BRIDGES 1998

Bridges Article December 28, 2000
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The Church Street Railroad Bridge was constructed in 1882 over the Providence and Worcester Railroad in the town of Cumberland, R.I. It is located in a combination residential and commercial area. By altering traffic flow over the bridge and employing some unobtrusive repairs and replacements, this 115-year-old truss bridge was upgraded to service modern highway needs.

The bridge is a single span double-intersection Warren through truss with a span of 107 ft and a spacing between trusses of 21 ft. Stone masonry abutments support the superstructure.

The original deck system consisted of wrought iron floor beams supporting timber stringers and a timber plank deck with a curb-to-curb width of 18 ft 8 in. The seven 22 in. deep built-up riveted floor beams spaced at 13 ft 4 in. were suspended from the lower chord by U-bolts. Asphalt was added at a later date as a riding surface.

The sidewalks are cantilevered by floor beam brackets (extensions) on the outside of each truss and also were composed of timber stringers and a timber plank walking surface. A lattice type railing with ornamental rosettes flank each sidewalk. Gas lamp posts were placed at each corner of the bridge. The minimum vertical railroad clearance was approximately 17 ft 4 in.

Bridge history

The Church Street Railroad Bridge is a highly significant example of early riveted truss construction at a time when most truss bridges were of the pinned type construction. Riveted connections became standard bridge practice in the 1890s; however, the bridge retained the use of U-bolt hangers to support the floor beams, a detail characteristic of pinned construction.

The bridge is the oldest of only three 19th century through trusses in the state and is exceptionally well preserved. The structure was nominated to the National Register of Historic Places on Jan. 10, 1989.

The bridge was structurally and functionally inadequate as determined by inspections and ratings of the structural elements. A report prepared by A.G. Lichtenstein & Associates Inc., Paramus, N.J., in 1986 revealed the floor system was inadequate. The highest as-built rating, controlled by a floor beam, was 9 tons for a HS-20 (36 ton) vehicle. However, the bridge was posted for 5 tons as governed by the existing deteriorated timber stringers. In addition, many elements of the truss had a substandard as-built rating with several diagonals having the lowest capacity at 25 tons.

Although not controlling the ratings, the lower chord had significant deterioration which was continuing to progress. Furthermore, the 18 ft 8 in. curb-to-curb width was not adequate for the two lanes of traffic utilizing the bridge. For these reasons, the report recommended replacement of the superstructure.

In 1987, the Rhode Island DOT (RIDOT) proposed to replace the bridge superstructure. However, because the structure was eligible for the National Register, Lichtenstein was selected to prepare the necessary Section 106 4(f) evaluation. As part of this report, an alternative analysis was performed to determine whether there were any feasible and prudent alternatives to replacing the existing bridge. The report presented five alternatives:

    1) Do nothing.
    2) New roadway alignment - leave existing bridge in-place.
    3) Structure rehabilitation - two lanes.
    4) Structure rehabilitation - one lane, local one-way traffic.
    5) Construct proposed bridge at new location.

Alternative one was considered not to be prudent since the existing substandard conditions would remain. Alternatives two and five were not feasible due to the extensive right-of-way takings and excessive costs involved. Alternative three would have required substantial alterations to the existing truss and would have adversely affected the historic integrity of the bridge.

Due to the relatively minor truss strengthening required, alternative four was considered feasible. However, this solution posed an inconvenience to the local traveling public, because traffic in the opposite direction would have to be permanently detoured approximately one half mile. But, this was considered secondary to the adverse affects to the truss itself.

The report was reviewed by several agencies including the RIDOT, State Historic Preservation Commission, Blackstone River Valley National Heritage Corridor Commission, Cumberland Historic District Commission and the Town of Cumberland. All parties agreed that alternative four would not significantly alter the historical integrity of the bridge and would meet the need to provide a safe and reliable crossing for the traveling public. Therefore, alternative four was chosen to be implemented.

Based on the responses of the reviewing agencies, the RIDOT requested that Lichtenstein perform an in-depth study to determine the feasibility of alternative four - limiting the bridge to local one-way traffic, capable of supporting HS20 truck loading.

Prior to commencing with this study, Lichtenstein contracted with a local testing firm, Thielsch Engineering Associates, Cranston, R.I., to perform metal testing of the truss material to determine its composition, and whether the material will perform adequately on the rehabilitated structure.

Tensile, charpy impact and chemical composition tests were performed on three samples removed from the bridge. The results of these tests indicated that the trusses were in fact composed of wrought iron and that it would perform in a satisfactory manner in the rehabilitated structure for one-way traffic.

Three rehabilitation alternates were studied. All alternates included the replacement of the floor beams, stringers and deck with some work required to the trusses.

The feasibility study also examined the effects of raising the profile of the bridge. The Providence and Worcester Railroad desired to increase the vertical clearance as much as possible during the rehabilitation. The four tracks running beneath the bridge are located toward the east with the centerline of the most easterly track approximately 10 ft away from the face of the east abutment.

Therefore, it was decided that the bearing elevations on the east abutment be raised by approximately 1 ft to gain additional clearance for the tracks below. The existing profile was situated on a sag curve with the low point adjacent to the east abutment. This configuration was well suited for the raising with a more desirable tangent profile as a result.

A traffic analysis was performed which examined the volume and direction of the traffic over the bridge. This was done to determine which direction, eastbound or westbound, should be maintained on Church Street in order to best service the existing traffic while minimizing any operational or safety impacts on the surrounding area. Meetings were held regularly with the local officials to obtain information and concerns particularly important to the community. After considering all the data, it was recommended that eastbound traffic be maintained as the one-way direction of travel.

After consideration of all the information including construction and future maintenance costs, the report recommended alternate three be implemented. The glu-laminated timber panel deck resulted in only minor visual differences from the existing timber stringer deck system. The lack of stringers resulted in a thinner superstructure depth and combined with the raised profile would provide an additional 1 ft 4 in. of vertical clearance for the railroad below. Finally, alternate three had the lowest construction and future maintenance costs of all the alternates.

Design

The design of the project consisted of three major categories:

  • Conversion of Church Street to one-way eastbound traffic and modifications to the east approach necessary for the profile raising;
  • Replacement of the existing floor system; and
  • Structural rehabilitation of the trusses and abutments.

The first consideration was traffic control during construction. Due to the types of repairs required, it was determined that both vehicular and pedestrian traffic would be detoured during construction. This would also provide for the shortest construction duration.
To facilitate one-way eastbound traffic, several approach modifications were necessary. Traffic had to be channeled from a 30-ft-wide approach roadway to a 14-ft 8-in. roadway on the bridge.

To accomplish this, the pavement was striped and a guide rail placed on a 15 to 1 taper on the east approach attached to the bridge railing. The approach sidewalks were widened to meet the new timber curb on the bridge. Advisory signs were placed at both intersections and at the bridge ends indicating the one-way arrangement.

To accommodate the profile raising, it was necessary for the DOT to acquire right-of-way from various properties along the east approach. This was to facilitate the resulting slopes from the raised sidewalks. The existing driveways were reconstructed and stone retaining walls at the bridge corners were capped. This was accomplished by removing the existing granite cap and adding stone courses with a similar appearance. The granite cap was then replaced. The sidewalks were reconstructed and granite curb added on both approaches.

Floor system

The existing timber stringer and timber plank floor system was replaced with longitudinal glu-laminated timber planks. Spanning between the floor beams, the glu-lam planks eliminated the need for stringers. A 2-in. minimum asphalt wearing surface with a waterproofing membrane was placed over the glu-lam panels. Structural tube railings flanking the traveled way were attached to the floor system.

Southern Yellow Pine timber commercial grade 20F-V2 having an allowable bending stress of 1450 psi was chosen for the roadway panels.

To get the top of asphalt to the required elevation, the glu-lam panels were placed on top of timber riser beams which are on the top flange of the floor beam. The panels and riser beams are attached to the floor beam top flange through the use of steel spring clips. These are L-shaped brackets which incorporate the use of springs to adjust for shrinkage or growth of the timber without loosening.
No holes were drilled into the flanges. At midspan, a transverse glu-lam stiffener beam is bolted to the underside of each panel to provide stiffness and transverse load distribution. Bituminous sealer was placed between the panels just prior to installation.
The new floor beam was chosen to be a W14X145 with an overall length of 34 ft 6 in. This beam represented an 8- in. depth reduction from the original floor beams. The cantilevered ends of the beam supporting the sidewalk are coped with a 1 1/16-in. plate welded onto the cut web.

This was done to maintain a similar appearance as the original. The floor beam is suspended from the truss by two 1-in. diam A325 steel rods at each end. The existing angles and plates used to support the hanger rods on the truss were replaced by removing the existing rivets and replacing them with high strength bolts. An elastomeric pad was added between the new floor beam and the bottom of the existing truss to accommodate the uneven surface of the lower chord and provide a uniform bearing surface.
The configuration of the proposed sidewalk was very similar to the existing. The existing 2 x 12 timber stringers and planks were replaced in-kind as calculations indicated this system was sufficient to carry modern pedestrian loading requirements. Timber shims were used to obtain the proper elevation for the top of the sidewalk.

Under the north sidewalk, the Pawtucket Water Supply Board requested that a new 12-in. diam steel water main be added across the bridge to upgrade their system. The main is supported on low-profile rollers attached to the top flange of the floor beam and situated between the timber sidewalk stringers. The new main was tied into the existing underground facilities at each approach.
An interesting feature of the existing bridge was the unique original ornamental railing system. The railing has a lattice configuration with cast iron rosettes at the intersection of the lattice bars. Many of these rosettes were missing.

Although not conventional in relation to modern railings, it was determined that the 3-ft 6-in. high railing would be rehabilitated and remain in place. The plans called for the railing to be removed and remounted after all the vertical posts at each floor beam were replaced with new 3-in. angles. The missing rosettes were replaced with new cast steel replicas. Because the railing does not meet the 4-ft 6-in. minimum height requirement for bicycles, signs were placed on each approach instructing bicyclists to walk bicycles across the bridge.

Ornamental gas lamps were originally placed at each corner of the bridge. Two of these were rehabilitated and two were damaged beyond repair and replaced with cast replicas of the original. All of the lamps were wired for electric lighting. The new bolts used in both the railing and lamp rehabilitation were selected to match the original including new threaded rivets to replace the existing rivets.

Trusses

The trusses were analyzed for the proposed single lane HS20 live loading and lighter dead load of the new floor system. Influence lines were generated and forces computed in all of the truss members. The metallurgical testing conducted previously indicated the truss material was wrought iron and had a minimum yield strength of 26,000 psi. Allowable stresses of 14,500 psi inventory and 19,500 psi operating were utilized.

The results of the truss analysis indicated that the as-built sections of the truss members were sufficient to carry the proposed loadings. The two tension diagonals at the midspan had the lowest inventory rating of 38 tons for the 36-ton HS20 design truck.

Structural inspections of the bridge revealed several areas of deterioration along the bottom chord. The most severe deterioration was found in the panels near the bearings with up to 75% of one vertical plate gone. Several other areas of pitting and section loss were found at various locations along the bottom chord.

Substructure

The existing abutments and wingwalls are of gravity stone masonry construction. A detailed inspection revealed them to be in very good condition with no signs of leaning, significant deterioration, or other signs of distress. Based on the original plans and data obtained in the field inspection, an analysis was performed on the abutments with the proposed loads from the rehabilitated truss configuration. This analysis revealed the abutments would be adequate to support the proposed rehabilitated structure.

Due to the profile raising, replacement of the deteriorated truss bearings, and the configuration of the new longitudinal glu-lam planks, it was necessary to recap the existing abutments with cast-in-place reinforced concrete. To accomplish this, raising of the trusses by jacking was needed.

The jacking scheme used was developed based on the constraints created by the railroad beneath the bridge. It was imperative that rail traffic not be interrupted for an extended period of time. In addition, clearance requirements also could not be violated during construction. For these reasons, it was not feasible to place the jacking structure below the bridge.

The final jacking method utilized jacking structures located at roadway level. First, after the existing floor system was removed, the portion of existing abutment adjacent to the truss bearings was removed. Now the new concrete cap was poured in these areas with the truss still bearing on the original stone of the abutment.

Next, jacking structures were constructed and attached to the top of the end portal diagonal. This was done by removing some of the existing rivets and attaching temporary angles and plates which in turn were connected to a jacking beam.

The jacking beam was lifted and consequently the truss raised by utilizing jacks bearing on the new abutment portions. Once lifted, the bearings were replaced and the new abutment cap under the truss bearings constructed. When this portion had sufficiently cured, the trusses were lowered, the jacking assembly removed and the rivets replaced with round head bolts.

Conclusion

The construction of the project was performed by J.H. Lynch and Sons Inc., Cumberland, R.I., and took one year with the bridge reopening to traffic in May of 1995. The total construction cost of the project was approximately $1 million or 10% under the engineer’s estimate. The design and construction was performed under the direction of Kazem Farhoumand, P.E., state bridge engineer and Robert Faraj, RIDOT project manager.

The successful completion of this project was the result of understanding the concerns of the various historical and public agencies in order to develop a rehabilitation scheme which safely accommodates the needs of the traveling public while maintaining the historical integrity of the bridge.

Complicating this was the unique structural challenges and the various requirements of the railroad below. The use of the glu-laminated deck system and innovative repairs to the trusses allowed for a cost-effective rehabilitation project. Furthermore, the aesthetic appeal of this 115-year- old historically significant truss structure was maintained and even enhanced to return its appearance to when originally built.

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