BRIDGE SEISMIC: Stronger every day
Retrofit on I-40 bridge progressing nicely
"I feel somewhat like a lone survivor, who gets the trophy bottle of champagne.”
Ed Wasserman, CE director of the Tennessee Department of Transportation’s (TDOT) Structures Division, has spent 21 of his 25 years as state bridge engineer dedicated to the seismic retrofit of the I-40 Mississippi River Bridge. Over his career, Wasserman’s commitment to the project has endured nine phases of construction and associated contracting processes, and he has worked with five different lead engineers with project partner Arkansas Highway and Transportation Department (AHTD). When Wasserman retired in August 2011 he had successfully overseen a longstanding, complex effort to not only secure the safety of travelers crossing the Mississippi, but to ensure the continued economic survival of the region.
Protecting the unprotected
Memphis, Tenn., is a major U.S. economic and transportation center. Spanning across the Mississippi River and into Memphis, I-40 carries 60,000 vehicles daily including much of the nation’s east-west interstate trucking traffic. As one of only two crossings of the river in the Memphis area, the I-40 Mississippi River Bridge, known locally as the Hernando de Soto Bridge, is a vital transportation, commerce and defense link between Tennessee and Arkansas. The structure complex is owned by both states, with costs for construction and maintenance (and the retrofit project) split 60% and 40%, respectively.
The I-40 Bridge is 3.3 miles long, including the main channel spans, approaches and ramps. The structure comprises 164 spans, 160 piers and 10 abutments. The main channel consists of five steel-box-girder spans and two steel-tied-arch truss spans. The west approach is precast, prestressed concrete I-girder spans and steel-plate-girder spans, while the east approach and connecting ramps are entirely welded steel-plate-girder spans.
Although the bridge sits along an active fault zone, it was constructed in the 1960s with little seismic protection.
“Following California’s 1989 Loma Prieta earthquake, AHDT suggested we jointly undertake a feasibility study to strengthen the bridge,” said Wasserman. “In 1992, the bridge and its approaches were given priority upgrade status in order to protect the long-term safety of the public.”
Since then, TDOT and AHTD have worked jointly through evaluation, analysis, design and construction to implement a cost-effective, structurally sound seismic retrofit strategy that maximizes the use of isolation bearings.
New Madrid’s fault
Situated at the southeastern edge of the New Madrid Seismic Zone, the I-40 Bridge crosses one of the most active seismic zones in the central U.S. The site of the bridge is characterized by soil deposits up to 2,500 ft deep that result in dominant long-period motion in the ground, and the potential for liquefaction exists at isolated locations along the structure.
In the winter of 1811-12, three earthquakes, each with estimated 7 magnitudes, occurred within the New Madrid zone. While there was little damage at the time because the area was sparsely populated, according to a recent USA Today article (“Central US has 1st multi state Shake Out,” April 28, 2011) a similar quake today would cause significant damage and disruption. The same article noted that the probability of another 7 quake occurring in the zone is 1 in 10.
Only minor damage
To assess the current condition of the bridge, and determine the retrofit needs for the structure, an in-depth analysis was undertaken throughout each phase of the project. Three-dimensional models of the bridge were analyzed using the existing structure configurations. They were reanalyzed for various types of trial retrofit strategies and analyzed once again for the preferred retrofit strategy.
For the main span, analysis included ground-motion input consisting of spatially varying displacement time histories at multiple supports and accounted for soil-structure interaction, foundation radiation damping and foundation rocking. Response spectrum analyses included the displacement-based “pushover” analysis.
“The targeted performance level for the retrofit,” said Wasserman,” is to ensure that in the case of a significant seismic event, up to and including the maximum credible event of 7.0 magnitude, that access across the Mississippi is maintained. This capability is vital, not only to the recovery of Memphis but to reduce impacts on transcontinental commerce.”
Originally, the I-40 Bridge was designed primarily to handle transverse wind forces, not earthquakes. To bring it up to modern safety standards, the retrofit was designed so that any resulting damage would be relatively minor, allowing the bridge to remain functional for emergency vehicles immediately after a quake. Also, it ensures any bridge closures last no longer than three days, and the structure remain operational for the general public following inspection.
Placed in isolation
Traditionally, the combination of “strength and ductility” was the accepted approach to provide maximum seismic resistance to bridge structures. The strength approach provides retrofitting of inadequate bridge components to transfer loads through the entire system (i.e., superstructure, bearings, piers and foundations, etc.)
Initially the I-40 Bridge retrofit strategy for the tied-arch and box-girder spans maintained all existing steel rocker bearings, requiring extensive strengthening or complete replacement of numerous components in the superstructure and substructure. As the cost of implementing this strategy skyrocketed, isolation emerged as a viable, but more cost-effective retrofit approach.
Isolation-bearing technology allowed forces to be reduced significantly in both the superstructure and substructure, so much so that 50% less retrofit work was required to be performed on the superstructure, compared to the strength approach. At Pier B large friction pendulum bearings (9 ft in diameter) were used to handle 19 in. of seismic movements. In conjunction with the friction pendulum bearings, modular expansion joints at the ends of the tied-arch bridge are designed to move ±22 in. in the longitudinal direction and ±18 in. in the transverse direction. The Pier B bearings have the distinction of carrying the highest axial load (11,300 kips) of any friction pendulum bridge bearing designed to date.
The estimated cost of the isolation bearing strategy was 30% less than the initial strengthening strategy, which maintained the existing bearings.
The retrofit of the Group G East Approach, which is composed of welded steel-plate-girder spans supported on concrete bent caps and multicolumn bents, also employed a retrofit strategy combining isolation with strength and ductility.
The substructure retrofit included:
Bent cap retrofit/widening;
Web-wall retrofit; and
Footing retrofit/enlarged cap with additional piles.
The superstructure retrofit included:
Bottom lateral retrofit;
Bearing replacement—112 bearings;
Expansion joint replacement; and
As an example, the footing at substructure Pier E8 had to be demolished and constructed in stages to avoid unloading the existing piles while traffic was maintained on the bridge. Additional 24-in.-diam. pipe piles were driven to withstand seismic uplift forces. The existing columns at Pier E8 were strengthened with No. 11 longitudinal bars (total 40) and ½-in.-thick steel column casings. The final diameter of the strengthened column was 7 ft 6 in. The lead core rubber isolation bearings are designed to move 16.5 in. in any direction.
Due to a significant amount of deterioration in the bridge deck, the Group B structure on the Arkansas West Approach was completely replaced with a new steel-plate-girder structure which was 2,536 ft in length and 107 ft in width to accommodate three lanes of traffic in each direction. The project required phased demolition of the existing structure and phased construction of the new structure to maintain two lanes of traffic in each direction. The new structure is a continuous steel I girder consisting of 17 spans with two expansion joints located at Piers 6 and 12.
The abutments were seat-type abutments founded on 24-in.-diam. pipe piles. Each bent consisted of integral concrete caps, six reinforced concrete columns and pile cap footings that were founded on 24-in.-diam. pipe piles. Modular swivel-type joints were placed at the expansion joints and abutments to accommodate the large seismic movements. Seismic analysis and design in this location conformed with the site-specific design criteria developed for the project.
Toward the future
According to Wasserman, the I-40 Mississippi River Bridge seismic retrofit “as originally conceived was envisioned to take six to 10 years, depending on funding availability.” When final construction is complete in 2014, the project will have been ongoing for 24 years, nearly Wasserman’s entire lifetime at TDOT, and even beyond.
Despite the extensive work and periodic traffic disruptions, public support for the project has been and remains high—a tribute to Wasserman’s steady leadership over the course of the upgrade. “Through the years,” he said, “we’ve been faithful to reminding the public why the work is being done.”
As one of only two river crossings in the area, the I-40 Mississippi River Bridge is a vital link for transportation, commerce and defense. Strengthened to withstand significant seismic activity in a dangerous fault zone, this lifeline structure stands ready to serve the traveling public for many more years to come.
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