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Rapid Replacement of the Back River Bridge Author Joseph E Krajewski T Y Lin International Portland OR jkrajewski ABSTRACT This case study describes the design of a new bridge superstructure and modifications to an existing bridge substructure that carries a heavily traveled urban arterial just south of Boston Massachusetts an area with little tolerance for increased travel delays and almost no tolerance for construction The Massachusetts Highway Department MHD required a rapid replacement design that did not involve experimental techniques or materials What the MHD wanted was a new structure designed and built with known materials and techniques in other words off the shelf technology The design consisted of using new steel stringers made composite with pre cast concrete and steel gird deck panels reconfiguring the existing 5 simple span arrangement into simple end spans and 3 span continuous middle spans and using a two stage construction sequence The design also included several time and cost saving ideas to live up to the FHWA s rapid replacement slogan Get in Get out and Stay out This case study demonstrates how a little extra design time and cost can reduce construction time and cost Considering the number of highway bridges that will need to be replaced in the near future any opportunities to save time money and reduce public inconvenience should be exploited LOCATION The Weymouth Back River Bridge is located 50 feet over the mouth of the river between the Towns of Hingham and Weymouth Massachusetts The Towns are located just south of Boston and are part of a major suburban area known as South Shore Route 3A is one of the main north south commuter roadways to and from Boston Average daily traffic in 1999 was 36 500 vehicles per day with 10 trucks By the year 2020 the estimated average daily traffic will be 49 000 vehicles per day assuming a 1 5 annual growth rate At the bridge location the Weymouth Back River is a Coast Guard Controlled navigable tidal waterway The average tide range is 10 feet Approximately a half mile from the bridge in both directions are major shopping centers with signalized entrances Underneath the bridge and inland along the shores of the Back River there are several protected salt marshes and estuaries which together form the largest protected wildlife area within the Boston metropolitan area 2008 ASCEStructures 2008 Crossing Borders Copyright ASCE 2008Copyright ASCE 2008Structures Congress 2008Structures Congress 2008 Structures Congress 2008 Downloaded from ascelibrary org by Changsha University of Science and Technology on 06 04 13 Copyright ASCE For personal use only all rights reserved EXISTING BRIDGE The existing bridge was built in 1942 to replace an existing structure that was built in 1900 The bridge consisted of 5 simple spans 46 9 85 90 85 46 9 of mixed structural types Spans 1 and 5 were 3 6 deep by 2 wide reinforced concrete tee beams spaced 9 apart Spans 2 and 4 were 10 deep steel deck trusses salvaged and modified from the 1900 bridge spaced 12 7 5 apart Span 3 was 36WF260 rolled steel wide flange stringers spaced 4 8 75 apart The main load carrying members in all spans support a reinforced concrete deck The bridge cross section consisted of reinforced concrete railings a 6 sidewalk along the north edge of the road a 4 median a 15 safety walk and 2 12 lanes in each direction The substructure consists of reinforced concrete stub abutments and solid piers founded on timber piles On top of and at the ends of each pier and abutment are large decorative reinforced concrete pilasters that rise above the concrete bridge railing Located just 3 feet off the north fascia is a steel castillated girder utility bridge that supports a 24 inch diameter high pressure sewer main The sewer main owners prohibited the use of the utility bridge for construction activities The most recent inspection reports before construction indicated the following Deck Condition Rating 4 Poor Superstructure Condition Rating 5 Fair Substructure Condition Rating 6 Satisfactory The 1987 Bridge Rating Report recommended the bridge be posted for 6 tons 2 axle 18 tons 3 axle 22 tons 5 axle From the above information the Massachusetts Highway Department MHD determined that the existing bridge superstructure needed to be replaced The MHD also determined that the bridge could not be closed to traffic during construction since the detour route was over 6 miles long and was also a high traffic volume roadway The MHD requested that we design a replacement bridge that could be constructed as rapidly as possible but did not involve experimental techniques or materials The MHD wanted a new structure constructed with known materials and techniques off the shelf technology SUBSTRUCTURE REUSE According to the Inspection Reports and our own inspection the piers and abutments did not have any major spalls or cracks The abutment backwalls and the pier pilasters were in good condition as well In order to save time no substructure demolition would be considered Removing portions of a substructure involves saw cutting neat lines around the removal area temporary earth support construction for abutment and wingwall removal jack hammering torch cutting existing reinforcing and disposal of debris In other words demolition equals time and money To save time and money the new superstructure was designed to be shallower than the existing to eliminate the need for demolition and require only pouring new beams seats and wall 2008 ASCEStructures 2008 Crossing Borders Copyright ASCE 2008Copyright ASCE 2008Structures Congress 2008Structures Congress 2008 Structures Congress 2008 Downloaded from ascelibrary org by Changsha University of Science and Technology on 06 04 13 Copyright ASCE For personal use only all rights reserved caps on top of the existing components In addition to a shallower superstructure we also were able to raise the roadway profile grade 10 inches We also took advantage of the existing concrete pilasters by using them as keeper blocks for the new superstructure under both regular and seismic loads The existing piers are solid wall reinforced concrete of substantial size compared to the superstructure they carry that they are too stiff to accommodate seismic movements Therefore we designed new steel laminated elastomeric bearings to allow for superstructure movement relative to the substructure The bearings were also sized to spread the seismic forces as evenly as possible to all the substructure components We analyzed the entire bridge structure using the multi mode spectral analysis technique with the bearings modeled as springs One final substructure concern was the difference in superstructure loading between new superstructure verses the old structure A conventional new superstructure would weigh more and have higher live load requirements than an existing structure In many cases the increased loading exceeds the capacity of the existing substructure When an existing substructure appears to have enough capacity an attempt should be made to design the new superstructure to be lighter than the existing and the capacity of the existing substructure should be checked As you will see in the next section we found a way to make the new superstructure lighter than the existing one NEW SUPERSTRUCTURE The new superstructure design had to meet the following criteria and requirements The superstructure was required to be replaced in stages in order to maintain 1 lane of traffic in each direction during construction We determined that 2 stages were required We also determined that the existing median had to be removed to provide adequate temporary traffic lanes Finally during construction no temporary sidewalk for pedestrians would be provided since there was no place for it Fortunately pedestrian traffic across the bridge is minimal During the winter construction shutdown in Massachusetts from mid November to end of March the MHD required that all 4 lanes on the bridge be open The main channel of the Weymouth Back River must remain open all times The main channel is a Coast Guard controlled waterway Once a construction stage is started it must be completed before the winter shutdown period The design we developed consisted of using new weathering steel stringers made composite with a pre cast concrete and steel grid deck panel system Steel Stringers The middle 3 spans were made continuous to eliminate expansion joints at piers 2 and 3 The end spans remained simple since the spans are too short in relation to the 3 middle spans to avoid causing uplift at the abutments Uplift forces would have required superstructure hold down devices at the abutments a foundation analysis for uplift and possibly substructure tension restraints Weathering steel was selected to minimize the amount of field painting to just the areas near the abutments and piers in accordance with MHD and FHWA Standards 2008 ASCEStructures 2008 Crossing Borders Copyright ASCE 2008Copyright ASCE 2008Structures Congress 2008Structures Congress 2008 Structures Congress 2008 Downloaded from ascelibrary org by Changsha University of Science and Technology on 06 04 13 Copyright ASCE For personal use only all rights reserved The stringers do not have cover plates due to the high truck traffic volume that would have required the cover plates to be practically the length of a span in order to satisfy AASHTO fatigue criteria for cover plate ends To save time and money the end spans were designed not only to support service loads but also crane loads which would allow for the erection of stringers for spans 2 and 4 and eliminate the need to mobilize a barge crane on the Back River The stringer segments for spans 2 and 4 were designed non compositely for crane loading to allow erection of the span 3 stringer segments which also eliminates the need for a barge crane Pre cast Concrete and Steel Grid Deck System A pre cast concrete and steel grid deck panel consists of a steel grid made composite with a concrete slab poured on top of the grid The panels come in standard widths of 4 to 8 feet wide by lengths up to approximately 40 feet The panels can be installed without concrete but with nearly all the stay in place formwork attached For this project 9 5 deep deck panels with 4 5 of reinforced pre cast concrete were selected Openings in the pre cast concrete panel at stringers fascias panel splice points and deck end haunches would allow for the installation of shear connectors stringers bridge railing fascias panel splices and end haunches These opening were filled with cast in place concrete that had a cure time of 3 days The pre cast panels were selected over conventional cast in place deck construction to save a significant amount of time and to save weight The panels are approximately half the weight of a conventional deck Bridge Railing and Median Barrier The new bridge railing is a pre case reinforced concrete Texas Rail A Texas Rail is a crash tested rail system that has 6 wide by 24 tall window openings spaced at 18 on center Normally the rail system is formed and cast in the field But the railing is time consuming to form and the resulting concrete finish requires lots of patching Therefore we specified the railing to be precast and mechanically connected to deck The F shape concrete median was also pre cast and mechanically connected to the deck as well A benefit to using pre cast components is that the finished pre cast unit is a higher quality product than conventional cast in place due to being made at a fabrication facility The difficulty with pre cast products is the higher level of construction precision and coordination between the Contractor and Fabricator to make the pre cast products fit properly in the field CONSTRUCTION TIMELINE April 15 2004 Contractor starts removing the existing concrete median June 1 2004 Route 3A is reduced to 2 lanes Stage 1 demolition and construction begins November 15 Stage 1 construction is complete except for bridge rail Route 3A reopened to 4 lanes for the winter shutdown May 1 2005 Installation of Stage 1 Texas Bridge Rail starts May 15 2005 Stage 1 Texas Bridge completed 2008 ASCEStructures 2008 Crossing Borders Copyright ASCE 2008Copyright ASCE 2008Structures Congress 2008Structures Congress 2008 Structures Congress 2008 Downloaded from ascelibrary org by Changsha University of Science and Technology on 06 04 13 Copyright ASCE For personal use only all rights reserved Route 3A reduced to 2 lanes Stage 2 demolition and construction begins August 15 2005 Stage 2 construction completed except for installation of bridge rail and medi