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Prefabricated Bridge Elements and Systems Cost Study: Accelerated Bridge Construction Success Stories
The 32.4–ft wide 5,000 psi precast reinforced high performance self–consolidating concrete abutments consist of 10 spread footing segments and 11 abutment wall and wingwall segments. All precast segments were fabricated at the J. P. Carrara & Sons plant in Middlebury, Vermont and shipped 170 miles to the jobsite.
Spread footings provide significant speed and simplicity to bridge construction when soil conditions permit their use as in this project and in many other New Hampshire bridge projects. The spread footings and other substructure components were fabricated in segments as determined by the contractor and precaster to facilitate shipping and handling, and were standardized to reduce fabrication costs. The precaster used a template in the plant fabrication to ensure adequate tolerances between the abutments, wingwalls, and footing segments. The contractor developed the assembly plan.
Following placement of the footings, a minimum 3–inch thick flowable grout bed was injected through grout tubes in the footings to provide a sound bearing surface for the roughened bottom surfaces of the footings. Proper grading was assured by using leveling screws cast in the corners of each footing segment. The abutment walls and wingwalls had splice sleeve connections to accommodate the reinforcing bars protruding from the tops of the footings. The walls were lowered into place, and the splice sleeves were then grouted to complete the bar splices. All horizontal joints are full–moment connections with grouted reinforcing bars, and vertical joints have grouted shear keys.
The erection of the abutments took 2 days, plus a 3rd day to cure the grout and prepare for the backfill. Similar conventional cast–in–place abutments would have required 6 separate concrete placements and two months to construct.
The 115–ft long and 28–ft wide superstructure consists of seven 4–ft wide adjacent box beams. The beams are made of 8,000 psi high performance pretensioned concrete. The use of HPC in combination with 0.6–inch diameter pretensioning strands stretched the 3–ft deep box beams to 115 ft, allowing the use of a single span. Following erection of the beams, a precast concrete pilaster was set along the top of the stem wall on each side of the outside box beams to provide lateral load transfer between the superstructure and substructure and to also provide a more finished look. Full–depth shear keys were then cast between each box beam, and the span was transversely post–tensioned in 6 locations to complete the connection between beams. A 3–bar aluminum railing was then installed. A waterproofing membrane was applied to the top surfaces of the box beams, followed by an asphalt overlay.
In spring 2006, the Lamprey River crested 1 to 2 ft above the bridge deck after heavy rains. Although the area has a significant flooding history, this level was the highest seen by Epping residents. The inundation battle–tested the bridge in its second year of existence, with no ill effects. The bridge is expected to see a service life of at least 75 years due to the use of HPC, the quality of its prefabricated construction, the attention given to connection details, and an aggressive NHDOT maintenance and preservation program.
Because the focus of this project was to develop project details and processes that will reduce the onsite bridge construction timeline and improve safety on future projects, the project was originally bid with a maximum 30–day bridge closure and a maximum 14–day bridge assembly. The 30–day bridge closure compared to a 5– month closure for conventional construction. The 14–day assembly limitation started with lifting the first footing and ended with opening to traffic. The limitation on assembly time allowed the NHDOT to evaluate the effectiveness of this new bridge substructure system by removing the site–specific conditions; the 14–day clock would start after the site was prepared.
Two separate incentive/disincentive clauses were also included in the project that was originally bid. The first was an incentive of $1,500 per day for completion before the 30–day closure limit and a disincentive of the same amount per day for completion after the 30–day closure limit. The second was an incentive of $5,000 per day for completion before the 14–day bridge assembly limit and a disincentive of the same amount per day for completion after the 14–day bridge assembly limit.
The low bid for the original project was $1.4M, which was 40% higher than the $1.0M budgeted for the project. The bid was not awarded.
The NHDOT modified the contract to eliminate the 30–day closure window and the incentive/disincentive related to the closure limit. The 14–day assembly limit and its incentive/disincentive clauses remained in the contract. The project was readvertised 3 months later.
The engineer's estimate for the re–advertised project was $ 0.95M. The low bid of $1.05M from R. M. Piper Construction was 10% or $97,000 more than the engineer's estimate. Since there were 6 bidders for the project and the second low bid was $152,000 or 15% above the low bid, the NHDOT awarded the contract. Partial funding was received from the FHWA Innovative Bridge Research and Construction Program.
The bridge assembly was completed in just 8 days. This was 6 days ahead of the 14– day limit in the contract. At $5,000 per day, the contractor received an incentive of $30,000.
The NHDOT plans to use the knowledge gained from this demonstration project to design and construct future bridges with their new precast abutment system. They are developing a plan to provide standard detail sheets for prefabricated elements that could be substituted for cast–in–place concrete designs at the contractor's option at bid, as they now do for partial–depth precast deck panels. The standard sheets in the contract plans will include prefabricated full–depth panels, multi–column bents, cantilevered abutments, and stub abutments. The contractor will be required to submit an assembly plan to pull the components together if he chooses the prefabricated standards option.
Each day the New Jersey Department of Transportation's Route 1 carries more than 50,000 vehicles through the city of Trenton in Mercer County on the western edge of New Jersey. Route 1 is a vital link to adjacent Pennsylvania as well as locations within New Jersey. Three bridge decks on the Route 1 Freeway through the capital city, one at the Olden Avenue Connector and two at Mulberry Street, were deteriorated and in need of constant maintenance. In 2005 the replacement of these three bridges was the NJDOT's first Hyperbuild project.
The term "Hyperbuild" was coined in 2004 by NJDOT Commissioner Jack Lettiere. Hyperbuild projects were initiated to shave years off road construction projects and save millions of dollars in design, construction, and road user costs. To qualify for Hyperbuild status, a project should have a well–defined scope and, if possible, require limited right–of–way acquisition, utility relocations and environmental impacts.
All three bridges in this project were replaced with no impact to rush–hour traffic. The Route 1 bridge over the Olden Avenue Connector was replaced during a weekend closure in August 2005. The Route 1 Southbound bridge over Mulberry Street was replaced during a weekend closure in September 2005, followed by the Route 1 Northbound bridge over Mulberry Street during a weekend closure in October 2005. Design and construction would have taken 22 months using conventional methods.