The spirit of bridge building

St. Louis has plans for the widest cable-stayed span in the world

Bridges Article November 28, 2001
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You can’t just barge in


You can’t just barge in.


Steering a heavy, wide load down the Mississippi River takes extreme precision and an experienced, delicate touch. The St. Louis area is more like a flooded forest for barge operators. Three bridges in particular—Martin Luther King, Eads and Poplar Street—stand relatively close together and require sharp senses.


Just north of those spans lies an area bracing for immensity. When completed in 2010, the New Mississippi River Bridge, at a cost of $370 million, will be the widest cable-stayed bridge in the world and the longest in the Western Hemisphere.


More importantly, it will take barges of all sizes, thanks to a design approach by Modjeski and Masters Inc., St. Louis, that includes a 2,000-ft-wide opening.


"St. Louis cares very deeply about its community and how it is perceived," Michelle Graham, public involvement manager for HNTB Corp., told ROADS & BRIDGES. "With such a signature plan it’s very important to this community that this new structure fit within the history of innovation and forward thinking. People feel very passionately about that."


Taking the world by committee


HNTB Corp. has served as the bridge’s top campaign manager, and the approval rating for such a structure remains high. Serving as the program management consultant for the project, HNTB kept the gateway of information open and essentially tied the public and DOT officials together by:


  • Creating communication policies;


  • Helping clients establish goals;


  • Involving the public in the bridge design and urban design of the Missouri interchange;


  • Developing ongoing communication with Illinois neighborhoods; and


  • Conducting public and committee meetings.

"The city of St. Louis had very strong feelings about the type of bridge they wanted," said Graham. "There were even discussions about a design competition. They wanted some type of commitment that it would be a world-class bridge."


To answer the call, the Illinois and Missouri DOTs, the city and HNTB worked out a process which formed two committees. The regional advisory committee (RAC) represents all the communities involved and impacted by the project. The executive branch includes district engineers from Missouri and Illinois, the mayor of St. Louis and the St. Clair County Board Chairman.


HNTB put education first, familiarizing the RAC with the technical aspects of building certain types of bridges. Members also had the chance to play barge operator. HNTB rented a boat and took the group to the bridge site so they could see the approach. When the actual bridge type was chosen, the RAC went to a barge simulator in Paducah, Ky.


After developing a definition of a "world-class bridge," six design consultants were interviewed and the RAC gave its recommendation to the executive committee. The Illinois DOT made the final decision.


The winning design team, Modjeski and Masters, now had the chance to work with the RAC on this world-class concept. Thirty initial designs were drawn up; 15 were presented at public workshops.


Community input led to a one-day design charrette, where the RAC, Illinois and Missouri DOTs, Federal Highway Administration, Corps of Engineers and the U.S. Coast Guard reduced the choice to six. All six went under extensive engineering analysis and were presented to the public via a newsletter. From there it took six months to determine the winner.


"At the end of the day in terms of aesthetics and the world-class bridge concept the RAC wanted a river-spanning bridge," said Graham. "They really wanted to respect that river by spanning the bridge across."


A lot to swallow


Jim Easterly knows how difficult it is to shake hands with the Mighty Mississippi—a body of water which has no intentions on cooperating.


"Any time you deal with the Mississippi River it’s a challenge," Easterly, a district engineer for IDOT who has been involved in a handful of bridge projects across the river, told ROADS & BRIDGES. "The river is going to do what the river is going to do, and you’re fooling yourself if you say I’m going to do this or that and disregard the river . . . it will eat you very, very quickly."


The New Mississippi River Bridge will not come dressed with parsley.


For the most part, the two 510-ft, inclined towers are clear of any immediate


danger, with one of the towers located at the bank of the river. The other will be "outside of the flow situation," John Kulicki, president and chief engineer of Modjeski and Masters, told ROADS & BRIDGES.


The main span will measure in at 2,000 ft, making it the longest cable-stayed bridge in the Western Hemisphere. The width covers a world-record 222 ft to hold eight lanes of traffic, with the possibility of 12 in the future. Fattening the bridge will be two 72-ft-wide roadways and an area down the center which will serve as a "central tower" to handle some of the immense weight.


"The purpose of the third set of cables is to support the transverse members at a more tolerable span," said Kulicki. "With that much suspended area the weight is significant, the size of the span is significant and the loads to be transferred are significant. What we’re doing is breaking up that span and supporting the dead weight in three places instead of two."


Each half of the suspended span is carried on three planes, and each plane will have 26 cables as large as 10-12 ft in diam.


"There are three planes in the navigational span anchoring against two planes on the back span, and the two planes on the back span go to the middle of the bridge," explained Kulicki.


Accommodations also had to be made to execute the idea of clustering side span cables into approximately 200 ft of space at both ends of the bridge. There, concrete cellular construction, located about 100 ft past the towers, will provide the needed weight to offset the cable groupings.


The ability to replace the bridge deck is rare in cable-stayed bridges, but is a necessity if you tag a structure like the New Mississippi Bridge with a 150-year service life. The public also viewed it as a top priority. To make deck replacement possible, high performance steel beams will be used.


"There are still loads that will be carried by the replaced deck," said Kulicki. "It’s not necessarily the case that 100% of the thrust gets transferred to the beams at that time, but a significant amount of it does."


The region as a whole, however, does not sit on stable ground. On the Illinois side, material is subject to liquefaction, and there is the potential of lateral spread during an earthquake. To address this concern, the design team decided to place the Illinois tower on a dredgewall caisson which will be set on solid material approximately 100-110 ft below the surface. The dredgewall caisson will be capped with reinforced concrete. A limestone formation is attainable at reasonable depth on the Missouri side. However, there may be the presence of solution cavities which hold pinnacles of rock. Crews have conducted extensive drilling to make sure future problems can be avoided.


"We don’t want to be sitting around an inverted ice cream cone," said Kulicki. "We do anticipate we’ll be able to place the tower on a spread footing on rock."


With the New Madrid fault line located just south of the St. Louis area, preparation for a major earthquake is essential. The New Mississippi Bridge is expected to handle a 2,500-year event. In addition to using rock motion data from USGS and the Mid-Atlantic Earthquake Center, special seismic evaluations were conducted during preliminary boring to produce parameters necessary for SHAKE 91 computer simulation.


"What we have found so far, because the vibration period of the bridge is relatively long, that we are far out on the earthquake spectrum where the loads become much lower.


"Also, when considering the wind and all other things the bridge must accommodate, preliminary calculations indicate that the towers will respond in a nearly elastic manner to these loads," said Kulicki.


Extensive analysis on the effects of those loads also was done on the approaches, and it was found that standard-type approach columns should be adequate.


Loads of difference


Dealing with heavy traffic is a constant battle, and St. Louis is in desperate need of another land-craft carrier. The New Mississippi Bridge will take significant strain off the Poplar Street and McKinley bridges.


There are fewer lanes spanning across the Mississippi River now than in 1965. Over 30 years ago the MacArthur, Poplar Street, Eads, Martin Luther King and McKinley offered 22 lanes. In 2001, there are 16.


The Poplar Street span is carrying the team. Three interstate routes—I-55, I-70 and I-64—cross the bridge into downtown St. Louis, and the average daily traffic is approximately 125,000 vehicles. Driver confusion runs high. The section is cluttered with various ramps and connections, and some exits are outdated from a design standpoint. In fact, the steep grade on the I-55 South connection has been known to overturn trucks. To keep the semis grounded officials plan to eliminate I-70 ramps which head north into downtown St. Louis and widen I-55’s to two lanes.


The Eads Bridge is being rehabilitated. First to cross the Mississippi in the late 1800s, the historic Eads is a double-decker structure that carries vehicles on top and rail down below. The vehicular deck is being replaced while light rail operates on the lower portion. However, when the deck is complete the Eads still won’t be able to hold a heavy volume of traffic, and it’s difficult to reach.


To further complicate the Mississippi crossing, the McKinley Bridge is on the verge of closing.


"It has an 8-ton load limit on it today," said Easterly. "The center of this region has a serious shortage of capacity to get from one side to another."


The New Mississippi River Bridge answers the cry and will realign the interstate system in both Illinois and Missouri.


East of the Mississippi I-70 will move north to connect with the new bridge, and there will be construction of a connection between I-64 and the "new" I-70. The "Tri-Level" interchange, where I-55, I-70 and I-64 meet, will be upgraded to eliminate a flood plain and State Route 3, a twisting, two-lane, north-south road running along the river, will be straightened and widened to four lanes.


"When the Tri-Level was built it was built under the water table," said Easterly. "Massive pumps are keeping that interchange dry. If our pumps would ever go out that interchange would be under water."


On the west end, in addition to the improvements on the Poplar Street Bridge, a new interchange will be built to connect the new bridge to the existing I-70 and local streets.


The total cost for the road work is about $640 million.


About the author: 
Bill Wilson is editor of Roads & Bridges
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