Built in 1938, the westbound bridge for state Route 10/US 78 over the Apalachee River at the Walton-Oconee county line was deemed eligible for preservation by the Georgia Department of Natural Resources’ Historic Preservation Division. Having worked extensively with Georgia Department of Transportation (GDOT) on past projects, GS&P was solicited by the Department in 2007 to investigate rehabilitation alternatives for the existing arch bridge.
“Very few bridges of this type remain in the United States, so preserving the historic structure was GDOT’s greatest concern,” explains senior transportation engineer Scott Shelton. “We were tasked with exploring all possible options for its rehabilitation and reuse. If none of those proved viable, we would then need to propose an alternate solution.”
“This kind of historic arch structure is rare because of the complexity of construction and the amount of labor required compared to other bridge types,” adds senior structural engineer Ted Kniazewycz. “It’s something that’s not built today due to the cost involved and the impact it would have on the surrounding environment. It took a tremendous amount of research just to find the proper technique to analyze the bridge for its adequacy and load-carrying capacity. Fortunately, we were able to locate the existing historic arch bridge plans that were put together by the engineer who originally designed the overpass. This allowed us to create a model to evaluate the load-carrying ability of the original structure. We also completed hand calculations at key support areas to assess the bridge’s overall sufficiency.”
Weighing the Options
GS&P’s analysis of the existing bridge revealed that it was constructed with reinforced concrete-deck girder spans that had a live-load capacity below the current AASHTO design standard for bridges. Moreover, the bridge components throughout the structure exhibited cracks and the substructure showed numerous surface failures, which exposed the reinforcing steel to the environmental elements leading to section loss.
“We spent a great deal of time determining if the existing bridge could be salvaged yet still bear the loads of modern-day traffic,” says senior transportation engineer Jody Braswell. “We explored various options for preservation, including building a new bridge above the existing structure and keeping the historic arches in place. However, all of the options would have resulted in damage to all or part of the historic bridge.”
After performing an exhaustive evaluation, only two feasible alternatives remained if the bridge was to be preserved: replace the existing deck of the structure to meet GDOT width and load requirements, or construct a replacement bridge to the north and evaluate the impacts to the existing bridge.
“With the first proposed alternative, the width of the bridge wasn’t sufficient to meet GDOT’s dimensional requirements,” says Kniazewycz. “One idea was to build a third arch element to give the bridge the added width needed. But the way the structure was configured would have meant placing the support in the middle of the river, which would have interfered with the river hydraulics at the site. To meet the load stipulations, we would have had to reinforce the bridge. That involved encasing it in concrete, which would have changed the aesthetics of the historic structure and defeated the entire purpose of the preservation. It would have also created a financial burden that was simply not viable for the project.”
Another issue with rehabilitating the existing bridge was its sufficiency rating of only 36 out of 100. Kniazewycz explains:
“A sufficiency rating is purely a mathematical calculation of various elements, including the superstructure, substructure, functional width, age and conditions. If a sufficiency rating is above 50, the bridge is likely to be considered for preservation. A score under 50 means the bridge will probably need to be replaced."
It’s All in the Planning
After assessing all alternatives, GS&P recommended the construction of a new westbound bridge parallel to the existing structure. Mitigation measures to preserve the historic arch bridge were proposed to the Federal Highway Administration and the State Historic Preservation Office (SHPO). GS&P worked closely with both GDOT and SHPO to identify the historical boundary of the property, along with permissible construction activities on the bridge itself and within close proximity to the structure.
“An essential part of the planning process was completing the proper research, studies and agency coordination to comply with the National Environmental Policy Act [NEPA] and Section 106 of the National Historic Preservation Act,” explains senior civil engineer Sandy Layne-Sclafani. “The GS&P team was able to draw on years of experience completing NEPA documents in multiple states, as well as effective coordination with natural and cultural resource agencies, to deliver an FHWA-approved NEPA document for GDOT. It allowed the undertaking to be completed without late-stage delays or objections from local, state or federal agencies so we could adhere to GDOT's schedule for construction.”
GS&P also assisted the Georgia Department of Natural Resources’ (DNR) Wildlife Resources Division with a comprehensive assessment of the aquatic environment around the historic bridge site.
“We conducted an aquatic protected species survey to determine the presence or absence of threatened or endangered aquatic species within the project area,” says Layne-Sclafani. “The survey revealed the presence of 18 different species of fish within the project area. One particular species—the Altamaha Shiner—is on the state’s list of endangered species. This discovery prompted DNR to issue a special provision restricting work within the stream from May 1 to July 1 due to the shiner’s spawning season.”
Configuring the New Bridge
Improving the hydraulic efficiency of the overall project site was a key goal in the design of the replacement structure. The new westbound bridge was placed at a parallel angle to the Apalachee River to better convey storm flow, while its substructure columns were aligned to deflect debris in the channel toward the center of the span to prevent any refuse from becoming entangled in the downstream bridge support.
“We designed a wider opening for the new bridge so the stream could flow freely beneath the road without being hampered,” says Braswell. “The design prevents a damming effect where the water would have otherwise backed up, spilled over the embankment, and flooded adjacent property.”
Additionally, the team’s Bridge Hydraulic Report, which required approval from Walton and Oconee counties, demonstrated that the new parallel bridge would not adversely impact the flood plain or the floodway, nor the Apalachee River’s flow characteristics.
“During the hydraulic analysis, we studied the potential effects of the proposed construction—as well as the possible impact of an old mill and weir wall located upstream—to make sure there would be no upstream or downstream flooding when the new bridge was put in place,” says Shelton. “Both Walton and Oconee counties approved GS&P’s report because the positioning of the proposed structure was designed to keep the flow of water the same. So there were no changes to the overall streambed.”
Jointly, the completed bridge evaluations, NEPA documentation, wildlife surveys and hydraulic analysis allowed GS&P to leave the historic arch bridge intact and commence construction on the new bridge in 2013.
“GDOT has fixed dollars to deliver projects, so any extra cost overruns the end result in a loss of funding for other programs,” notes Shelton. “For this effort, the GS&P team successfully estimated the construction costs, which prevented GDOT from having to reallocate monies from other projects.
“From the start date through to completion, GS&P developed, updated and monitored the project costs. When the engineer's final cost estimate was compared to the contractor's, it was in a very close range. This allowed the Department to go to construction per schedule without having to wait on additional funding.”
Meeting the Needs of the Future
Opened to traffic in May 2014, the new westbound crossing of the Apalachee River consists of a 410-foot-long by 40-foot-wide bridge, with the adjoining roadway comprising a typical section of two 12-foot travel lanes in the westbound direction and a 10-foot rural shoulder. The preserved historic arch bridge was permanently closed to traffic, with the new two-lane overpass serving all vehicular circulation traveling westbound.
“This project is unique in that it was part of the first GDOT bridge contract that GS&P won in Georgia and ultimately allowed us to expand our structural services in the Atlanta office, enabling us to secure future GDOT bridge work,” says Shelton. “It was also special because it gave us a whole new perspective on preserving a piece of history since it was a quality design performed in the past by our civil engineering peers. Every time we went into the field we stood in awe of the historic arch bridge. It was an honor to preserve that past design while at the same time executing a design of our own to meet the needs of the future.”
“The GS&P design team did an excellent job delivering a quality bridge replacement design on time for the Department and successfully worked with the GDOT Historic Preservation Division to preserve the adjacent historic arch structure during construction of the new bridge,” concludes Derrick M. Brown, District 1 Program Manager for GDOT. “I look forward to working with the GS&P design team in the future, as I know they will be responsive, proactive and timely with deliverables.”