Conceptual Design and Construction Support of a First-of-Its-Kind Pedestrian Bridge in the U.S.

• Environmental studies and conceptual design of the bobtail swing span bridge
• Cost-estimating and grant support to secure an $18 million BUILD grant
• Feasibility studies to guide standalone bridge choice
• Review of cost, serviceability and navigation impacts of bridge options leading to bobtail span selection
• Owner's representative to the city of Charleston's Capital Projects Team

Two bridges take U.S. Highway 17 over the Ashley River in Charleston, South Carolina, connecting the city's largely residential West Ashley neighborhood to downtown. Narrow sidewalks and bustling traffic on the two bridges have posed safety concerns for pedestrians and bicyclists crossing the river for decades.

To improve multimodal connectivity and enhance community wellness, the city is building a new pedestrian bridge over the Ashley River, adjacent to the current U.S. 17 northbound bridge. The new bridge will be the first cable-stayed moveable pedestrian bridge in the United States.

Delivering Trusted Support

HDR has been a trusted advisor to the city and other local governments for more than a decade on the project. Our involvement began in 2010 with feasibility studies to improve mobility using the existing bridges. Several options were reviewed by our team, including a cantilevered pathway attached to the northbound bridge and the repurposing of an existing lane of traffic for pedestrians. Both were passed over for a standalone pedestrian bridge that would avoid congestion from eliminating a travel lane and stress to the U.S. 17 bridge from the additional weight of a cantilever. As the project progressed, we provided early grant support, leading to a successful federal BUILD grant in 2019. We teamed with the city to develop the conceptual designs and required environmental studies. Complicating the design was the requirement that the center of the bridge be movable to accommodate boat traffic on the river’s navigation channel.

Our movable bridge engineers performed an extensive alternatives analysis that ranked span type options based on serviceability, including capital cost, maintenance, navigation impacts and aesthetics. This process led to a bobtail swing bridge as the preferred movable bridge type. This less common option includes an asymmetrical design with a short span on one side and a longer span crossing the navigational channel.

We also provided cost estimating, environmental review, design-build document development and assisted in the procurement of a contractor.

With a design-build team identified, we have provided reviews of the final design. Throughout the $75 million project’s construction, which began in January 2025, our team will serve as the construction engineering inspector (CEI) for the city. Those CEI services include roadway inspection, bridge inspection, movable bridge commissioning services and environmental compliance reviews until the scheduled completion in 2027.

Enhancing Connectivity

The design of the bridge, stretching more than 4,000 feet across the Ashley River and its marshes, closely mirrors the original concept developed by our movable bridge engineers. Unlike the adjacent bascule bridges that carry vehicle traffic, the bobtail swing bridge features a center span that swings horizontally to allow marine traffic to pass. The bobtail swing design is not common but was chosen as a more affordable option that still complies with the U.S. Coast Guard’s requirement for a 100-foot-wide navigation channel. The asymmetrical bobtail has a 150-foot front span and a 50-foot back span with a single moveable piece, costing less than a symmetrical bridge by requiring fewer mechanical and electronic components. The cable-stayed towers in the bobtail design will act as a counterweight to balance the span while also providing an aesthetic design that mirrors the nearby Ravenel Bridge. While being cost effective compared with other span types, the aesthetics also pleased local historic preservation groups.

Once complete, the Ashley River Crossing will serve as one of the final pieces of the East Coast Greenway, a network of multimodal greenspaces stretching between Maine and Florida. To improve the connection between the West Ashley Greenway and Ashley River Crossing and the overall user experience, adjacent intersections are also being improved through enhanced crosswalks, pedestrian ramps, widened sidewalk and pedestrian signals.