The Serious Business of Birdwatching (I95)
Bob Muir, P.E. of First State Section of ASHE
Harry Roecker of DMJM+HARRIS
By their very nature, birdwatchers are meticulous observers. In this case, that diligence proved to be a lifesaver.
Looks can be deceiving. And by most appearances, the bridge over Brandywine Creek on I-95—a stretch of interstate that carries 100,000 vehicles daily—looked fine. That is, until a birdwatcher wandering through Brandywine Creek State Park happened to look up from his position below the bridge. He discovered daylight glinting through an inch-wide crevice between the span and one of its supporting girders. Not exactly sure of the implications of his discovery, the birdwatcher called the Delaware Department of Transportation (DelDOT).
Knowing exactly what that observation could mean, DelDOT officials responded immediately. In fact, they were on the scene within minutes. As soon as they examined the bridge, DelDOT officials knew they had a serious problem on their hands: An acute fracture had stretched the exterior girder supporting the northbound traffic lanes of the I-95 Brandywine Creek Bridge.
“I couldn’t sleep that night after seeing the crack,” explains DelDOT Bridge Engineer Jiten K. Soneji. “I still remember that night. Our main concern was the first interior girder. We didn’t want the first interior girder to get overstressed. Everyone was asking why and how this happened. I told them, ‘At the moment, “why” is my least concern. I’m most concerned about what we can do to keep our motorists safe.”
DelDOT immediately closed the third, outer lane of traffic. Upon deploying an Under Bridge Inspection Vehicle (UBIV), it quickly became clear that this crack was no minor anomaly. The 8-foot-deep steel girder had cracked from its bottom to approximately 7 inches from its top. The edges of the crack had widened to approximately 1 inch at the bottom of the girder, and the girder had dropped almost 2 inches from its original position. But DelDOT needed more in-depth information. They had to determine whether the bridge could remain open at all.
“We assessed the load-carrying capacity of the damaged bridge. And the analysis revealed that the bridge could safely carry only two lanes of traffic until permanent repairs of the girder were completed,” explains John Milius, P.E., a senior structural engineer for DMJM+HARRIS. “But we needed to apply a temporary fix as well. What we really wanted to do with this temporary fix was to arrest the progression of that crack. We did not want to lose the girder. If we were able to close that crack, we knew that we could restore the girder to its original strength.”
Even before the crack came to light, a project team consisting of DMJM+HARRIS, contractor Greggo & Ferrara, DelDOT, and the Federal Highway Administration (FHWA) was already performing repairs and architectural improvements on the bridge. In addition to creating concrete barriers with synthetic stone and adding ornamental street lighting, the project team was repairing severely spalled parapets and median joints. Originally the work was to be done in two phases. Phase I called for the third traffic lane to divert to the outside shoulder. Phase II would shift traffic toward the median to allow for construction work in the shoulder. But as soon as the crack was discovered, the girder repair became the top priority. The JD Eckman firm was added to the team, specifically to fix the cracked girder. But a question naturally arose: with engineers actively working on the bridge, how could a crack like this go unnoticed? Milius explains.
“There had been a bridge inspection just two months earlier and this crack was not discovered. At that time, it could not have been visible. Something of this magnitude—even something much smaller than this—would certainly have been discovered. So, once this crack began to form, it must have progressed very quickly, probably within a matter of a few days. Fortunately, someone in the park just happened to see it and had the excellent good sense to call DelDOT when they discovered it.”
Later inspection proved Milius’s belief to be correct. The steel girders are I-shaped elements constructed of a vertical plate (or web) located between top and bottom flange plates. Analysis revealed that the crack began in a weld in a nonstructural stiffener plate attached to the web of the girder on its outside vertical face. During construction of the bridge in the 1960s, the weld was used to attach a decorative element to the bridge. Temporary changes in loading caused this weld to fracture. Once the weld cracked, the crack propagated quickly through the girder web and flange. But the analysis revealed more.
The decorative weld that fractured was not unique to one point on the girder; similar welds existed every 25 to 30 feet along both sides of the 1,875-foot bridge. Milius and his project team inspected the entire bridge for evidence of similar weld failures. It took two weeks, but every problematic weld was located and drilled out of the structure. But before that could be completed, the girder problem needed to be addressed.
To immediately arrest the progress of the crack, the project team drilled several small-diameter holes at the tips of the crack. Then, using specially fabricated steel splice plates, the project team reunited the bottom flange of the steel girder. Less than 48 hours after the fracture was discovered, the project team had the plates securely in place and had ensured the temporary safety of the bridge. Now it was time to make the temporary repairs permanent.
A team of researchers led by Dr. Michael Chajes—Chair of the University of Delaware’s Civil and Environmental Engineering Department—began to test the amount of live load (strain from vehicular and truck traffic) that was stressing the bridge girders and the temporary splice. After clamping strain gauges onto the fractured girder as well as onto undamaged nearby girders, the team brought in large trucks with specific, known weights. Results from the damaged northbound lanes were compared with results from the undamaged southbound lanes. The tests revealed that the live load was being distributed to the bridge girders normally; the temporary splices were helping the fractured girder function normally. That addressed the live load. But what about the weight of the bridge itself? Again, Milius explains.
“We improved the girder to take the live load. And that saved the girder. But the one thing that we hadn’t done in the temporary fix was to restore capacity for the ‘dead load,’ the weight of the bridge itself. The only way to bring this load capacity back was to restore the girder to its former position, and that’s the main difficulty with a permanent fix. We had to find a way to get the girder back into its original position.”
A challenge unto itself, there was added pressure as well. The July 4 holiday weekend—a period of extremely high traffic volume—was only weeks away. Any permanent solution had to be as quick as it was effective.
“What was unique about this was that we were already under contract,” explains Lance Wilgus, P.E., the project engineer for FHWA. “When you have something unexpected like this come up, your main focus has to be the safety of the traveling public; you have to fix the problem. But if at all possible, you also want to make sure that you don’t delay the original project that is under way either.” Every day of delay costs people time, lengthening their daily commutes. Getting the emergency repairs done before July 4 would make sure that the original project wouldn’t get delayed either, and it would keep all three lanes available for one of the busiest travel times of the year.
Executing the repairs meant that the project team would have to jack the mammoth structure. They considered five options for this challenging task: (1) lifting the bridge using a beam extended above; (2) jacking with a beam connected to the other girders below the bridge; (3) post-tensioning, with draped cables and lever arms on both sides to lift the girder; (4) jacking the bridge from towers constructed below the fractured beam; and (5) complete girder replacement.
The team quickly determined that both post-tensioning and jacking with a beam were not practical. And complete replacement of the girder would require removing the bridge’s concrete deck, a time-consuming and fiscally prohibitive option. Jacking the bridge was the only practical, time-sensitive, cost-effective solution.
Two jacking towers were placed on either side of the crack. Erected from below the bridge in Brandywine Creek, the jacking towers would rise approximately 80 feet from a temporary foundation in the stream. The two towers consisted of four columns separated by 8-foot-long supports. Once the girder was jacked, the permanent repair splice would be attached and minor modifications would be made to the diaphragm crossbeam adjacent to the crack. After quickly obtaining a permit from the Delaware Department of Natural Resources and Environmental Control (permitting the team to secure the towers in the creek), the project team put their plan into action. Working at a frenetic pace, the team completed the permanent splice in time for the July 4 holiday.
In addition to providing holiday travelers with a safe trip, the Brandywine Bridge Project now serves as a platform for education. In a paper presented at the New York City Bridge Conference (Oct. 20–21, 2003), Dr. Chajes documents the contribution this project will make in the study of how load redistributes to other areas when a major area of a bridge is lost.
“In addition to the technical insights, this project and this paper demonstrate the clear benefit of joint partnership,” explains Dr. Chajes. “And the partnership between the university, DelDOT, DMJM+HARRIS, and the FHWA was invaluable. When something like this happens, the important thing is to fix it quickly. Unfortunately, working at such speed often prevents researchers from studying the problem and its causes. But this project serves as a great example of how both can be accomplished.”
FHWA’s Wilgus agrees. “This was a great demonstration of how a close partnership can develop an effective solution in an efficient and timely manner. It took a lot of cooperation to make that happen, but the end result speaks for itself. The state was able to implement a repair solution without adversely affecting another project already under way. And that’s remarkable.”
Close, effective, innovative teamwork made this project successful. But one team member may never know how important his contribution really was. For most birdwatchers, meticulous observation is just part of their hobby. But in this case, that diligence proved to be a lifesaver that actually helped advance the state of the art in bridge repair. And that certainly wasn’t for the birds.
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