To further support community buy-in, the team proposed a pedestrian path from the bridge level down to the street below. The path helped to make the project more worthwhile for the street-level businesses, including a restaurant and a bar, affected during construction. Now that the bridge is complete, residents and visitors of the affluent Gulch district can reach these businesses with a 10-minute walk, bringing extra appetites and deeper pockets to their front door. Ahead of the curve The team also anticipated technical challenges associated with constructing a curved steel bridge. The tight radius of the bridge curvature required a more rigorous analysis model to look at the total superstructure system behavior, as opposed to the more typical line girder analysis. And typical for most curved girder bridges, Division Street caused increased torsional forces and displacements, differential deflections, and girder flange lateral bending. Additionally, the bridge required a bearing design to accommodate displacement and girder end rotations. All of these issues created bigger and subsequently more costly member sizes in the cross frames, girder flanges, and bearings. One unique design aspect used by the design team to reduce fabrication costs was to locate field splices at the beginning and end of the horizontal curve. This allowed for fabrication of tangent girder sections at the beginning of the bridge with the curved portion located between the field splices. This was a tremendous benefit to both girder erection and fabrication. Additionally, girder framing was radial, which allowed for standardization of the cross frames. The curved bridge also impacted construction documents. Division Street required increased detailing due to the varying girder lengths. Additionally, the team had to identify fit methods such as girder web position and the condition under which the position was achieved. Other construction issues included increased erection preparation that was required to get the bridge to “fit-up” in the field, and an inherent lack of stability that required increased temporary shoring and heavy lifts of two-girder units that were required for stability during erection. The tight radius of the bridge curvature required a more rigorous analysis model to look at the total superstructure system behavior. To reduce fabrication costs, the design team located girder field splices at the beginning and end of the horizontal curve.
The extension bridge is multi-modal, accommodating vehicles, bicyclists, and pedestrians.
Green elements The multi-modal nature of the extension already made positive contributions to the health of the community — by encouraging active lifestyles through walking and biking — and to the health of the environment — by reducing the number of vehicles on the road. Its design also incorporated some progressive green elements. The project features rain gardens and bioswales, reducing the speed of stormwater runoff and thereby decreasing the amount of pollutants that reach tributaries and rivers. The design also includes narrow traffic lanes, which minimize the overall impervious area. Vibrant LED lighting and native landscaping add to the project’s aesthetics and make it visible from nearby Interstate 40. The project was originally planned to include landscaping and bioswales on each side of the street and in the median for the entire length, but in an attempt to decrease bridge width and thereby minimize nearby property impacts, the team decided to include plants and bioswales on just one side where needed. Connecting the dots Contrary to its name, the Division Street extension bridge has set the stage since its grand opening for many important new connections in the city. The team’s vision was to create a structure that would improve quality of life for everyone who travels through the area and support essential development along this dynamic corridor. The bridge is already encouraging more foot and bicycle traffic, making it easier for drivers to access major interstates, reducing congestion on nearby roads, and providing an essential link between growing neighborhoods. It is truly opening new doors in the community and helping us take another step forward into Nashville’s future. DIANE REGENSBURG, P.E. , is a senior transportation engineer and project manager at design and consulting firm Gresham, Smith and Partners (www.gspnet.com) in Nashville, Tenn. With 20 years of experience, she has deep knowledge in roadway layout, drainage design, traffic control design, and erosion control design for an array of interstate, arterial, and local street projects. Contact her at diane_regensburg@gspnet.com.
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april 2018
csengineermag.com
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