ACTT Workshop: Rhode Island
Pawtucket Bridge No. 550 - Building a Foundation for the Future
Chapter 2: Project Details
2.1. Project Scope
The scope of the Pawtucket Bridge No. 550 project is to 1) either rehabilitate or replace the 50-year-old bridge, and 2) make interchange improvements to alleviate chronic traffic congestion. To eliminate the safety and congestion problems caused by the interchange ramp configuration, the project also features construction of a new collector-distributor (C-D) road along the northbound side of the bridge. Traffic control during construction will be a major challenge.
Rehabilitating or replacing Pawtucket Bridge No. 550 will be a daunting task: it is a five-span, two-girder, pin and hanger (suspended cantilever) steel bridge. The fixed end spans are situated on reinforced concrete cantilever abutments, with the three interior spans supported on four reinforced concrete column piers. The bridge consists of two separate structures (one northbound and one southbound) spanning west to east, with a one-inch-wide open joint between the median barriers along the bridge centerline. The overall span of the bridge is 694 feet five inches between bearings, as measured along the I-95 centerline.
The current configuration has three 12-foot travel lanes in each direction. On the northbound structure, the bridge flares at each end to accommodate a variable-width acceleration/deceleration lane for the George Street on-ramp at the southwest corner and the School Street off-ramp at the southeast corner. The southbound structure features an increasing width deceleration lane for the George Street off-ramp at the northwest corner of the bridge. The typical bridge deck width out to out is 99 feet six inches, and the concrete bridge deck is seven inches thick.
Figure 1: Project Location
The bridge superstructure consists of three primary framing components: 1) two main girders along each bound, 2) transverse floor beams that are attached to these girders, and 3) longitudinal stringers spanning between every floor beam. Four pin and hanger joints located in spans one, three and five allow for thermal expansion and contraction.
RIDOT rehabilitated the bridge in the 1980s, replacing the bituminous wearing surface and the waterproofing membrane, repairing several sections of the concrete deck, and raising the southeast corner of the bridge to accommodate roadway superelevation. In 1994, the agency replaced all of the deck joints; retrofitted the existing carbon steel pins and hangers with stainless steel assemblies; and replaced the existing rocker-type girder bearings with lead-core elastomeric isolation bearings.
Figure 2: Bridge No. 550 Aerial View
The challenges facing RIDOT today are numerous. Many of the bridge steel framing components show advanced corrosion. The concrete deck requires replacement. Several of the riveted girder connections must be retrofitted to improve inadequate fatigue resistance, and the bridge rails need to be replaced with crash-tested systems. The labor costs involved with repairing this type of structure are great; therefore, RIDOT must decide whether to rehabilitate or replace Pawtucket Bridge No. 550.
And there are major traffic issues as well. The northbound section of the bridge carries the George Street on-ramp at the beginning of the bridge and the School Street off-ramp at the end. This causes merging on-ramp traffic to weave with pending off-ramp traffic in the bridge-shoulder lane, which is only 695 feet long. The resulting congestion and numerous accidents have prompted RIDOT to incorporate interchange improvements as part of the Pawtucket Bridge No. 550 project.
Prior to the workshop, RIDOT investigated the interchange options that they believed would relive congestion and improve safety while facilitating staged construction of Pawtucket Bridge No. 550. The selected option incorporates a separate C-D road to take the George Street on-ramp and the School Street off-ramp traffic out of the mainline traffic stream. This option requires the widening of several upstream and downstream bridges, as well as a significant amount of retaining wall construction along the widened sections.
2.2. Workshop Priorities
As the project currently stands, three key decisions remain:
- The scope of the Pawtucket Bridge No. 550 project must be finalized. The options include a major superstructure repair with associated widening to accommodate interchange improvements or a full replacement with a wider structure.
- The rehabilitation option would cost an estimated $30 million, including interchange improvements, and would be completed in five phases over six years.
- The replacement option would cost an estimated $40 million and would be completed in six phases over five seasons.
- The effect on local traffic patterns due to permanently closing the George Street on-ramp should be evaluated to determine if this is a viable option.
- The possibility of temporarily closing the School Street off-ramp, at least during part of construction, should be evaluated. Temporary closure of the ramp would affect construction phasing and bridge widening.
Figure 3: Pawtucket Bridge No. 550
2.3. Project Constraints
The current average daily traffic (ADT) through the interchange is approximately 172,000 vehicles. The George Street/I-95 northbound on-ramp carries approximately 12,600 vehicles per day. Approximately 16,000 vehicles use the School Street/I-95 northbound off-ramp each day. What's more, I-95 serves as the major corridor between New York City to the south and Boston to the north, and there is no practical detour around the interchange. This means that traffic must be maintained during all phases of construction.
Because queue analyses show excessive travel delays with only two lanes open in each direction, RIDOT is requiring that all three travel lanes remain open in each direction throughout construction. Additionally, the School Street/I-95 northbound off-ramp must remain open, as it is used to access the downtown Pawtucket area as well as a nearby medical center, Pawtucket Memorial Hospital. The George Street/I-95 northbound on-ramp will be closed during construction, and the traffic and roadway skill sets were instructed to investigate the effects of closing that ramp permanently.
Figure 4: Existing and Preliminary Bridge Cross Sections
As any type of construction work causes delays, RIDOT's goal is to minimize the amount of time that roadway traffic patterns will be affected. Where possible, RIDOT wants to use advance construction for components such as foundations. The agency is open to installing temporary traffic barriers so that all construction work could be performed behind the barriers during normal working hours, avoiding major lane closures. Temporary lane closures, if necessary, would be allowed only at night or on weekends.
2.3.3. Work Area
The existing bridge features on- and off-ramps on three of its four corners, and one, the School Street off-ramp, must remain open throughout the construction cycle. This not only affects the phasing of the project, but it also serves to limit the space available for the contractor's work area. Further, the unique two-girder construction of the bridge does not allow phased construction in the traditional sense, as the structure cannot be dismantled within lane widths: it must be dismantled as a complete bound (northbound and southbound).