BRIDGE REHAB: No longer trailing

Nov. 11, 2014

PennDOT gets in front of deteriorating Freeport Bridge

The Freeport Bridge, also known as the Donald R. Lobaugh Bridge, carries S.R. 0356 over the Allegheny River approximately 25 miles northeast of Pittsburgh, Pa.

Built in 1965, the bridge is vital to commerce and serves as a route for tourists and outdoor enthusiasts utilizing the extensive nearby rails-to-trails network, including the Kiskiminetas-Conemaugh water trails. Several of the region’s original railways that parallel the Allegheny River were built to serve Pittsburgh when it was established as a major industrial hub in the mid-Atlantic region. Those railways are now being converted via rails-to-trails facilities for hiking and biking and contribute to the increased regional traffic.

During a routine NBIS bridge inspection, significant superstructural corrosion and section loss were discovered. These findings needed to be addressed in order to prolong the bridge’s life span and ensure the safety of the traveling public. It also was discovered that several existing factors, acting together, were contributing to the deterioration. Continuous drainage through a 1-in. open median joint and relief joints at alternate panel points in the truss span’s deck, free-fall roadway drainage from slotted curbs, and simple-span construction for the approach spans all contributed to cumulative deleterious effects on the bridge. This was a prime concern for PennDOT in prompting repair.

Dealing with an emergency

In 2006, significant emergency repairs were designed by bridge engineering firm Modjeski and Masters Inc. (M&M) and were completed in order to temporarily keep the bridge from being load-restricted, or posted. Otherwise, all heavy vehicles, including school buses and emergency vehicles, would have been banned from crossing the bridge, resulting in a 20-mile detour. Significant reinforcement of deteriorated steel plate girder approach spans and portions of the continuous deck truss span’s floor system was completed, but in order to preserve a safe crossing more extensive repairs would be needed in the near future.

Beyond the need for structural repairs, improvements to geometric and safety features were required. Those substandard features included curb-to-curb width, bridge rails, sidewalk width and lack of roadway barrier protection for the sidewalk. Design for a modernized bridge would need to not only address remaining corrosion and section loss, but also bring many of the original bridge’s features up to current standards and simplify the alignment of the north approach structures. A revised north approach alignment would improve safety by reducing the number of intersections and weave areas, and eliminate skewed intersections. Additionally, the revised alignment would reduce travel time with a complete S.R. 0356 bypass of Freeport Borough.

Ambitious success

In 2010, contract documents were submitted to PennDOT for a $63 million project focusing on the rehabilitation of the three-span deck truss and complete replacement of the north and south approach spans—an ambitious project with an overall length of 2,443 ft from abutment to abutment. The final documents also included plans and specifications by team subconsultant SPK Engineering for realignment of the north approach spans, reconstruction of approach roadways, and a new short-span bridge over a tributary branch of the Allegheny River, along with street improvements in the Freeport Borough. American Geotechnical & Environmental Services Inc., another major subconsultant, provided geotechnical engineering and foundation design.

The new structure’s cross-section accommodates four lanes of traffic and utilizes modern steel-plate girder design for the approach spans. This modern superstructure design, capitalizing on advances in strength of materials, was instrumental in reducing the number of approach spans from 26 to nine.

Working together, the M&M/PennDOT engineering and construction teams have encountered and overcome significant challenges throughout the course of planning, design and construction. Because a full bridge closure would impose a 20-mile detour, PennDOT required that two-way traffic be maintained during all construction phases.

One of the most significant challenges was the temporary widening of the original northbound travel lanes to accommodate two-way traffic during construction operations. This widening impacted the south approach roadway and bridge spans, the three-span deck truss and several of the north approach spans. The temporary northbound widening was 1,860 ft in total length and included the removal of the existing sidewalk, barrier rail and a portion of the concrete deck. This initial temporary widening was necessary to accommodate the required 24-ft curb-to-curb temporary cartway for two-way traffic. It also enabled construction to proceed on the southbound portion of the truss spans and the new southbound north and south approach structures.

The temporary widening with its cantilevered slab required several modifications to maintain structural stability and ensure safety of construction crews and the traveling public, including:

  1. A significant amount of truss floorbeam strengthening was initially required prior to the temporary widening because of the large floorbeam cantilevers. Additional demand on the floorbeam’s final condition also was imposed by an increase in design live loading to PHL-93 and from a wider final deck configuration. Strengthening of the floorbeams, including redundancy improvements, was primarily accomplished by bolting inside and outside strengthening plates to the top and bottom flanges;
  2. As a result of the truss being continuous, some members required reinforcing prior to completing any deck removal. Several truss members also required strengthening for the increased live loading which was completed by bolting full-length plates to one or both sides of the webs and developing the plates into the gusset plates;
  3. The temporary widening required strengthening of several original approach span exterior girders; and
  4. The original south abutment approach slab was replaced with a wider slab and attached barrier cantilevered beyond the original abutment’s northbound wingwall.

Due to strength issues, construction staging and PennDOT’s desire for a joint-less deck, the truss span’s stringers were replaced with full-length continuous stringers. The new joint-less reinforced concrete deck will extend the service life of the bridge and minimize future maintenance requirements for PennDOT.

Avoiding interferences with the existing structures resulted in additional challenges. Several of the original piers for the north and south approach were two-column bents supporting both superstructures for the northbound and southbound roadways. These piers had to be maintained in service while the new southbound structure was constructed. This affected the location selected for some of the new piers, the overall depth of some new approach superstructures, and required some of the new spans to be erected overtop of existing in-service structures. A portion of the existing northbound structure had to be removed at an early stage to avoid interference with construction of the new southbound superstructure.

Another challenge facing the teams was the abnormally positioned and seismically vulnerable rocker bearings at the north end of the truss that needed to be replaced to improve bridge safety and functionality. A temporary structural frame supported on the pier’s footing was erected and jacks were used to raise the truss bridge for individual replacement of the rockers with pot-type bearings. The remaining truss expansion and fixed bearings had retrofits installed to improve their seismic resistance.

Staging of work activities and loading/unloading patterns and associated construction live loads were of significant concern throughout the design and construction phases, in effort to maintain structural integrity. Since the deck was being replaced under traffic, multiple analysis models were created to confirm the bridge could safely accommodate construction loads and two-way traffic patterns. Preventing a repetition of the I-35 disaster was at the forefront of designers’ concerns. The lessons learned there established a precedent that such an event would not be repeated during the Freeport Bridge revitalization. Thorough 3-D FEM analyses were completed to evaluate the cumulative effects of the increased, decreased and varying loading patterns and intensities during the rehabilitation. Construction staging and loading patterns and limits throughout the various stages of construction were evaluated. The results of these analyses confirmed that the trusses were not overburdened. A mandatory maximum construction live and stockpiling load limit, in addition to a detailed and specific construction sequence, were provided in the contract documents to ensure that the bridge did not become overburdened.

Revitalizing the Freeport Bridge required close collaboration among multiple Pennsylvania counties and PennDOT engineering districts, Freeport Borough, and design and construction teams. By providing floor-system continuity and a joint-less bridge deck, the bridge should see a substantial increase of service life and decreased maintenance costs for PennDOT. The bridge is now fully opened to traffic with improved geometric alignment and increased safety for the traveling public. R&B

About The Author: Ahlskog and Smith are structural engineers with Modjeski and Masters. Irwin is a project manager and senior associate with Modjeski and Masters.

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