Engineers in the US claim to have broken a record for shifting the world’s longest bridge laterally onto a set of new piers.
The Milton-Madison Bridge serves as a crossing for US Route 421 over the Ohio River, connecting the towns of Milton, Kentucky and Madison, Indiana. It provides the shortest route between Indianapolis, Indiana and Lexington, Kentucky, and sees traffic of approximately 10,000 vehicles per day.
The original two-lane bridge was a continuous truss structure with a main span of 180 metres, and a total of length of over 970 metres.
The structure was built 85 years ago and had deteriorated significantly over the course of its operational life, prompting the construction of a $104 million replacement.
In order to permit continued operation of the bridge during the process of building its replacement, engineers constructed a pair of temporary piers approximately 17 metres away from the original structure where the new bridge could be assembled.
Polished steel sliding plates were then fitted onto the permanent refurbished piers so the new bridge could be pulled over from the temporary piers using steel cables and a set of eight hydraulic jacks.
Each of the jacks had a capacity of 364 tons and was equipped with 31 strands of 5/8-inch diameter cables. One jack was mounted on each of the bridge’s two land piers, while two were placed on the three water piers.
A computer system called BRAVO was used to synchronize the complex sliding process to within a three one-thousandths of an inch. Each of the jacks was equipped with lasers that surveyed reflective targets attached to gusset plates on the truss in order to ensure the utmost level of precision.
The new structure is a steel truss bridge measuring 740 metres in length and over 12 metres in width – twice that of its predecessor. The new steel bridge is equipped with two 3.7-metre lanes and shoulders nearly 2.5 metres in width.
According to the Indiana Department of Transportation, it is the “longest bridge in North America – and perhaps the world” to have been fitted into place using this lateral pulling technique.
The use of the lateral pulling technique to put the bridge in place enabled engineers to reduce the bridge closure period from nearly to year to under six weeks, serving to greatly diminish the economic and transportation disruption caused by the construction process.