By Adam Davidson, Tennessee Bridge Department Leader, Gresham Smith
Accelerated Bridge Construction (ABC) is increasingly being implemented by Departments of Transportation(DOT) across the country. The uptick in the use of this non-traditional bridge construction methodology can be tied to both increasing traffic volumes and roughly one-third of the country’s 620,000 bridges needing major repair work or replacement, with more than 43,500 bridges in poor enough condition to be deemed structurally deficient (according to the ARTBA).
ABC may well be the solution for rural and urban agencies that are looking to address their own failing bridge infrastructure. Growing municipalities and rural counties alike can benefit from a number of advances made in ABC construction over recent years, since much of the fabrication and assembly can be completed off-site long before any road closures are announced.
Depending on the project, location, and multiple other factors, ABC has proven to be beneficial when taking into account the reduced disruption to community residents, as well as user, economic, and societal impacts. Instead of dealing with partial road closures for months or even years borne out of traditional construction processes, ABC can limit the disruption to motorists to just a few weekends or non-peak hours.
Some key considerations when exploring an ABC approach in a rural or urban setting are:
- Traffic patterns;
- Impacts of potential closures on drivers, freight and emergency services;
- Site conditions;
- Environmental impacts;
- Ability to standardize, repair or replace plans.
Let’s take a look at a couple of examples of ABC projects in a rural setting.
Tennessee State Route 49
The Tennessee Department of Transportation (TDOT) has embraced ABC and alternate project delivery philosophies in the completion of a number of diverse projects. In the case of the Tennessee State Route 49 bridge project, a TDOT rehabilitation project was needed on two rural Robertson County bridges originally built in 1929 and rehabilitated in 1964. Using ABC techniques, both the Millers Creek Bridge and the Calebs Creek Bridge were converted from two-span to single-span structures, with most of the work completed during two single weekend closures, limiting traffic disruptions to just 56 hours.
The State Route between these two bridges represents the main access point for Coopertown Elementary School, Coopertown Middle School, and the Coopertown Police Department. The replacement of these structures over two single weekend closures eliminated disruptions to school traffic, as well as access and minimized delayed response times from the police department.
Another example of ABC techniques in a rural setting is the replacement of the State Route 16 bridge over Kelly Creek in rural Rutherford County, Tennessee, that was originally built in 1940 and rehabilitated in 1957. This main thoroughfare runs through Eagleville connecting the small town to the greater Middle Tennessee region.
If a long-duration, phased-construction approach had been implemented, it would have resulted in significant impacts to the mobility of this local community. By using ABC techniques, this aging and deteriorating two-span bridge was completely replaced in a 56-hour weekend closure using precast concrete bridge components. In addition to minimizing the construction impacts to the community, the conversion to a single-span bridge improved the overall hydraulic performance of Kelly Creek.
ABC In an Urban Setting: Fast Fix 8
Originally built in the late 1960s, the eastbound and westbound I-40 bridges in downtown Nashville were showing advanced signs of deterioration, required near-constant maintenance and needed immediate attention. To minimize the impacts on Nashville’s central business and entertainment districts, TDOT elected to use ABC versus a more traditional bridge replacement method. By employing ABC, it was estimated the project could be completed within 13 weekend full-closure periods instead of up to three years of reduced traffic lanes.
To meet the aggressive schedule, numerous innovative engineering techniques were employed. For example, structural steel superstructure units were used on four of the eight bridges. These units were fabricated off-site at a “Bridge Farm” utilizing existing TDOT right of way in the median of I-40, and then transported and installed during the weekend closure.
Project challenges in this urban setting included:
- Project site congestion with all bridges located within a half of a mile of the I-40 segment, with frontage roads that parallel both sides of the interstate limiting available working room.
- Existing structures had complex geometry including horizontal and vertical curves, tapering widths and exit ramps leading to challenging construction and fabrication.
- The existing cross-slope on the bridges was substandard by current standards and contributed to the drainage issues along this stretch of interstate highway.
- A CSXT railroad track was the only connection heading west out of Nashville to other regional transportation hubs and available open-track periods were severely limited.
- Availability of work weekends were dependent on minimizing conflicts with major downtown, neighborhood, university and professional sporting events.
Despite these challenges, the project utilized only 10 weekend road closures making Fast Fix 8 an ABC success story that evidences just how well accelerated bridge construction can work in an urban area.
The Trade-offs of Using ABC
Of course, there are pros and cons to everything, and it’s worth looking at some of the trade-offs of using the ABC methodology. The ABC method shortens the total project duration, minimizing the disruption on the community, and reduces the impact on public transportation, and emergency services. However, the ABC method increases the initial project cost in terms of design and construction due to the specialized nature of the work. It also is a new process, so the industry is still evaluating the long-term performance of the structure.
ABC Checklist
As touched on earlier, the following checklist can help an agency determine if the ABC approach is the right solution for repairing or replacing a bridge—whether it’s part of a two-lane rural route or in the middle of a sprawling metropolis:
- Consideration of Traffic Patterns Early in the Process: No matter the project’s size, roadways near a repair or replacement project will be closed for a period, requiring site-specific plans and lane assignments based on peak-traffic movements. Minimizing the impact to the average roadway user is key. When tackling multiple nearby projects, detours must be considered so that drivers aren’t experiencing simultaneous closures. Construction schedules and timing should be thoroughly considered and planned in advance.
- Minimized Impacts on Drivers: A major benefit to using ABC is the minimization of risk and improvement of safety for the immediate area. Bridge and roadway closures have an immediate impact on society and daily users. The length of time and traffic congestion for the relocation of traffic onto a detour are also important factors. How will this affect emergency routes? Business and school access? What about the local environment?
- Factored-in Site Conditions: To save time and headaches, prefabrication of certain bridge units can be constructed off-site by the contractor, then installed and joined together during a weekend road closure. Geometrics, and conditions such as overall state of the bridge, all impact the feasibility of ABC.
- Keeping the Environment in the Mix: With proper planning, several components on a bridge project can often be reused. These can include substructure units from the existing bridge. By limiting the time spent at a project site, environmental impacts such as emissions and noise pollution can be significantly reduced.
- Standardized Plans: When dealing with multiple bridges along a roadway, the contractor will need a bridge-specific removal plan for each structure, but the repair details can be standardized so that the contractor can fabricate replacement items identically. This reduces cost and makes installation more seamless.
Keeping all these factors in mind, it can be determined whether the benefits of ABC outweigh additional considerations, like increased construction costs. If you’re still wondering if ABC is right for your project, the Federal Highway Administration (FHWA) offers an ABC decision-making tool to contrast the benefits of using ABC versus traditional construction. Whatever method you choose, ABC technologies are rapidly changing the rehabilitation and construction game by taking an innovative approach to a traditional process—especially as bridges across the nation continue to age. R&B
