The new Virginia Street Bridge, in Reno, Nevada, is designed to accommodate the Truckee River's 100-year flood level. Jacobs Engineering Group, Inc.
In replacing an earth-filled arch bridge that is listed in the National Register of Historic Places, designers had to consider aesthetics as well as function.
April 30, 2013—At roughly 160 ft long, the Virginia Street Bridge, in Reno, Nevada, is not a large structure, but its location makes it an important one. A primary identifier of Reno, the bridge not only marks the site at which the city was founded but also carries Virginia Street, the city’s main thoroughfare, over one of the desert region’s few rivers and into the city’s historic downtown. So when city officials decided to replace the 107-year-old bridge, a great deal of consideration was given to the new crossing’s functionality and design.
A two-span, earth-filled concrete arch bridge, the Virginia Street Bridge is listed in the National Register of Historic Places as an example of early-20th-century design. Some community members wanted to save the bridge because of its historical importance, but replacement was deemed necessary for two reasons: the bridge is structurally deficient and restricts the flow of the Truckee River, exacerbating flooding in the area. “The Truckee River is very low [during] a big part of the year—it is a desert, after all—but in the spring and during most of the snowmelt season in the Sierras, it’s not,” says Fred Gottemoeller, P.E., AIA, M.ASCE, the principal architect of Bridgescape LLC, an architecture firm based in Columbia, Maryland, that is responsible for the architectural design of the new crossing. “In fact, there’s been some pretty substantial floods over the years in Reno, and they were the result, in part, of the bridge blocking the water flow.”
So the City of Reno put out a request for proposals in 2010 for the design of the replacement bridge, and on the basis of a competitive qualification process it selected a team comprising Jacobs Engineering Group, an engineering firm headquartered in Pasadena, California, and Bridgescape LLC for the project. The city’s chief request was that the new bridge be above the Truckee River’s 100-year flood level. That meant increasing the height of the bridge over the river and, in turn, raising the profile of Virginia Street. Many community members were concerned that raising the road’s profile would affect the sidewalks that adjoin the historically important buildings along the street, and at their request the designers agreed not to raise the road’s profile by more than 3 ft above its current elevation at the flood walls. “That kind of created our upper envelope of what we were allowed to work with,” says Bryan Gant, P.E., a senior project manager for Jacobs Engineering. “And then the lower envelope, of course, was based on passing design flood flows.”
The new Virginia Street Bridge will have sidewalks that will
measure 18 ft at their widest point, providing plenty of room for
pedestrians to take in the views of the Truckee River. Jacobs
Engineering Group, Inc.
In addition to those requirements, the team had to follow The Secretary of the Interior’s Standards for the Treatment of Historic Properties, guidelines designed to promote the responsible preservation of such historically significant buildings as those located near the Virginia Street Bridge. Taking all of the requirements into consideration, the team developed bridge types that were acceptable from both an engineering and a historical standpoint: three static bridges, all of which would require raising Virginia Street, and two movable bridges, which would not require the street to be raised. After stakeholders and city officials examined the options, they determined that the movable bridges were less desirable because of their cost and maintenance issues over time, Gant says. Of the static bridge options, the consensus was that the through arch bridge was the most visually compelling, and that type was therefore approved.
In contrast to the existing bridge, the new bridge will not have a skewed alignment. It will be approximately 166 ft long and will vary in width from 84 to 98 ft, the widest part being at the center, where 11 to 18 ft wide pedestrian walkways will bow out on either side. The bridge will carry two 12 ft wide traffic lanes, and there will be room along the shoulder for parking or a streetcar line should the city decide to add those features. The structure will be roughly 17 ft over the river when the water is lowest and will allow unrestricted flow of the Truckee even as the water rises.
The bridge’s arches and tie beams will be structurally integrated with the abutments, an engineering innovation that will make it possible for the superstructure to be just 3 ft deep. “The structural continuity of the arch and the integral abutments gives us the stiffness necessary to keep the floor structure relatively thin,” Gottemoeller explains. “So the stiffness and structural support of the bridge are really supplied by the arches and the integral abutments, which in turn support the floor system.” The design obviates the need for piers in the water that could obstruct the river’s flow.
The new through-arch bridge will be wide enough to accommodate
parking or streetcars in the future. Jacobs Engineering Group, Inc.
The bridge is designed to have a low profile so as not to obstruct views up and down the river, which is a recreational resource for kayakers, anglers, and those traveling downstream on rafts made with inner tubes. The concrete arches will be approximately 17 ft tall from the centerline of the deck to the centerline of the arch, or roughly 13 1/2 ft high from a pedestrian viewpoint. The arches will be tapered to impart visual appeal and will range from approximately 3 ft deep at midspan to about 4 1/2 ft deep at the ends, and 13 cables formed from steel wire strands will extend from each arch to the bridge deck, Gant says. “One of the issues that arose in our public input process was that having vertical interest is good but not to the point that it starts to impede the view up and down the river,” Gant says. “So compared to a traditional arch, [these relatively stout arches are] not carrying all of the weight and stiffness of the bridge. The integral abutments are doing some of that.” The abutments will be founded on drilled shafts.
While the bridge design meets the flood flow requirements, raising the profile of the road and the adjoining sidewalks will still be necessary. “The road has to taper back down to the north and south on either side of the approaches, which isn’t a big deal from a road standpoint, but then the adjoining sidewalk has to move with it,” Gant explains. “That becomes problematic in an urban area where you have buildings at the back end of that sidewalk.” To minimize the effect, the team has designed split sidewalks; thus, the portions directly in front of the buildings can remain at their existing elevation. “We’re kind of splitting it down the middle,” Gant says. “The sidewalk adjoining the roadway will rise with the road, and then we’ll have steps and ramps and other connections to make up for that difference” in elevation with respect to the other half of the sidewalk.
Construction of the bridge is expected to begin in the spring of 2014 and, depending on the weather and river conditions, to take 12 to 18 months. The designers say the Virginia Street Bridge is critical to the city’s identity and economy, and they are taking steps to ensure that the new structure is just as distinctive as the existing one. “The normal assumption is that only the really big bridges get or deserve the kind of thoughtful attention that you think of for a signature bridge or landmark bridge, but in a lot of places—and Reno is one of them—the small bridges have just as much importance and they deserve just as much attention,” Gottemoeller says. “I’m optimistic that once people see this bridge and start to photograph it and be around it, it’s going to become the city’s new postcard bridge.”