipating the quake's damaging energy. The bearings can withstand a lateral load of 249,000 kilograms (550,000 pounds) and a lateral displacement of 57 centimeters (22.5 inches) during an earthquake.

Lead-rubber bearings have been installed on more than 100 bridges and 70 buildings worldwide and proved their effectiveness during the 1994 earthquake in Northridge, CA, which measured 6.7 on the Richter scale. The University of Southern California (USC) University Hospital, for example, uses lead-rubber bearings. Although other buildings and infrastructures in southern California were damaged severely, the hospital survived the quake without harm to the structure or its contents.

Spans in the Main River Channel

FHWA and the two State transportation agencies subdivided the retrofit construction into several contracts. The first two, for work in the main river channel, were awarded in December 1999 and December 2000. The contractor completed work on these two contracts in January 2003 and March 2003, respectively. Construction under a third contract, awarded to the incumbent contractor in December 2002, is progressing rapidly, and the team expects the project to be completed by the end of 2005.

Workers pour concrete around a new swivel joint to complete the joint installation.

All 18 friction-pendulum bearings and the 12 lead-rubber bearings were installed on the spans crossing the main river channel. The contractor used hydraulic jacks during installation of the new bearings to maintain traffic flow on the bridge. Hydraulic jacks with vertical lift capacities ranging from 4.5-10.9 million metric tons (5-12 million tons), placed on either side of the existing bearings, lifted and supported the bridge while the contractor removed the rocker bearings and replaced them with new isolation bearings.

Stiffener plates and other strengthening components were added at several locations to provide adequate jacking points. The contractor extended many of the column caps to accommodate the new bearings and retrofitted four of the webwalls by extending them vertically and connecting them to the column caps.

The workers performed the work on the superstructure and deck while maintaining four lanes of traffic at all times. (The bridge accommodates six lanes of traffic under normal conditions.) One key feature of the superstructure retrofit was the replacement of the existing finger joints with swivel joints. Swivel joints will foster additional ductility in the superstructure by allowing 57 centimeters (22.5 inches) of longitudinal movement and 46 centimeters (18 inches) of transverse movement during an earthquake.

Additional retrofitting activities for the superstructure included strengthening diaphragms, adding direct connections between the deck and box girders, stiffening the connections between stringers and the floor beams, and replacing cross frames and lateral bracing members. The contractor used a moveable suspended work platform to perform the majority of the superstructure retrofit. The platform enabled the crew to work underneath the structure rather than from above, which would have required removing large portions of the deck.

Strengthening the footings and columns also is underway in the main river channel. To date, workers have retrofitted two footings and begun constructing a second pair. Cofferdams-some of the largest ever constructed in the State of Tennessee, standing up to 17 meters (55 feet) above the riverbed-are required to perform the substructure work "in the dry." The cofferdams were constructed on a river barge and then launched from the barge into place around the piers. Once the cofferdams are completed and dewatered, the footings and columns will be prepared for concrete encasement. Each of the footing encasements consists of approximately 2,190 meters (2,400 yards) of concrete, poured continuously over a 12-hour period.

On the suspended platform under the superstructure, workers are removing the existing lateral bracing.

Spans in the East Approach

In addition to work on the main river channel, the team awarded the contract for five spans on the east approach in October 2001, and the job was completed in March 2003. The retrofit work on the substructure included strengthening the footings, columns, and pier caps. The contractor installed cast-indrilled- hole piles around the perimeter of the existing footings, drilling and bonding bars to the footings, and then forming and placing reinforced concrete to encase the new piles, bonding bars, and footings.

The columns were retrofitted by erecting a steel casing around the existing column and then forming and placing reinforced concrete between the existing column and the steel casing. Workers braced the pier caps by casting reinforced concrete around the existing cap to increase the width.

Retrofit work also included replacing the existing bearings with Disktron bearings, custom designed for this application, and strengthening the cross-frame system. The cross frame consists of upper and lower bearing plates mounted to the superstructure and resting on a slide plate. The bearing plates (with the superstructure) are allowed to move longitudinally across the slide plate up to 30.5 centimeters (12 inches) but are restrained in the lateral direction. The new bearings are designed to withstand a maximum vertical load of 72,500 kilograms (160,000 pounds), a transverse load of 113,400 kilograms (250,000 pounds), a longitudinal load of 31,700 kilograms (70,000 pounds), and an uplift load of 4,500 kilograms (10,000 pounds).

Jacks support the superstructure after workers removed the existing bearing and before the installation of the new friction pendulum bearing. Friction brackets have been installed around the column to accommodate the jacking system.

As with the main spans, hydraulic jacks enabled the workers to install the bearings in the eastern approaches under normal traffic flow. The jacks-supported by the newly retrofitted footings and cross frames-required a maximum jacking load of 45,400 kilograms (100,000 pounds).

As with the main spans, hydraulic jacks enabled the workers to install the bearings in the eastern approaches under normal traffic flow. The jacks-supported by the newly retrofitted footings and cross frames-required a maximum jacking load of 45,400 kilograms (100,000 pounds).

The contractor removed and replaced the existing cross-frame system. The new cross frame, located at every pier location, consists of builtup steel I-beams bolted between each steel girder and drilled and bonded to the bridge deck. Shear blocks also were installed at several locations to control lateral movement during a seismic event.

Seismic Monitoring

In conjunction with the retrofit, The University of Memphis will install a seismic instrumentation system to monitor the performance of the Hernando DeSoto Bridge during an event. The system will consist of 114 sensors at 38 locations on the main channel spans. The Hernando DeSoto Bridge will be the first existing long-span bridge in the New Madrid Seismic Zone to be instrumented for seismic monitoring.

The data collected from the sensors during small seismic events will be used to verify or enhance the current mathematical models that researchers use to predict movements under larger events. The project not only will improve predictions of the performance of this particular bridge but also will help advance seismic codes and future bridge designs.

"To date, there is little data available describing the response of longspan bridges to seismic activities," says Paul Sharp, team leader for technical programs at FHWA.

"The installation of a seismic instrumentation system to monitor this structure's response to smaller events should prove to be invaluable toward predicting the effectiveness of an isolation-based design for future retrofit projects. Future plans may include an early warning system that would be based on exceeding certain thresholds, alerting DOT engineers of a need for response."

Looking Ahead

Work completed to date on the bridge already has made this project a pioneer in seismic design and construction. Upon completion, the Hernando DeSoto Bridge will safely endure an earthquake with a magnitude of 7.0 on the Richter scale with little to no damage.

Presently, State and Federal sources have invested almost $72 million in combined funds for the construction.

FHWA and the Arkansas and Tennessee agencies are working closely to achieve goals that can benefit the transportation system as a whole, preparing the region to endure and recover quickly in the event of an earthquake. This project demonstrates that new technology and teamwork within the transportation community can build a healthier, safer future.

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Rebecca Jaramilla, P.E., currently serves as the assistant bridge engineer in the FHWA Tennessee Division Office. Her role covers all aspects of bridges including design, construction, and in-service inspection. Prior to her current position with FHWA, Jaramilla worked for the U.S. Army Corps of Engineers as a structural engineer in Chicago. She holds a professional engineering license in Tennessee and has a B.S. in engineering from the University of Illinois at Chicago.

For more information, contact Becky Jaramilla at 615-781-5758 or rebecca.jaramilla@fhwa.dot.gov.