By: Bill Wilson
No two cable-stayed bridges are alike in Kentucky. Of course, the state will have its first pair up and running by 2002. But instead of just forming duplicates with the same contractor, a unique situation has developed.
As soon as designer preferences took hold, contrasts between the spans in Maysville and Owensboro became prevalent. The prime contractor on both jobs, Traylor Bros. Inc., Evansville, Ind., has juggled all the requests—and harmony has prevailed. In fact, problem solving on one bridge has put minds at ease on the other.
"There’s been a transfer of information from Maysville to Owensboro to try to make things go smoother," Bill Caroland, chief bridge engineer at American Consulting Engineer, Lexington, Ky., told ROADS & BRIDGES. "It’s been the best construction job I’ve ever been on because usually you have the contractor versus the inspector and they’re at each other’s throats. It hasn’t been that way at all."American Consulting Engineers teamed with Michael Baker Jr. to design the Maysville bridge, a $36 million project which is expected to be completed by Thanksgiving. Parsons Brinkerhoff produced the blueprints for the $55 million Owensboro bridge that could be open to traffic in late 2001. Both structures span the Ohio River.
Thanks to the countless amount of information sharing, the Kentucky Department of Transportation (KDOT) has received a good education on cable-stayed bridge construction. One well-known fact, however, could not be overlooked—teamwork conquers all.
"You need good people, you need to have a good design and you need to have good suppliers," Steve Waddle, transportation specialist with KDOT, told ROADS & BRIDGES. "We’ve had all that. I have learned so much and I hope to use all this knowledge again."Fraternal twins
The two bridges were chosen for the same reason. Cable-stayed spans are more economical than those of the steel truss variety and often take less time to erect.
But each designer applied their own touch, and the differences were significant.
The Maysville bridge has two lanes with the possibility for a third in the future. It’s 2,420 ft long from end-to-end and has a roadway width of 48 ft.
Two H-shaped towers help support the bridge and made it easier to "keep the stays in line between the deck and the tower," said Caroland. The tower legs are post-tensioned from the beginning of the cables to the top to counteract the tension force from the cables.
Longitudinal post-tensioning also was used throughout the deck panels.
"At certain live loading conditions there is tension in the deck, and we know once you exceed 7 ksi on the reinforcement steel you’re going to get a crack in the concrete. We feel that post-tensioning longitudinally along the entire length of the bridge would keep the deck from cracking," said Caroland.
The cable guide pipe connection to the girders at the bridge deck level is different at Maysville. There, the cable anchors are all below deck and are connected to the girder with a bolting connection throughout the web. It’s all post-tensioned and was primarily done to prevent fatigue.
Because both bridges sit near a major fault line, seismic designs were used to counteract a minor earthquake. Maysville has force regulating units located on each tower. The units serve as big shock absorbers which allow the temperature and quake forces to be released without much resistance.
"We felt like, from all the earthquake data we have read and all our fears on earthquakes, that the best thing to do was make as many of the substructure units as we could sharing the earthquake force," said Caroland. "When you have earthquakes, or even high winds, these units lock up and force both towers to share the loads.
According to Caroland, Maysville is the first bridge in the U.S. to use force regulating units. Several other bridges are beginning to follow suit, including the Golden Gate Bridge in San Francisco.
A minor, but significant problem in Maysville involved the reinforcing steel for the barrier walls in the deck panel. The decks were cast in vertical precasting beds, and the steel was detailed perpendicular to the precast panels. The panels were placed side-by-side at each location on the bridge. When on a cross slope, the barrier wall steel is supposed to be vertical to the bridge. But when installed on the cross slope it was at 90û to the panel.
"There was a slight angle, so the barrier wall was really bending out," said Waddle.
The barrier wall needed to be vertical to the center line of the bridge. To correct the problem, steel had to be bent in place by hand, costing at least four days work.
"What we could have done if we would’ve known the problem is when we cast the precast panels we could have put the barrier wall steel in at an angle equal to the cross slope, then when it was on the bridge deck it would’ve been perfectly verticle," said Waddle.
A splice in the barrier wall will help deal with a similar problem in Owensboro.
Built for four lanes, the Owensboro bridge is 4,510 ft long from end-to-end and offers a roadway width of 66 ft, 6 in. For aesthetic reasons, plans called for A-framed towers.
Post-tensioning was not utilized in the tower legs. The upper cable connection at Owensboro uses a steel frame, so the contractor was able to cast the concrete tower legs around the frame, which resists tension forces.
"That’s something they could do because of the A-frame design," said Waddle. "Each side of the tower had the cable connections connected to a steel frame."
The cable guide pipe connection to the girders at bridge deck level consists of anchors located above deck and no post-tensioning.
"That’s a designer preference," said Waddle. "It may be designed such and have loads that they don’t expect stress reversal."
Transverse bumpers are used on each tower for earthquake protection in Owensboro. Elastomeric pads allow for slow, but not sudden, movement.
Inconsistent geology also made construction of the foundations difficult. Some of the 8-ft-diam. drilled shafts were 135 - 140 ft long and had to be inserted deep into the river bottom.
"There was a logistics problem," Henry Phillips, transportation engineer specialist for KDOT, told ROADS & BRIDGES. "We were working on the Ohio River, on a wide section of the river, and we had problems trying to drill that deep. It was just a chore getting that much steel reinforcement down in the shaft and getting concrete in the hole."At Maysville, the river bottom is overburdened with sand and gravel over limestone. The towers are supported on 16, 72-in.-diam. shafts drilled 10 ft into solid rock.
Self taught
First timers often go through a trial-and-error phase. KDOT experienced more of a trial-and-enhancement cycle with the two cable-stayed bridges. The following is an outline of the learning process.
Polyethylene pipe
The cable system of each bridge, which is a multiple mono-strand system, consists of a series of individually greased and sheathed seven wire strands and is placed inside a polyethylene pipe.
The polyethylene pipe was originally set up to be a black pipe coated with tedlar tape. After further review, a co-extruded pipe, popular in Europe, was used. The co-extruded pipe has a black layer and a thin white outer layer. It serves the same purpose as the tape, which is to provide ultraviolet protection. Maysville installed the tape version before converting over to the co-extruded pipe. It was added to the contract in Owensboro.
"It just felt like we got a tremendously better product," said Waddle. "The tape was a nightmare to install and there was potential maintenance problems down the road."
Utilizing a spiral bead
A spiral bead about 1/8 in. in diam. was wrapped around the pipe to "facilitate runoff rainwater."
"There have been past experiences where if you get light rain and light wind the cables start vibrating," said Waddle. "Even though we felt like we were pretty far inland, we felt like it was good insurance."
Revision of pipe connection
Originally the guide pipe, which is the bottom connection to the cables, was going to be welded to a 4-in. anchor plate that would be connected to the main girders. KDOT, however, detected a problem.
"If that guide pipe was to experience any vibrations, the first thing that would go would be that stiff weld," Waddle explained.
Instead, they connected the guide pipe to a 1-in. plate and moved the weld 4 in. up the pipe.
"Then we took that 1-in. plate and bolted it to the original 4-in. plate," said Waddle. "That created a very fatigue-resistant design."
Cable installation
According to KDOT, the strand-by-strand cable installation method was used with the co-extruded polyethylene pipe for the first time in the U.S. The tricky part was pulling a cable with a shoe through the pipe. The shoe had a tendency of getting caught on a spiral spacer connected to a plastic weld tab located inside the pipe, and when this happened workers had to start the process all over again, which usually took half a day.
"To correct this problem we took all those weld tabs and we ground them down to where they were smooth, and that would cause the shoe to knock back in towards the center of the pipe," said Waddle.
The tabs were smoothed in Maysville. Before installation took place in Owensboro, the spiral spacer was cast as part of the pipe and the shoe snag was all but eliminated.
Strand fabrication
Each strand is made up of seven wires, and each cable is made up of 55 strands. During testing in Chicago, one wire broke during fatigue testing.
According to code, it still passed, but the incident brought crews from both bridges together for discussions.
"We realized during the fabrication of the wires there could be very, very slight imperfections," said Waddle.
After it was agreed that the acceptance criteria would be increased, 110% eddy testing was done on all fabricated wires.
Revision of cable wind tie connections
On the Maysville bridge, it was discovered that there was a very rigid connection where the wind ties were connected to the polyethylene pipe. To prevent future cracking, a re-designed connection called for neoprene collars.
"It still provides support, but it also provides a little give in the connection so we don’t put stress on the pipe," said Waddle.
As of press time, it was not known if neoprene collars were needed in Owensboro.
Progress report
The Maysville bridge is near completion. On Oct. 23, concrete barrier walls were in place on each side and they were beginning to erect a Texas truck rail. Strand tails in the towers had to be cut and caps on the anchorages needed to be installed. Electrical work for aviation beacons, a ventilation fan and various lighting also was expected to take place. Crews began grouting the cables Oct. 31.
In Owensboro, the cast-in-place tower on the Indiana side of the bridge has been constructed and work crews were in the process of installing cables and precast deck panels.
About The Author: Wilson is managing editor of ROADS & BRIDGES
