Since late February, the East Stadium Boulevard bridge over State Street has funneled vehicles across the span in just two of the available four lanes. The lane reduction is a strategy to protect the fifth – counting from the southern edge of the bridge – of the 16 beams in the structure. That fifth beam is “sagging” 7/8 of an inch lower than other beams in the bridge. So traffic is currently restricted to the northern lanes. [Previous Chronicle coverage of the bridge provides additional background.]
In her update to city council in early March, Sue McCormick, the city’s director of public services, indicated that the bridge was being closely monitored in order to verify the safety of the bridge. Here at The Chronicle, we wondered what “monitoring” entailed.
An emailed request to Homayoon Pirooz, project management manager with the city of Ann Arbor, resulted in The Chronicle spending part of a rainy Wednesday morning under a bridge with a handful of staff from the field services unit and two engineers: Michael Nearing, senior project manager for the city of Ann Arbor’s project management unit, and Jonathan Drummond, with Northwest Consultants Inc.
The weekly monitoring entails regulating traffic under the bridge to allow for a truck with an elevated platform to park and lift the inspectors right up to the underside of the structure. Traffic control was done by a two-man crew, born and bred in Ann Arbor, one wearing a hardhat stickered with a UM winged helmet. They let cars through the one-lane gap on an alternating basis.
Nearing briefed us on the basics. The idea was to check the bridge weekly for additional displacement of the fifth beam to determine if the traffic re-routing had allowed the beam to stabilize at the 7/8-inch deflection, plus give the rest of the bridge a visual once-over. After 6-8 weeks, if the beam doesn’t show any further sagging, the plan is to reduce the frequency of the inspections to every other week.
Although the word “beam” is, to a layman, perhaps suggestive of a solid block of concrete, the beams have a hollow box as a cross section. Their strength derives from the steel strands in the bottom of the box. Such box beams are manufactured remotely and brought to a construction site fully cured. At the manufacturing facility, the steel strands are stretched under tremendous tension and the concrete is poured over them. When the concrete cures, the force of the pre-tensioned strands is applied to the concrete, which gives the beam its strength. Without tension the strands don’t add the same strength.
Thus Nearing said that the seven exposed pre-tensioned strands on the fifth beam were no longer contributing to the strength of that member. But on Wednesday morning, the fifth beam was holding steady at a 7/8-inch deflection. The measurement was taken at a point marked with a metal strip fastened into place along the beam to ensure the measurement was taken consistently at the same point from week to week. This week, at least, the beam hadn’t measurably sagged any lower. Still, Nearing and other field services staff took the opportunity to chip away bits of loose concrete from the area, so that they would not fall on their own – possibly onto the windshields of State Street traffic.
And it’s definitely the traffic under the bridge that Nearing said he was concerned about – as much as the traffic going over the bridge. In fact, Nearing did not assess a dramatic collapse across a whole lane’s width as very realistic possibility. He was more concerned about pieces of concrete falling onto State Street traffic.
Here’s why a whole-span collapse isn’t realistic. Even though the beams are tied together, Nearing emphasized that each beam was designed to bear the whole load of its section of the bridge. And given that the beams are not individually wide enough to allow a car to fall through, even the failure of a single beam would not result in cars falling through the air.
In addition to the fifth beam, the eighth beam had caught Nearing’s attention. He said it was starting to show signs of a longitudinal rust stain, very similar in appearance to how the fifth beam looked before the pre-tensioning strands started to expose themselves.
That eighth beam doesn’t figure in the current plan for a bridge repair scenario. That would entail replacement of just as many beams as necessary to get at the weakest one – the fifth one. And that would mean the five beams on the southern side of the bridge would be replaced. Nearing said that although a repair plan had been drawn up, whether it gets implemented depends on how soon funding is available [possibly via the second round of federal stimulus package money] for a complete reconstruction, including the adjacent bridge over the railroad tracks. If funding for complete reconstruction isn’t forthcoming in the next couple of years, the repair scenario (replacement of some limited number of beams) could be implemented.
Nearing walked us around to the top of the bridge and pointed out where it’s not just the box beams that are an issue for the bridge. The wingwalls are in poor shape (daylight is visible where there should be concrete), and the sidewalks are starting to slope downward where the embankment is gradually giving way. The crater-like asphalt surface of the bridge lets water through – wet spots on the underside of the bridge were visible on that rainy morning. And the drain holes in the beams are to some extent clogged by an artifact of their construction technique. Namely, the hollow areas inside the concrete boxes were created with cardboard forms when they were poured, and the cardboard, as it gets wet, falls apart and winds up clogging the holes that are meant to let the water out. Water inside the boxes, combined with freeze-thaw cycles, accelerates the deterioration of the bridge’s condition.
In the coming few weeks, there’ll be additional monitoring trips. We’re not planning to accompany Nearing on every one of them. But we’ll try to keep readers posted on any new developments.