Everybody wants in-and out

May 20, 2004

Every once in a while, the mayor has to get a key made. The Tampa-Hillsborough County Expressway Authority, designer Figg Bridge Engineers and prime contractor PCL Civil Constructors Inc. are in the process of executing the perfect cut, one that will keep the doors open into downtown Tampa for years to come.

Every once in a while, the mayor has to get a key made. The Tampa-Hillsborough County Expressway Authority, designer Figg Bridge Engineers and prime contractor PCL Civil Constructors Inc. are in the process of executing the perfect cut, one that will keep the doors open into downtown Tampa for years to come.

Faced with the constricted problem of congestion on the four-lane Lee Roy Selmon Expressway, then-Mayor Dick Greco knew the walls were closing in at a fast rate. Additional capacity appeared to be the only way out. Initial plans called for the construction of two more lanes, but the Expressway Authority viewed that approach as more of a bandage instead of a permanent stitch.

“We really needed to turn this into a 10-lane facility,” Pat McCue, executive director for the Expressway Authority, told Roads & Bridges. “That’s when we started investigating other ways to do it.”

When it was all said and done, the one way was like no other in North America. Working closely with Figg, the Expressway Authority proposed taking up just 6 ft of median for a three-lane segmental reversible bridge. And to avoid the obtrusiveness of an elevated concrete structure, aesthetics became a critical element of design.

“The mayor was very interested in artistic statements downtown and he liked the design,” said McCue. “We did some drawings and then created a video showing how it would be built and that it wouldn’t be a major traffic disruption. He liked it so much he actually opened the video by telling people that this was the future of Tampa.”

The final plan was the positive talk of the town. It fulfilled the Florida DOT’s request of a system that would tie into I-4. Tolls allowed the Expressway Authority to cover the entire $300 million cost of the 5-mile bridge, leaving Tampa free of any debt. Through the use of an underslung truss system, crews would only close one lane of traffic in each direction during off-peak travel times. The EPA has designated Tampa as a non-attainment area, but the bridge will upgrade the system from a level-of-service F to a level-of-service B, hence serving as a cleanser to the environment. And, perhaps most important, rounded corners and curved flares appeared easy on the eye.

“In the public meetings we showed that video to probably 20,000 to 30,000 people at various functions and they loved it,” said McCue. “Usually opponents just come out of the woodwork when you do something like this. Nobody opposed this project.”

Sliding into place

Figg caught the eye of the Expressway Authority at the right time. The Tallahassee-based design firm was the top choice to execute plans for three miles worth of feeder roads in the suburb of Brandon Parkway which would link to the city’s interstate system. With the presence of wetlands nearby, environmental sensitivity was high. Other designs called for the use of fill sections to get up over existing interstate which would have affected the flow of water through the area and required substantial litigation. Figg stuck with its concrete segmental bridge design, and instantly won over the Expressway Authority.

“The board liked the design so much they decided to use the same bridge to go downtown,” said McCue.

Crews first broke ground for the reversible three-lane downtown bridge in the summer of 2003, inserting drilled shafts 6 ft in diam. 70 ft deep into solid limestone. A total of 226 shafts, with a maximum load of 2400 kps, have been used for the job. Drilled shaft collars are located just 2 ft below ground surface.

Pier construction followed. The cast-in-place structures, which are 6 ft wide and range in height from 25 to 60 ft, have been poured in two sections. An 8-ft stub is poured at the base of the pier, then a portion which is shaped more like a champagne glass is set into place. The curved flare at the top has a maximum width of 13 ft 8 in. to match the bottom slab of the box girder and accommodate the bearings. The bridge will be held up by a total of 206 piers, and 164 are expected to be constructed by the end of June.

The pier segments for the superstructure are being precast at a yard a few miles south of the jobsite in the port of Tampa. A total of 11 casting cells are being used. Each cell consists of eight typical segment beds and three specialty segment beds. The specialty beds consist of one expansion joint segment cell. The bridge segments are being cast with an extra 1?2 in. of concrete on the top slab so when erection is complete the riding surface can be milled to meet rideability criteria.

Using the match-casting process, PCL is currently producing nine segments a day. The 59-ft-wide, 70-ton pieces are then hauled by truck to the construction site.

The segments are actually heavier per foot than a typical segment because of the weight of the concrete internal diaphragm. The internal diaphragm actually supports the entire weight of the structure between the columns and transfers the load down to the columns.

“It’s also a place where we anchor our post-tensioning,” Jose Rodriguez, senior project director for Figg, told Roads & Bridges.

The segments are delivered to 160-ft self-launching erection girders located in the median of the expressway. Following the span-by-span erection method, cranes lift and place the pieces onto the girders, which are equipped with a chain drive system that pulls the segments down the girder into place.

According to Rodriguez, PCL had to work through a learning curve when it came time to operate the erection girders. Two are used for each span, and setup time was a little slow at the start.

“Once the contractor handled that initial learning curve construction turned quick and they were building two spans per week,” said Rodriguez. As soon as the erection girders slide the segment pieces into place, an epoxy glue is applied to the base of each segment, and at the end of each span crews fill 6-in.-wide closure joints with a concrete mix.

Post-tensioning also is conducted. In each web the typical span has four 27-in. x 0.6-in. strand tendons with a 12-in. x 0.6-in. strand tendon in interior spans or a 19-in. x 0.6-in. strand tendon in exterior spans.

Transverse post-tensioning will be executed on the top deck. Transverse post-tensioning will provide compression in the top slab of the concrete, making it a durable long-term structure. In all, the job calls for 6.3 million lb of post-tensioning.

Each 142-ft span will consist of a total of 14 segments. PCL was on target to have 100 spans completed by the end of June, which would mark the project’s halfway point.

High-performance, high-early-strength concrete mix, which contains super plasticizers, is being used for both the piers and the superstructure. The strength over a 28-day cure time is 6,500 psi for the piers and 6,000 psi for the superstructure. According to Rodriguez, PCL is achieving 10,000 psi strength on the deck during the same time frame.

There are three locations where the existing roadway cannot be avoided. Two of the trouble spots are on each side of the toll plaza for the existing roadway, and the third is where the expressway turns into actual city streets. Figg elected to use straddle bents as a solution. Essentially, a transverse beam is placed on two columns so the elevated structure can span over the existing roadway.

“This bridge is being built for $65 a sq ft,” noted McCue. “That is the most economical bridge project built in the state in the last 30 years. The average bridge costs $90-100 a sq ft. We saved about $50 million on the cost of the project. It’s also designed for a 100-year service life.”

The sinker

With the erection girder experiment out of the way, PCL was cooking. The prime contractor was six months ahead of schedule when Mother Earth decided to create a mess. During the early morning on April 13 a sinkhole dropped one of the piers about 20 ft. Fortunately, the only injury was suffered by one of the spans.

“The bridge performed as it was supposed to: It moved as a complete unit and in one piece following the line of the pier,” Figg President Linda Figg told Roads & Bridges. “Once it happened I knew we had to take quick action to come up with a plan to make sure everything would be safe for everyone.”

At the time of the accident, temporary supports were being constructed under two spans and PCL created two temporary lanes in both directions to keep motorists out of harm’s way. Crews were in the process of disassembling the two spans so geotechnical specialists could conduct an investigation. Figg said the group planned on re-examining a variety of different locations along the route for safe measure.

“The presumption is the sinkhole was about 100 ft into the ground,” said McCue. “So far the conclusion is that this is just a bizarre event. It’s like hitting a needle in a haystack, but we hit one and it temporarily slowed us down.

“This is the strongest bridge ever built in Florida. The trusses put on these piers to support the segments during construction add another 700,000 lb to the weight. That’s five to 10 times greater than the loads this bridge will handle when it opens,” McCue added.

Future viewing

The bridge has certainly drawn crowds like a bodybuilder, but most are interested in catching a view of the future in transportation. “We’re receiving calls from visitors all over the world wanting to come and look at it,” claimed McCue.

The structure will be an open-road facility, with Florida SunPass users paying a $1 toll and cash customers handing over $1.25. Intelligent transportation systems, like cameras, will be able to detect violators, who will be billed for the use.

A transportation managment center also is under construction. The Expressway Authority will share the space with traffic engineers for the city of Tampa. Here, they’ll be able to respond to stalls or accidents on the bridge and coordinate signal timing throughout the metropolitan region.

“When we have special events downtown we can control all the traffic lights and the bridge to facilitate the flow of traffic coming in and then reverse it if need be,” said McCue.

Air Force technology will be installed on the bridge to catch those confused about the direction of traffic. Safety nets, like the ones used on aircraft carriers, will be ready to go at strategic points. If someone breaks through one of the wrong-way gates a net will cross the entrance ramp to catch the intrudor.

The bridge is expected to be complete in July 2005, and should provide an answer to a controversial and challenging problem for years to come. “This bridge is really the solution to increasing urban transportation capacity in existing right-of-way,” said Figg. “People from all over have visited the project to examine it as a solution to meet their capacity challenges.”

“This is a very novel project in the sense that all of the things that we are doing have been done before individually, but nobody has ever combined all of the features into one,” added McCue.

About The Author: Wilson is Editor of Roads & Bridges.

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