Transforming Charleston

May 22, 2024
An innovative bridge should improve pedestrian safety

By Richard Kirkman, Contributing Author

The link between a city’s economic growth and the state of its transportation systems is undeniable.

Heavy civil engineers don’t just design critical infrastructure. They also create oppor- tunities, enhance connectivity, and improve access to employment, educational institutions, healthcare services, and recreational facilities.

Bridges, roadways, and cycling and pedes- trian paths stimulate an area’s economy and are indicative of its growth. They can either support further development or hinder it.

The Ashley River Crossing in Charleston, S.C. aims to address the city’s evolving needs and provide residents with a safe and reliable connection between West Ashley and down- town Charleston.

Designed to be the country’s first cable- stayed movable pedestrian structure, the crossing’s aesthetics and engineering ingenuity are as paramount as its purpose. The design of the 4,100-foot-long bridge contains a 204-foot center moveable span that swings outward to allow marine traffic to pass.

Superior Construction Company Southeast will lead as design-construction manager, with Johnson, Mirmiran & Thompson (JMT) leading

the design. JMT will collaborate with COWI and TranSystems as specialty subconsultants.

Safety Challenges

The Ashley River Crossing will address the ongoing safety concerns pedestrians have experienced over the past few years.

Funding comes from the city of Charleston, Charleston County, Medical University of South Carolina (MUSC), the South Carolina Department of Transportation, and the Federal Highway Administration.

“The existing bridges and roads do not have sidewalks that meet current standards for people to ride their bike or walk safely,” said Jim O’Connor, JMT vice president and lead design engineer for the project.

Pedestrians traveling on U.S. Route 17 between West Ashley and downtown must cross the Ashley River on two parallel bridges with narrow sidewalks.

More than 63,000 cars cross these shared-use bridges daily. The high traf-
fic volume resulted in over 100 bike- and pedestrian-related crashes between 2014 and 2019 — necessitating alternate transportation infrastructure for pedestrians.

“Providing safe, connected, equitable and enjoyable ways for Charlestonians to walk and bicycle will both save lives and improve quality of life, especially because our region consistently ranks as one of the most dangerous for non-motorized travelers,” said Katie Zimmerman, executive director of Charleston Moves, a local nonprofit organization that encourages multimodal transportation.

Superior and JMT developed a preliminary design for the pedestrian bridge that will separate foot and car traffic and mirror the functionality of the existing adjacent highway bridge.

Functionality Meets Aesthetics

Charleston’s original request for qualifications (RFQ) stated one of its top goals was that the new design be iconic and aesthetically pleasing, which contributed to the owner opting for a cable-stayed movable structure.

“There are other movable vehicular bridges in the country. There are also stationary, long-span cable-stayed bridges, but we don’t know of any other bridge in the
U.S. that combines all these complex elements,” said O’Connor. “We looked to similar European bridges as the basis of our design. That’s where we found examples of similar bridges by COWI and brought them into the cable stayed span design.”

One of the team’s primary proposed innovations is the use of a slewing ring, which will make the bridge more compact, efficient, and easier to construct. A slewing ring configuration allows for the pre-assembly of mechanical components and the structure towers. It also enables erection as a single unit

The Ashley River Crossing’s hydraulic swing span components will allow the bridge to open horizontally rather than vertically, improving marine clearance to greatly reduce on-site and over-water assembly time. This eliminates the use of balance wheels and center wedges and the need to install and precisely align open gearing.

An iconic steel V-shaped tower will provide a bold architectural statement. Its outward-leaning stay cable plane addresses the city’s concern about any ice on the cables falling onto the pedestrian path below.

Delivery and Approach

The project team will construct the Ashley River Crossing using a segmental approach. The five geographic segments span from West Ashley to the east end of the project limits at Lockwood Drive.

Segment three comprises the movable swing span and piers and will be constructed concurrently with segments two and four. This allows ample time for the design, procurement, instal- lation, and testing of all mechanical, electrical, and structural swing span components.

“The first bridge work Superior will perform will be installing the fender system extension to protect the new swing span construction and enhance the overall marine safety for the boating public and our workers,” said Brian Tolbert, project director at Superior Construction. “We also plan to build the swing span bridge in the open position to minimize marine traffic impacts.”

Moveable Bridge Operations

After completing the approach span foundation work, power and control conduits will be connected to the new main span by the fourth quarter of 2025. The new movable bridge operation will be tied into the existing movable bridge operations by the third quarter of 2026.

The project team will isolate the swing span operation during construction to prevent any premature operational activity from the existing operator’s house.

The functioning span will come online once the required testing cycle is run from the pivot pier controls. The operator’s vision views are acceptable between the tender’s direct sight and the camera views.

The existing controls will remain largely untouched, except for the installation of the proposed interlocks, to prioritize movable span operation and its associated bypasses. The new movable span will be controlled by a touchscreen display and discrete pushbuttons for the gates.

Tolbert said that the work also includes a trailhead with benches, water stations, and bike repair stations.

Timesaving Solutions

The project team is working to achieve substantial comple- tion ahead of the owner-approved contract time of 1,277 calendar days.

To accomplish this, they proposed constructing the West Ashley approach as a boardwalk structure, significantly reduc- ing the construction duration by avoiding delays associated with relocating overhead utility lines.

The proposed solution also minimizes the risk associated with constructing an on-grade pathway adjacent to U.S. Route 17.

By utilizing longer approach spans in segments two and four, the team also reduced the number of foundations they would need to build in the river.

Opting for prefabricated items — like segment caps, prefab cap forms for cast-in-place concrete and tying steel cages off-site — will also accelerate construction.

The project team is working with the United States Army Corps of Engineers and the South Carolina Department of

Health and Environmental Control to reduce any environmen- tal impacts associated with construction activities within the salt marsh.

Project Timeline

The team received a project award in September 2023 and held a groundbreaking ceremony a month later. Charleston Mayor John Tecklenburg attended the event, as well as city council members and community advocates like Zimmerman of the Charleston Moves nonprofit.

“Thousands of community members, including officials from multiple levels of government, worked together for many years to bring this project to fruition because they recognize the Ashley River Crossing will transform our region,” Zimmerman said.

David Cole, the MUSC President also attended the ground- breaking ceremony, demonstrating his support for the project, which will serve as a recreational facility and a valuable trans- portation link for MUSC commuters. Construction is estimated to begin by the end of 2024, with expected completion in 2027.

Designing for Future Generations 

The Ashley River Crossing has the potential to transform Charleston’s bustling urban landscape. By prioritizing transpor- tation upgrades, cities can create more accessible and resilient urban environments for everyone.

Zimmerman said this is a critical step in creating safer and affordable access to amenities everyone needs.

“The Ashley River Crossing will bridge a major gap in equitable mobility that the community has advocated to fix for decades,” Zimmerman said. RB

Richard Kirkman, PE, is Superior’s division manager for the Carolina Region. You can reach him at [email protected].

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