Crossing The Mississippi

Massman Construction Co./Traylor Bros., Inc., a Joint Venture, and HNTB have been working together since 2001 on what will be the third-largest cable-stayed span in North America. The new US 82 bridge in Greenville, Mississippi, is replacing a narrow, outdated bridge that crosses the Mississippi River

The new $110-million bridge will be concrete piers with a steel superstructure. The cable-stayed portion of the bridge will have a total of three spans. The main span of the bridge is 1,378 feet long, a significant improvement over the 840-foot span of the steel truss bridge, that will also make it the longest cable-stayed structure in the United States. The outside spans are each 591 feet long.

The twin towers that make up the main support will be the most impressive feature of the bridge. They come at a price of $20 million per tower. Once complete, they will be 425 feet tall each. At the time of our visit, the Arkansas tower, pier 37, was 60 feet away from its completed height, and the Mississippi tower, pier 38, was 100 feet away from its completed height. On our visit to the site, Gowen Dishman, project engineer for HNTB gave us a tour of the site, taking us to pier 38 by boat. We climbed nearly 300 feet to the bow-tie structure of the tower, where workers were preparing to stress the post-tensioned strands. In the time it took us to climb from the bottom of the tower to the bow tie and back down again, a tow boat had moved from beneath the old bridge, so far down river it could barely be seen in the distance.

"Our contract was originally to oversee the construction of the entire project. The state has decided that they want us to oversee the construction of the Mississippi approach. So far they've told us that we will also be overseeing the Arkansas approach," says Mike Caulfield, assistant project engineer for HNTB. "It's been a fantastic experience for all of us here. As an engineer it's the kind of project you dream of doing when you're in school."

The old Greenville Bridge, built in 1940, is a steel truss bridge, a popular design for its time. Its main span is 840 feet, which was the longest span for a highway bridge on the Mississippi River until 1943, when an 845-foot bridge at Dubuque, Iowa, was completed. The old bridge was designed by HNTB and built by Massman. Both design firm and contractor are working on the current project.

According to Caulfield, the old bridge is structurally sound. "There are two reasons we're replacing the old bridge. One reason is the bridge is functionally obsolete. The more significant reason is its location makes it the most hit bridge anywhere, struck by tow boats coming downstream."

The old bridge has two 12-foot lanes and no shoulders. Crossing the narrow original bridge is an anxious experience. Its location could have provided a perfect view of the new cable stay's construction, but taking my eyes off the road for even a brief moment caused a bridge vertigo — it felt as if the car might veer into the side of the bridge. It is easy to imagine an oversized load, such as a house trailer, crossing the old bridge might pose as a game of chicken to oncoming traffic, if the opposite lane were not shut down in advance.

The old bridge is also a crash course for marine navigation. Walker Bend, a 90-degree turn in the Mississippi River, lies just above, making it difficult for barge traffic to maneuver around the structure. "Coming south, the river current's forces strike west, and the tow boat has to make a hard turn to catch the navigation span," explains Caulfield. "It's a very tricky maneuver for them. Since 1972, there have been 47 recorded impacts on the second pier."

A camera was mounted on the old bridge to record barge collisions, when studying options for a future bridge. Gowen Dishman shared a video of a barge hitting the second pier on the Arkansas side. In the video, the bundle of barges nicks the side of the pier. The force of the impact breaks the cable that binds the barges together, and they scatter apart. Fortunately, the bridge only sustains superficial damage in such an impact, and the tow boat usually only loses the time it takes to reign in the barges and tie them together again.

Though the old bridge is structurally sound, it is a navigation hazard, and will eventually be removed. When the new bridge and approaches opens in 2008, the old bridge will be scheduled for demolition.

A major consideration in the location of the new bridge was that placement had to be far enough from Walker Bend to allow easy navigation for marine traffic. "The location of the span was determined by the U.S. Army Corps of Engineers Waterway Experiment Station in Vicksburg," says Caulfield. "A scale mock-up of the river, and remote-control tow boats were used to determine placement. They decided that 3,000 feet south of the old bridge was a good distance for the tow boats to make the curve."

Other improved features of the new bridge include four 12-foot lanes, which will make it considerably wider than the old bridge. The structure will also include a 12-foot outside shoulder, and an 8-foot inside shoulder.

Steel girders were fabricated by PDM Bridge. The girders are 50 feet long and 100 feet wide. There are 56 field sections. Steel will be placed off barges. Massman/Traylor will work both piers at the same time. The first steel was scheduled to go up the week of May 1st, just days after our visit to the site.

Cables for the bridge are made of 270-ksi steel strand, with 28 on each pier. All together, there will be a total of 112 stays.

Caissons for the new bridge are not traditional pneumatic caissons. These caissons were open dredged — river bottom material was removed through the open top of the long caisson box. The bottom of the caisson, or the cutting edge, is a 122-foot by 80-foot steel box. Air domes help the box stay buoyant and keep it watertight. This working chamber is floated down the river to the location of the piers. Concrete is poured into the exterior walls of the cutting edge and the interior walls around the air domes. Concrete is raised 10 feet at a time. Each concrete placement makes the steel box heavier, sinking it into the water along the caisson guide, a structure that protects the caisson from strong currents and vessel collision. When the caisson reaches the river bottom, the air domes are removed, and cranes with clamshell buckets then excavate riverbed material. Divers went down inot the murky waters to inspect the installation and insure its integrity. As the soil and materials are removed, the caissons sink further into the river bottom. Completely sunk, the caissons reach 120 feet into the riverbed.

Steve Underwood, project manager for Massman/Traylor JV, has been with Massman for 25 years. This is the first cable-stayed bridge he's worked on. Underwood says the biggest challenge has been floating the caissons. The process is unique to the construction of this bridge. "If you don't know how to track one [caisson], then the first one won't be a good experience. It's not unusual to have to refloat the caisson."

Gowen Dishman has worked on other bridges for HNTB, including the Skyway in Tampa, Florida, and Baytown in Houston, Texas. In his office, he displays pictures he's taken of the impressive bridges he's worked on. Dishman attributes HNTB's success with bridge design to their practical approach. "You don't design without thinking about the construction — how are you going to build it? HNTB designs from a construction standpoint. It doesn't matter if you say a cubic yard of concrete, if it takes a different crane to place it."

According to Dishman, workdays are 10 hours long, work weeks are six days a week. A very natural fear of heights, coupled with the fear of the strong and steady current of the Mississippi below, makes it hard to imagine working on these towers day in and day out. Those factors aside, high winds and flooding can cause problems on the job site. Winds of 30 miles per hour limit crane use. When the Mississippi rises 48 feet or more, it limits and even shuts down work. Since work began in August of 2001, flooding has caused work delays on four different occasions.

Underwood says he uses around 125 workers at a time, but finding experienced labor for this job has been difficult. "There's nothing in this area that's been built of this proportion in several years," says Underwood. "Being on the river in addition to that makes it more difficult. And then, you get 400 feet high over the water, and it makes it even more difficult. I've had several people come out and tell me they are willing to work high, until we get out on the water. When you put them on the water, it changes their whole perspective."

The curve in US 82 will be broadened to catch the new bridge. The Mississippi approach will be 2,970 feet of roadway, and 6,406 feet of approach bridge. The contract has been let to Hill Brothers of Mississippi. Work began in April of this year, with a completion date of April 1, 2008.

According to ADOT Public Affairs, the Arkansas approach is expected to let in August of this year. It will be 3,752 feet of roadway and 4,602 feet of approach bridge.

Total length of the project, including bridge, approaches and new roadway, is 3.84 miles.

Though some delays due to flooding have put the project behind schedule, Underwood says he believes they will complete the job within the timeframe. Both the design team and contractor agree that partnering has worked really well on this site. "We spoke in front of a group when we started, and one of their comments was 'we'll see how it's working two years from now.' Well, it's two years from now, and we're still working." Adds Dishman, "They [Massman/Traylor] had to have confidence in us, and trust in the design of the project, to do a good job."

That good working relationship has served them well in the past, and may serve them beyond this project. The short list for the cable-stayed Mississippi River Bridge at St. Francisville was recently released. HNTB and MRB Constructors, a joint venture of Traylor Bros., Massman Construction and Gilbert Southern Corp., is one of the three teams asked to submit proposals. A team for this project should be chosen by the end of the year.