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.