Crews have completed the widening of a segmental fly-over bridge in Miami-Dade Countya rare construction feat and the first job of its kind in the mainland United States.
Construction of the project that doubled the 29-ft width of the 3,019-ft-long precast concrete, segmental box overpass merging northbound traffic from the Palmetto Expressway onto northbound Interstate 75 began in March of 2003. Davie, Fla.-based Astaldi Construction Corp. took 17 months to complete the $6.4-million contract, increasing the bridge from one lane to two lanes with shoulders while keeping one lane open to traffic.
Segmental bridges are not unique, but the task of widening such a bridge called for a design approach that needed to be innovative sinceaccording to the American Segmental Bridge Instituteit was the first such widening of a segmental bridge. Project engineers, however, say one other widening was done in Hawaii.
The designer, Tallahassee-based Corven Engineering, produced a model from existing plans and field visits. We looked at the [original] design and noticed that the design had some reserve capacity, says John Corven, president of Corven Engineering, also the engineer of record. He notes they were concerned about what would happen once the cantilevered wings were lengthened. Would they be able to carry the traffic loads when they moved all the way out to where the new parapets were? Corven says. So there was a transverse design consideration, which we found the bridge had reserve capacity to handle. We were real excited about that and it gave us confidence to go forward, to now being able to carry the bridge longitudinally to carry the two lanes of traffic.
Radius curves at either end of the bridge added another challenge. We had to go back and mimic [the curves] in the computer the way the bridge had been built, including the effects of curvature torsion overturning stability, says Corven. When you build it cantilevered, you lock in certain stresses at different stages as you close each span.
This would not have been a concern, if not for the original design of the bearings between the deck and the columns. They were very, very close together, so that any new eccentricity would cause uplift on the bearings and the bridge could turn over, Corven says. That gave us great concern. Typically a bridge like that [the bearings] might be spaced 10 feet on center; these were spaced 4 feet 6 inches on center.
Corvens next analysis showed that if the bridge could be widened symmetrically, the bearing issue would solve itself because the loads became symmetrical. The bridge had the strength. It was how do you get there from point A to point B, from a narrow bridge to a wide bridge, allowing the traffic to run on it at all times? Corven says. Engineers had to get counterbalancing loads on each side to keep the bridge from tipping over.
Dealing with the existing structure, built in 1985, and how it varied from drawings presented another challenge. This bridge had no as-built type drawings or as-built documentation of any sort, says William Garcia, project manager for Miramar, Fla.-based Consul-Tech Engineering, Inc., the project engineer. During the [widening] design, the designer went through the original bridge plans. Thats all he had, the original design plans. And thats what he based his design on.
When construction began and partial demolition of the bridge started, we discovered that the as-built condition of the bridge was much different than the plans. And that caused us a considerable amount of grief, not only in the widening portion, but as well as the expansion joints and some of the other areas, adds Garcia.
Post-tensioning had to be retrofitted into the job and external tendons placed on seven of the spans. In addition, the footprint bearings at the abutments were widened for stability.
When you apply torsion, or eccentric loads to [this type of bridge], the abutments have a tendency to suck those reactions up and to twist, so the bridge gets transferred down to each abutment, Corven says. So what we had to do was add external blocks to the bridge, post-tension them so that we could then jack the bridge up, install new bearings that were further apart--giving it more overturning capability--and then cut out the old bearings. The external blocks post-tensioned to the webs of the abutment segments allowed new bearings to be placed.
Will this project lead to future widening projects to segmental bridges? I think what [the project] has exposed is that these bridges have more flexibility [in terms of being widened] than people maybe traditionally think, Corven says. Its definitely not for every bridge, there are some that just wont take widening, but I think yes, there is applicability elsewhere.