The massive steel delta frames that support the Shenandoah River Bridge rise from solid concrete wall piers founded on spread footings. The legs of each frame measure 212 ft along their lengths and connect via top and bottom knuckles. Keith Philpott
A bridge across the Shenandoah River in West Virginia’s panhandle features what may be one of the largest steel delta frames in the world.
June 4, 2013—When the West Virginia Department of Transportation set out to construct a new bridge across the Shenandoah River in the state’s panhandle, it had a conventional deck truss structure in mind. But when bidders began considering alternatives, an unlikely design was deemed to be at once more visually striking and more cost effective: a long-span steel delta frame bridge.
The Shenandoah River Bridge was constructed as part of a larger effort to upgrade Route 9. When the highway was built, in 1930, it was designed as a rural collector road, but it has since become a major artery for commuters traveling to Washington, D.C. Indeed, it carries more than 22,000 vehicles a day on the 5 mi stretch between Charles Town and the Virginia state line. To ease congestion and improve safety, this portion of the highway has been widened to four lanes and realigned, taking it along the hilltops of the Shenandoah Valley and over the new bridge.
The Department of Transportation advertised a design/bid/build contract in 2009 for a deck truss bridge, but when bidders began proposing alternative designs, the department changed the contract to design/build. The decision to consider alternatives was also influenced by the collapse in 2007 of the deck truss bridge in Minneapolis that carried Interstate 35W over the Mississippi River, as the collapse occurred when the Shenandoah River Bridge truss design was being finalized. (See “Bridge Collapse Prompts Investigations, Raises Questions,” Civil Engineering, September 2007.) A team of Trumbull Corporation, a firm based in Pittsburgh that specializes in bridge and highway construction, and HDR, Inc., an international engineering, architecture, and consulting firm headquartered in Omaha, Nebraska, studied several steel and concrete options for the new crossing. The team proposed a steel delta frame as the most cost-effective design, and it won the contract.
Delta frame bridges were popular in the United States in the 1960s and 1970s but eventually fell out of favor. Although the bridge type is no longer common, its ability to support long spans at a significant height with few piers made it an ideal fit for traversing the Shenandoah. “Geometric and environmental constraints on the project limited where your substructures could land in the river valley, so the spans got a little too long for traditional plate girder bridges,” says Jason Fuller, P.E., a senior project manager for HDR. “We had to look at something a little more innovative for the steel to get appropriate spans to make it work, and that’s [how] we ended up with the delta frame.”
The Shenandoah River Bridge carries West Virginia Route 9 over
the Shenandoah River Valley in the eastern panhandle of West
Virginia. Keith Philpott
The 1,650 ft long and nearly 85 ft wide bridge looms over the Shenandoah at a height of approximately 200 ft. It has seven spans: two approach spans and five main spans, including the tops of the two deltas. “If you look at an elevation of it, it’s a span, a triangle, a span, a triangle, and a span,” Fuller explains. The two massive steel delta frames rise from solid concrete wall piers founded on spread footings. The legs of each frame measure 212 ft along their lengths (150 ft vertically), from the plate girder superstructure to the piers. The connections at these points are referred to as respectively the top knuckle and the bottom knuckle. The horizontal distance between the tops of the legs of each frame is 300 ft. “The uniqueness is how large they are,” Fuller says. “We think this is one of the largest delta frames built in the United States, if not the world.”
No codes exist to govern delta frame design, so engineers designed the bridge using a mix of three-dimensional modeling and intricate calculations. “The code doesn’t really cover the leg design,” Fuller says. “So we had to go through the code and interpret it.” The team paid particular attention to the load transfers at the knuckles. For the top knuckles, inclined flange plates at the ends of the legs are bolted to the bottom of the girder by an end plate and inclined stiffeners transfer the loads. For the bottom, the legs converge on bearings embedded in the substructure, and an extensive array of longitudinal, transverse, bearing, and auxiliary stiffeners transfer the loads. “A lot of load was coming into that single point, so we did a lot of stiffening and a lot of stress calculations to make sure we weren’t overstressing the elements,” Fuller says. “Then we actually did additional finite-element modeling of those areas to make sure we weren’t missing anything.”
Temporary falsework and stay cables supported the delta frames as they rose from the foundations during construction. Jacks and shims were then used to adjust the legs and ensure proper geometric placement of the girder components. All in all, the construction progressed smoothly despite delays caused by natural phenomena, including hurricanes, flooding, and an earthquake. Now complete, it’s clear that the delta frame was not only the most cost-effective design but also the best fit for the site, Fuller says. “By using the delta frame, we were able to span the large valley, and we had the depth to create the delta,” he says. “The geometry just worked perfectly.”