The bridges that bookend the Land Between the Lakes National Recreation Area in southwestern Kentucky will each feature a basket-handle steel tied-arch main span. Michael Baker International
Kentucky replaces two bridges near a national recreation area at roughly the same time, the nearly identical designs resulting in a cost savings.
April 22, 2014—It is not often that two large bridges located within a few miles of one another can be replaced at the same time. But that’s exactly what is happening with the two bridges that bookend the Land Between the Lakes National Recreation Area in southwestern Kentucky. As a result, the replacement bridges will feature a similar design that ensures that they not only look nearly identical but also save the state money.
Kentucky Lake Bridge and Lake Barkley Bridge, as the two bridges are known, are located on either side of the Land Between the Lakes National Recreation Area, which is bordered to the west by Kentucky Lake and to the east by Lake Barkley, making it the largest inland peninsula in the United States. Kentucky Lake was created in 1945, when the Tennessee River was impounded, and Lake Barkley was formed in 1966, when the Cumberland River was impounded. The bridges are located approximately 8 mi apart on U.S. 68/KY 80, and each traverse the lake for which it is named.
The existing bridges were constructed in the 1930s and were later elevated when the rivers were impounded to accommodate lake vessels. Each bridge is more than 3,000 ft long and has a through-truss main span over what are now the navigational channels of the lakes. Nearing the end of their useful lives, the bridges are too narrow to accommodate modern vehicular traffic and their main spans are not long enough to accommodate modern lake traffic; their superstructures measure just 20 ft curb to curb and their navigational channel spans measure only about 320 ft in length. In 2012 the need to replace the bridges was magnified when part of the Kentucky Lake Bridge was struck by an oceangoing vessel and collapsed, says Michael McGregor, P.E., the chief district engineer in the Kentucky Transportation Cabinet’s (KYTC) Highway District 1 office, which is responsible for the bridges.
A public input process was held in 2009 to determine a new bridge type for the crossings, and a variety of designs were considered. Residents wanted the bridges to act as gateways to the recreational area, so state officials selected a bridge type that would be both architecturally appealing and cost effective—a basket-handle steel tied arch. “The cable-stayed bridge probably had the highest ranking as far as the public input, but it was also more expensive,” McGregor says. “Ultimately, the cabinet decided to strike a balance between the cost and having a signature bridge type, and that’s how it was determined to build a basket- handle tied arch.”
The Louisville office of Michael Baker Jr. Inc., an operating unit of Michael Baker International, which is based in Moon Township, Pennsylvania, is designing both bridges. McGregor says the state is realizing a cost savings by using the same bridge type and designers for both crossings. “The ability to have these two structures built relatively close to each other in distance and in time allows us to have the same designers for both of them and to utilize some economies of scale,” he says. “The bridges are not exactly the same in every way, but there are a lot of things that are similar enough that when you’ve designed it once, you’ve designed it twice.”
Although the bridges will be alike and are referred to jointly as the Lake Bridges Project, their construction is being procured under separate contracts. The Kentucky Lake Bridge design has been completed and construction is scheduled to start in May, while the Lake Barkley Bridge is in the final design phase and construction is expected to commence in 2015. Each bridge will be constructed on new alignments parallel to the existing bridges, which will remain open to traffic throughout construction and then demolished once the new bridges are completed. The bridge replacements are part of a decades-long effort to upgrade U.S. 68/KY 80 to create a four-lane, high-speed economic corridor through the southern half of the state.
Kentucky Lake Bridge will be slightly more than 3,600 ft long, its basket-handle tied-arch main span measuring approximately 550 ft long and 60 ft high, and offering a 67 ft high clearance over the navigational channel. The bridge’s 75 ft 4 in. wide deck will be substantially wider than that of the existing bridge to allow for four lanes of traffic. It will also have a 10 ft wide multiuse path. The superstructure will be supported by nine in-water bents, two of them the main-span bents that will delineate the new navigational channel. It will be founded on 6 ft diameter steel pipe piles driven deep into the dense sand and gravel layers below the riverbed. “The uniqueness is in the size of these piles,” says Gregory D. Stiles, P.E., M.ASCE, a bridge technical manager for Baker. “The depths of the water through which they have to free stand as well as the long span arrangement resulted in pipe piles of this large diameter.”
Still in design, the Lake Barkley Bridge will also be more than 3,000 ft long and have a basket-handle steel tied-arch main span measuring approximately 550 ft long and 60 ft high. So far, the primary difference between the Kentucky Lake and Lake Barkley bridges will be the foundation. The limestone bedrock at Lake Barkley is much closer to the riverbed than that at Kentucky Lake, so Lake Barkley Bridge will be founded on 6 to 8 ft diameter drilled shafts or caissons. However, the area has potential for karst formations, so contingencies will be in place to mitigate for fissures and voids if needed during the foundation construction, Stiles notes.
The new bridges will each be approximately 75 ft wide and carry
four lanes of traffic. They will be substantially wider than the
existing bridges, which measure just 20 ft curb to curb.
Michael Baker International
The bridges are located approximately 80 mi east of the New Madrid Fault Line and are also impacted by the Wabash Valley Seismic Zone to the north. They are being designed to withstand seismic ground motions that correspond with a 7 percent probability of exceedance in 75 years—equivalent to a seismic return period of 1,000 years. The bridges are had to be analyzed using Seismic Design Category D as defined in the publication American Association of State Highway and Transportation Officials for LRFD Guide Specifications for Seismic Design. The seismic design is based on the potential for liquefiable soil and lateral spreading, and on one-second design spectral accelerations that are greater than 0.50g for the site-specific response spectra used in a multimode spectral analysis. A nonlinear time-history analysis was conducted to determine the displacement demands for the design event and a nonlinear static pushover analysis was used to calculate the displacement capacity of the piers. Either friction isolation bearings with a “fuselike” keeper assembly or lead-core elastomeric isolation bearings will be used on the main spans, Stiles says. “The bridges will be able to reopen for emergency vehicles and as an evacuation route very soon after a seismic event because all structural elements will remain essentially elastic,” Stiles explains.
Both bridges are also designed for ease of inspection and maintenance. The floor stringers will be integrated with the floor beams to minimize the structural depth and reduce maintenance needs by eliminating stringer bearings, Stiles says. H-shaped plate girder sections will be used for the arch ribs and the end-floor beams instead of box sections, which are notoriously difficult to inspect. The plate-girder design will enable the inspectors to more easily see all of the welds and connections. The bridges will also feature several built-in elements to help inspectors access the structures. “Snooper trucks for future inspections could be costly and lift equipment from floating barges on the lake is impractical, so Baker designed safety cabling, steps, and tie-off points for inspectors to climb the entire structure using only rope access,” Stiles explains.
The new bridge piers will not line up with the existing bridges, so the navigational channels will be reduced to as much as 200 ft during construction. The contractors and the KYTC will coordinate closely with the U.S Army Corps of Engineers, U.S. Coast Guard, and commercial towing industry representatives to ensure barge traffic is maintained throughout the project. “Certain phases of the project will involve restricted clearances for the flotillas—some of which are over 900 feet long and have 15 barges loaded with coal, sand, gravel, and other commodities,” Stiles says. “The contractor will be required to have a helper boat on call to assist tows safely through the bridge construction site when the navigational channel is encroached upon.”
The KYTC has developed an aggressive schedule for completion of the bridges. Completion of the Kentucky Lake Bridge project, including demolition of the existing bridge, is anticipated in 2016. The Lake Barkley Bridge schedule is still being evaluated but will likely have similar durations because the construction activities will be similar. The total cost of both bridges will be approximately $330 million. Once completed, the two crossings will not only meet modern vehicular and waterway traffic criteria but also complement the beauty of the recreational area, McGregor says. “We’re constructing bridges that are not only bigger, better, and safer but also aesthetically fitting to the very special and unique area that is the Land Between the Lakes.”