The turbine design that will complete Charlotte, North Carolina’s outer loop of interstate highways is a two-level interchange with spiraling, semidirectional flyover ramps supported by 18 simply supported bridges. Courtesy of STV, Incorporated
A “turbine” interchange design will replace an existing “trumpet” interchange to connect Interstate 85 with Interstate 485 as Charlotte, North Carolina, completes its outer loop.
November 1, 2001--New York-based STV/Ralph Whitehead Associates worked with design/build partner Lane Construction Company, headquartered in Cheshire, Connecticut, to design an innovative two-level turbine interchange in North Carolina that will join Interstate 85 with Interstate 485 to help complete Charlotte’s “outer loop,” an interstate beltway around the city. The $92-million turbine interchange will connect I-85 to I-485 at the point at which a future section of I-485—currently under construction—will meet the two existing roadways.
The turbine design will replace an existing trumpet interchange where I-85 now meets the temporary end of I-485. (A trumpet design, named for its resemblance to trumpet tubing, is commonly used to end one freeway at another, and includes at least one looped ramp to connect traffic flow entering or leaving the terminating expressway with the far lanes of the continuing highway.) The new turbine design is a two-level interchange with spiraling, semidirectional flyover ramps supported by 18 simple-span bridges. Construction is set to begin this fall and is scheduled to be completed in 2014. The turbine design, which is rare in the United States, is a first of its kind in North Carolina; another notable example exists at the interchange of I-4 and I-75 in Tampa, Florida.
The North Carolina Department of Transportation (NCDOT) sought to convert the existing trumpet interchange as it designed and built the last, 7 mi leg of I-485. The NCDOT estimates that the outer loop will carry about 130,000 vehicles per day by 2035.
The turbine interchange directs all left-turning traffic around the center of the intersection in a clockwise direction. Turbine interchanges are rare in the Unites States because they use wide, sweeping, high-speed ramps, requiring more land than is required for a typical interchange. (Watch a YouTube animation of the interchange here: http://www.youtube.com/watch?v=8czDkgzpSco.)
But the NCDOT had purchased plenty of the right-of-way in the early 1990s for a modified clover interchange that was originally planned for that site. STV studied the cloverleaf design and discovered that “the design had some of the components we needed,” says John Johnson, P.E., a civil group leader and senior project manager for STV/Ralph Whitehead Associates. “It had some high-speed ramps, but it had two small, tight loops as well. We knew if we could replace the two small loops with high-speed ramps, then we’d have the solution.”
The turbine design solution is expected to save the state $30 million by replacing a costlier four-level “stack” design that was previously considered for the site. This cost savings derives from the need for less earthwork and the use of simple-span bridges instead of the longer, spliced-girder bridges—some as high as 70 ft—that were required for the stacked interchange. “Eliminating the need for spliced girders eases complicated construction means and methods and improves safety for the motoring public as well as the construction crews,” says Johnson.
The two-level turbine will also cost less to maintain and reduce impacts on traffic during construction. The two-level turbine is also advantageous in that it can be widened relatively easily in the future.
Because the turbine interchange will utilize simple-span bridges, which are common to NCDOT projects, they can be built more easily by local contractors, Johnson says. The new design also offers wider shoulders. “One issue with a four-level stack is that the bridges would require large-diameter columns, which would reduce the amount of the existing interior shoulder,” says Johnson.
Additionally he adds, “We would have had to haul in two million yards of borrow for a four-level stack,” and construction crews would have tied up traffic on the two interstates to do that. “Removing construction traffic from the interstate was a huge positive for the project,” Johnson says.
Had the four-level stack been built, interstate traffic would have been detoured at night during construction so the girders could be erected, generating added contractor costs and creating inconvenience for travelers, Johnson adds.
The turbine interchange also had environmental advantages. It gave STV the flexibility to align a ramp inward to avoid Stoney Creek, a creek with impact on the local ecology. “Besides the ability to reduce construction cost, you can lessen environmental impact,” says Johnson. “In the four-level stack interchange, the ramp would create a much more significant impact to the creek.”