A three-hitter

Nov. 8, 2004

The new U.S. 82 bridge currently under construction over the Mississippi River near Greenville, Miss., will be the longest cable-stayed bridge on the Mississippi River and the second longest in the U.S. when it is completed in 2006. The new bridge will replace a 1940s structure that bears the dubious distinction of being struck by more barges than any other bridge on the Mississippi River. That bridge was opened to traffic in 1940 at a cost of $4,447,000.

The new U.S. 82 bridge currently under construction over the Mississippi River near Greenville, Miss., will be the longest cable-stayed bridge on the Mississippi River and the second longest in the U.S. when it is completed in 2006. The new bridge will replace a 1940s structure that bears the dubious distinction of being struck by more barges than any other bridge on the Mississippi River. That bridge was opened to traffic in 1940 at a cost of $4,447,000.

The new bridge is located near Greenville on the Mississippi-Arkansas border approximately 150 miles south of Memphis, Tenn., 120 miles northwest of Jackson, Miss., and 150 miles southeast of Little Rock, Ark. At this location the Mississippi River drains approximately 1?3 of the U.S. The 13,700-ft bridge includes a steel composite cable-stayed span of 1,378 ft providing approximately 65 ft of vertical clearance over the navigation channel.

Striking appearance

The existing Greenville bridge bears the dubious distinction of being the most struck bridge on the lower Mississippi River. In an Arkansas Democrat article dated March 17, 1996, Randy Tardy reported that in the period from 1972 through 1995 there were a total of 100 barge collisions on the lower Mississippi and Arkansas Rivers. The breakdown of those collisions is as follows:

Mississippi River

  • Caruthersville, Mo. - 10 Collisions
  • Memphis, Tenn. - 18 Collisions (four bridges)
  • Helena, Ark. - 19 Collisions
  • Greenville, Miss. - 39 Collisions

Arkansas River All Bridges - 14 collisions

It has been determined that one of the primary reasons for the large number of collisions at the existing Greenville Bridge is its location relative to an upstream bend in the river. The river pilots refer to the bend not as a curve, but as a left turn against the current. The navigation opening is located on the east side of the channel (the left descending bank) and in order to transit the opening, the pilots must begin maneuvering far in advance of the bridge. With up to 1,500 ft of barge out front and one of the swiftest currents along the entire river, managing control of the vessel becomes difficult. As the operators attempt to line up for the bridge, the current drives them toward the Arkansas bank of the river and toward the westernmost main pier.

Because the incidence of collisions was so high, the Mississippi Department of Transportation (MDOT), along with the Arkansas State Highway and Transportation Department (AHTD) and the Federal Highway Administration (FHWA), contracted with the U.S. Army Corps of Engineers Waterways Experiment Station in Vicksburg, Miss., to evaluate the relationship between the pier locations and the effect of current on the tow operators. The Waterways Experiment Station extended the model of the Greenville reach of the river and conducted model studies at various flows with remote control barges. By carefully studying the location of the piers relative to the current and the operator’s ability to control the vessels, the pier locations and navigation span clearance envelope were experimentally determined.

Once the navigation clearances had been set, the force of potential collisions had to be computed. This was done in accordance with the AASHTO Guide Specifications for Vessel Collision Design of Bridges. The design vessel was determined to be a 1,500-ft barge tow, up to six barges wide and five long. By considering a powered, loaded tow moving with the current, the equivalent static collision force was 6,600 kips.

Taking blows

The same year that the existing Greenville bridge was opened to traffic, another better-known bridge opened in the Pacific Northwest. The Tacoma Narrows Bridge, better known as “Galloping Gertie,” under moderate wind loading crashed into the Narrows.

In order to determine the wind effects on the new Greenville Bridge, HNTB retained RWDI of Guelph, Ontario, Canada, to make a series of studies to determine the wind environment and the design wind speed for the bridge. Their wind performance evaluation was a process which began with an analysis of historical records of wind data and concluded with a report describing the anticipated wind loads both during construction and after completion of the bridge. The historical data was based on hourly observations at the Greenville Municipal Airport as well as nearby recording stations in Mississippi, Arkansas and Tennessee. From these data it was determined that the design wind speed for stability should be the 10,000-year, 10-minute mean wind speed of 99 mph at the level of the deck. The 100-year fastest mile wind speed for structural design was set at 80 mph.

RWDI then made a review of the preliminary plans for the bridge and established criteria to prevent aerodynamic instability. This preliminary assessment of the bridge was based upon past experience with similar structures and available published data regarding the performance of other long-span bridges under various wind conditions. The criteria to prevent vortex-shedding oscillations were based upon the ASCE-recommended practice of limiting peak accelerations to 0.05 g for winds to 30 mph and 0.10 g for winds in excess of that value. For flutter, the criteria were established as a torsional amplitude of 1.5°. Recommendations for modifications to the bridge structure were made that were incorporated into the final design process. Both drawings and computer models were provided to the wind engineers to begin development of model design for the project. At the conclusion of model construction, final confirmation was made between the structural and wind engineers that the model was in general agreement with the current state of the design and that the scaled mass and mass moment of inertia of the model were verified. The wind tunnel testing program was then started.

The wind tunnel tests were performed for two primary reasons. The first was to determine the dynamic performance of the structure under wind loads. This included an evaluation of the deck cross section and its behavior under various wind speeds applied normal to the longitudinal axis of the bridge and an assessment of potential measures to improve the performance of the bridge. The second reason for wind tunnel testing was to quantify design wind forces to be applied by the bridge designers in order to correctly size the members for the static and dynamic effects of wind loads. These wind forces for design are a combination of shape, wind speed and drag coefficients combined into a single, simple factor to be applied to the various members during the analysis stage.

The factors are similar in application to the standard force coefficients found in design codes; however, the wind data provided through wind tunnel testing is applicable to a specific structure at a specific location. Additionally, these factors include dynamic effects and variations in force on different structural elements due to buffeting, or the random vibration due to unsteady wind loading. The wind report produced for this project recommended appropriate wind load combinations, considering the buffeting response of the structure, the variation in water surface elevation and a complete range of wind speeds and directions. Thus, through a parallel evaluation of the structure and the testing program, the effectiveness of structural modifications was fully evaluated.

Tough place to stand

Perhaps one of the most complicated tasks of the design was the geotechnical investigation. The Greenville Bridge is located in one of the swiftest sections of the Mississippi River. Further complicating the investigation is the water depth, which varies from 60 to 120 ft at the location of the tower piers. In order to provide a stable drilling platform, an offshore, jack-up drilling platform was brought in from the Gulf of Mexico. Anticipating a large, open-dredged caisson foundation, four borings were taken at each tower pier location at the approximate locations of the foundation corners. Samples were taken and laboratory tests conducted to establish the allowable bearing capacity of the material.

As a result of the geotechnical investigation, it was confirmed that a dredged caisson foundation was the appropriate foundation type for this location. Dredged caissons are a type of foundation where the method of construction is as much a part of the design as the bearing capacity itself. Because of the depth of water, it was decided that a “floating” caisson would be necessary at Greenville. In this method, the contractor is required to progressively construct a large perforated concrete box in the river and keep it afloat until it is tall enough to rest on the river bottom and extend above the water surface. Once safely on the bottom of the river, the contractor may then begin to excavate the material from within the caisson in order to reduce friction and cause the caisson to “sink” to its final bearing elevation. Finally, the contractor may seal the bottom of the excavation and start up with his construction process.

Completed caissons

The current construction contract for the main span (595 ft-1,378 ft-595 ft) includes two anchor piers on drilled shaft foundations, two deep river caissons, towers and the cable-stayed steel superstructure with precast deck panels.

As of this writing, the contractor, a joint venture of Massman Construction Co. of Kansas City, Mo., and Traylor Brothers of Evansville, Ind., has completed the construction of the two caissons in the river and begun construction of the two tower piers in the middle of the river. They have progressed to an elevation approximately 30 ft above the level of the bridge deck and are on schedule for completion in 2006.

There will be two separate contracts let for bids for the approach spans. For the Mississippi side, the total length of the project will be 8,475 ft including 6,535 ft of bridge. For the Arkansas side, the total length of bridge will be 7,504 ft including 4,660 ft of bridge. Both approach span contracts will include a combination of welded steel plate girders and precast prestressed concrete beams spans with a cast-in-place concrete deck on drilled shaft foundations. The expected letting dates for these contracts are late fall 2004 for the Mississippi side and late spring 2005 for the Arkansas side. Both contracts will be let by the Mississippi Department of Transportation.

About The Author: Hague is an associate vice president with HNTB Corp., Kansas City, Mo. Carr is a state bridge engineer with the Mississippi DOT, Jackson, Miss.

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