Murray Johnson had performed this sort of heavy lifting before, but the previous feat was just a warm-up. This time, the executive engineer for North Vancouver, British Columbia-based Buckland & Taylor Ltd. is shooting for a world record. He and his colleagues believe that the Milton-Madison Bridge, now being built over the Ohio River between Kentucky and Indiana, would involve the biggest truss slide in history.
"We have specialized over the years in many crazy—I mean high-end—erection schemes. From that point of view, we have confidence," says Johnson. As he and I talked over the phone in early July, Johnson was being careful not to overstate the riskiness of sliding an approximately 2,400-ft-long truss. But after some gentle nudging, he soon admitted that the job was "definitely interesting and exciting and a little bit scary when push comes to shove and you actually lift this over the river."
Procedurally, the Milton-Madison Bridge is a repeat of an earlier slide but much larger in scale. Johnson designed the earlier scoot to facilitate replacement of the old Capilano River Bridge in West Vancouver. In June 2010, he watched as workers there used hydraulic jacks and threaded bars to yank the 430-ft-long, 1,280-ton, two-span truss an average of 43 ft laterally in relative silence. Riding on Teflon pads and polished metal plates, it slid onto a temporary pier in only six hours and instantly became a detour to make way for construction of a new crossing. At Milton-Madison, the contractor is building the new bridge on temporary piers next to the old one. Traffic will switch to the new bridge while workers demolish the old one. Then, the new bridge will slide sideways on a steel runway 55 ft and land on the original piers, which are being refurbished under live traffic.
For some civil engineers, bridge-sliding is old hat. It complements one of the industry's latest fashion trends—prefabrication and modular building—and cuts down on service interruptions. But Capilano also served as an unusual example of applying the method to an octogenarian erector set. Rather than moving steel behemoths, engineers prefer to use the graceful technique for small highway overpasses, railroad crossings and other nimble structures that consist of simple beams and girders. New bridges that must keep their old alignment—such as urban links where space is limited—also make good sliders, engineers say.
Milton-Madison is no urban bridge, but the 1929-vintage truss does connect two historic small towns—Milton, Ky., and Madison, Ind., whose residents needed a modern structure but wanted the look and feel of the original. At 44 ft, the new bridge will be twice as wide—still retaining one lane in each direction but with shoulder and pedestrian access. The supporting caissons originally were hand dug down 80 ft to bedrock and are being heavily reinforced with bundled rebar and grout, as well as 2- to 6-in.-thick encapsulating concrete jackets and new pier caps.
The slide idea, scaled up to fit a bridge that would be nearly six times longer and 12 times heavier than Capilano, took some by surprise. When the project went out to bid as a design-build contract in the summer of 2010, around the same time as the Capilano move, "I don't think this whole sliding idea was on anybody's radar screen," says Tom Bolte, project manager for Columbus, Ohio-based Burgess & Niple Inc., the engineer-of-record for the Ohio River crossing. "It's not totally new, but it has not been done to our knowledge for a bridge of this size."
The winning bid, led by the Crown Point, Ind., office of Walsh Construction Co., reflected just how fresh the idea was. Engineers for the owners—Indiana Dept. of Transportation and Kentucky Transportation Cabinet—originally estimated that replacing the 83-year-old bridge would cost taxpayers $130 million and require a continuous year of traffic shutdown. The bridge serves approximately 12,000 vehicles a day with no alternative over a 72-mile stretch of the Ohio River, so the owners also expected the yearlong shutdown would require expensive and cumbersome ferries to shuttle people and their cars across the river. That plan came with a touch of irony, as the original bridge was built to replace overstressed ferries in the 1920s.
The bid was structured as an "A+B" equation in which one half of the formula would represent the contractor's hard-dollar price and the other a $25,000 assessment the owners would charge for each day the bridge was closed to traffic (for a maximum of 365 days, or roughly $9.1 million). The Walsh team—with Burgess & Niple designing the approaches and pier rehabilitation and Buckland & Taylor taking care of the design and construction engineering of the steel spans—proposed doing the job for just $103.7 million while shutting down the bridge for only 10 days. The contractor estimated five days to reroute traffic onto the new bridge, built in place using a combination of truss prefabrication, floating, strand-jack lifting and cantilevered "stick" construction on temporary steel piers. The job would take five more days to slide the roughly half-mile truss onto the permanent piers.
Four other bidders proposed traditional in-situ construction and didn't put a dent in the 365-day traffic shutdown. Prefabricating the new bridge and sliding it into position "thrilled" the owners' representatives, says Kevin Hetrick, project manager at INDOT's central office in Indianapolis. "The state of Indiana has a lot of projects in the queue, and this just allows another project or two to be built," he explains. Looking just at the hard numbers, Walsh was technically the second-lowest bidder, but the huge time difference allowed it to jump to the front of the line.
From a profit standpoint, Walsh could justify the bold move, its engineers say. "We looked at the value of all our temporary falseworks and appurtenances required to do the slide, and it was pretty close to equal the value of those B days. So, we could be competitive with this plan," explains Brad Koester, business group leader in Walsh's Crown Point office. Indiana Gov. Mitch Daniels hailed the solution as "a great example of innovation that makes a great outcome even better."