To residents of Birmingham, Alabama, work on the downtown section of I-59/20 has been a major “adjustment,” to say the least. While it might not equal the shutdown of L.A.’s 405 in 2011 (Carmageddon, it was dubbed); the gridlock after the 2007 collapse of the Mississippi River Bridge near downtown Minneapolis; or traffic headaches related to Boston’s “Big Dig,” in terms of overall disruption, it was huge. In downtown Birmingham, the main span that carries the interstate and literally dissects the city was recently demolished to make way for a new structure designed to better accommodate current traffic volumes.
Facing a daunting one-year deadline to make the segmented span happen, Johnson Brothers Corporation, a Southland Holdings Company (JBC, Roanoke, Tex.), has employed some impressive techniques. Helping address the tight tolerances mandated for both the casting and assembly of the more than 2,000 individual segments that make up the structure, is an arsenal of theodolites, laser levels and robotic total stations. Their accuracy is helping JBC bridge the gap between west and east, suburb and downtown, the past and the future.
"... we committed to an aggressive time frame: one year from the time traffic was diverted off the bridge to the time it is opened.”
Jason Anderson, JBC’s survey manager
Outdated and Overused
Like much of this country’s infrastructure, Birmingham’s I-59/20 bridge had outlived its usefulness to the point where it was sorely in need of replacement. Built in the 1960s with a designed capacity of 80,000 vehicles per day, traffic flows increased steadily over the years to nearly double that volume: about 160,000 vehicles per day. In addition, features of modern roads and bridges such as wider breakdown lanes were lacking, resulting in frequent backups, according to Jason Anderson, JBC’s survey manager.
“The project, which also includes several approach ramps, is valued at $475 million — the largest project Alabama Department of Transportation (ALDOT) has ever let,” he said. “The main downtown span is being replaced after being determined that it was both structurally deficient and functionally obsolete. However, because the thoroughfare is so key to the downtown area, ALDOT needed things done quickly. So, we committed to an aggressive time frame: one year from the time traffic was diverted off the bridge to the time it is opened.”
Maintaining such a compressed schedule meant choosing the construction technique best suited for quick structural assembly. With that in mind, a segmental bridge design consisting of 2,316 individual pre-cast, post-tensioned segments was chosen. With expedience in mind, more than a year before the old structure was even leveled, work was ramping up at JBC’s casting yard to begin creating segments.
A Cast of Thousands
Located near the Birmingham-Shuttlesworth Airport, the casting yard consists of twelve concrete casting machines, capable of producing eight segments, two piers and two expansion joints per day. The segments themselves vary in width — anywhere from 32’ to 45’ wide — but are 9’ tall and are cast in 12’ lengths.
JBC is using a process called “match casting,” a precision manufacturing process that creates each segment to be a perfect fit with the segments that will be installed adjacent to it on the finished structure. To ensure accuracies are maintained, each casting bed is overseen by a spotter in a tower using a Topcon theodolite and an auto level, both of which are mounted atop a concrete column. According to Brian Mayorga, one of JBC’s casting bed spotters, their role is critical for quality control.
“Right out of the gate, we have a target on each tower so that, when we set up, we line up precisely on that target,” he said. “With that verified, we have numbers that we site in order to establish a benchmark. Then, using the theodolite, we shoot the corners of where a segment will tie in to the next one, and use the auto level to lock down the slope between the columns and the ends of each pre-cast section to ensure accuracies. Small bolts in the concrete act as elevation points and are used to obtain the geometry needed to verify the correct superelevation.
Because the concrete is being bucketed into the molds, there are always cranes rolling past these beds. That causes serious vibration, which, with other instruments we’ve used in the past, led to a tendency to ‘float.’
“That’s not the case with these units,” said Mayorga. “They are very stable and not affected by the vibration at all.”
With that one-year deadline always at the forefront, additional steps were taken to streamline the overall workflow, including pouring the substructure well in advance of the actual start of construction.
“We had about 60% of the column footers in before we even tore the old bridge down,” said Anderson. “Then, to further move things along, we minimized the amount of cast-in-place work, opting instead to precast as much as possible — including the columns and caps. Doing so meant that, often within three days of them coming on site, we could have a series of completed columns and caps up and ready to support segments. That was a huge timesaver.”
In addition to alleviating Birmingham’s traffic woes, JBC had a real incentive to keep things on track: there was a $250,000 per day “disincentive” for every day the project extended past the deadline. Conversely a rather sizeable “incentive” was in place for finishing the project within that one-year time frame.
“Having the digital levels connected to the data collectors made things so much easier than in the past.”
Jason Anderson, JBC’s survey manager
In determining the best survey-grade instruments needed for setting the columns, erecting the shoring towers and falsework, and for accurately placing each segment (as well as for the casting yard work described above), JBC turned to Earl Dudley & Associates, a Birmingham-based firm with an 80-year history supplying survey instruments to the region. Having worked with them in the past, Anderson was confident that they would understand their needs and give them the support a project of this size demanded.
“I’ve had a relationship with the Dudley company since we were on a job together in 2006,” he said. “The fact that I knew them, felt comfortable with them and they were right here in town was huge for us in making our decision. Because of the tight time frame, we couldn’t afford to have to go to Atlanta or elsewhere to get a calibration or a fix if something went wrong. Now, if a guy breaks a bipod in the morning, we know we’ll have a replacement that afternoon.”
That vote of confidence, said Brian Singleton, Dudley’s sales rep, is the result of a concerted effort on their part to make it happen. ”We always try to look at our relationship with the customer as a partnership,” he said. “If they have a problem and we can’t make it right, it hurts us both. So, we’ve been working with them from very early on, helping them integrate the use of survey gear at the precast plant, and everything moving forward. That’s just the way we do things.”
Perhaps one of the most critical facets of the I-59/20 job is the actual placement and alignment of the individual segments. It is in that part of the project that JCB has relied heavily on the use of Topcon GT-1000, one-second robots with FC-5000 data collectors and MAGNET Field software for positioning, as well as the digital levels also running MAGNET Field for elevation.
The full scope of the segmental process starts with establishing grade and laying out three corners of the crane mats which serve as the base for the shoring towers. With those in place, the tops of the structures are “as-built” to verify elevation and position.
“Robotic is really the best way to do a job like this,” said Art Buckman, one of JBC’s survey crew members. “To get our control, we set up a robotic total station over an established center line on either an already-completed section or a standalone column. At that point, the first segment — there are generally between 10 to 12 per span with either a pier section or expansion joint piece at either end — is craned up and positioned. Once up, we shoot the centerline and verify two line check points and four elevation points which are based on elevations entered into our data collector by Jason. Once we make any necessary adjustments needed to maintain that 1/16” tolerance we survey the corners and move on.”
Anderson said that he was impressed with the sophistication of the digital levels from Topcon. “Having the digital levels connected to the data collectors made things so much easier than in the past,” he said. “Because we are using the same software to communicate with both the robot and the level, we have great on-site connectivity. That makes a huge difference, in that we can easily switch back and forth between the two. It also removes the risk of human error and increases accuracy. They’ve been some of the most impressive instruments out here.”
The Right Stuff
With the first segment up and set it in place, the next two sections are set and secured using both a two-part epoxy glue on the matching faces and temporary post-tension bars. A JBC surveyor will then go topside to make any necessary adjustments to ensure the grade and line are where they need it to be. That process is repeated until all 12 segments are in place, at which point cables are run through them and post-tensioned. With the structure verified, the shoring towers are removed, and work moves on the next of the structure’s 168 spans.
According to Anderson, the choice of robotic total stations proved the right one from the casting yard throughout the course of the project. “We purchased seven of the robotic units and probably could not have done it any better way,” he said. “The proximity of our work area would have made use of any GPS instruments a real challenge. And, from both a quality and efficiency perspective, the robotic total station was the way to go. It allowed us to keep our crew size down to two men versus three or four, yet still get the same amount of work — or more — done.”
The final segment for the I-59/20 structure was placed on 10/14/19, putting JBC well on pace for an on-time or early completion.