Tyler J. Barile, E.I.T. City of Philadelphia, Streets Department, Bridge Section
Audrey A. Corrado, P.E. Michael Baker International, Philadelphia, PA
Christopher J. Menna, P.E. Jacobs Engineering Group, Former City of Philadelphia, Streets Department, Bridge Section
James B Miller, P.E. Michael Baker International, Philadelphia, PA
Christopher J. Renfro, E.I.T. City of Philadelphia, Streets Department, Bridge Section
Michael J. Schickling, E.I.T. City of Philadelphia, Streets Department, Bridge Section
IBC – 08-75
Abstract: The project involved the reconstruction of a unique, three-span, shallow depth, steel stringer bridge in the Chestnut Hill West Section of Philadelphia. The structure had reached the end of its useful life, having already undergone two emergency repair projects. Innovative reconstruction concepts were advanced to satisfy historic preservation criteria and to restore the structure’s place in the neighborhood.
The original Willow Grove Avenue Bridge over Southeastern Pennsylvania Transportation Authority (SEPTA) tracks was a three-span, simply supported, shallow depth, steel stringer bridge with asphalt deck, as rehabilitated in 1962, with stone masonry abutments and wing walls that dated to 1884. The original double iron channel and timber superstructure was built by the Edge Moor Bridge Works for the Pennsylvania Railroad and designed for a live load of horse and carriage. Built during the industrial revolution, the structure was built to provide grade separation for a street crossing in a very affluent, new neighborhood in Philadelphia – a suburban setting to house railroad and industrialist corporate executives and their families. The original and existing bridges featured materials appropriate for the area – timber, metal, and Wissahickon Schist stone. Though decidedly inelegant, the first and second bridges provided a practical solution to challenging design features– a short hump crest vertical curve nestled between two driveways and incorporated into the site design and functionality of a commuter railroad train station. Originally spanning over two mainline tracks and a siding for an ice house, the current bridge spans over two electrified tracks between the piers. The bridge is contained within the Chestnut Hill Historic District, but is not a contributing element, nor is it individually eligible for listing in the National Register of Historic Places. The National Historic Preservation Act (NHPA) Section 106 review for the project resulted in a No Adverse Effect finding for the Chestnut Hill Historic District.
Inspection and Emergency Repairs
Prior to 1994, the bridge was on a regular five-year inspection cycle, per Federal Highway Administration (FHWA) standards. However, due a metallurgical flaw of the 1962 replacement steel and the constant infiltration of water and snow salts through the asphalt deck, deterioration aggressively accelerated beginning in 1995. Because of this, stabilization repairs were required, and inspection reporting was increased to a two-year inspection cycle. For the repairs, the City’s Bridge Section performed in-depth inspections to develop rehabilitation and reconstruction strategies. The major findings were as follows:
- Severely corroded steel, including the pier steel bent frames.
- Seized and malfunctioning bearings and expansion dams.
- Abutment and wingwall stonework in need of repointing and/or rebuilding.
- Moderate spalling of the drivable asphalt deck surfaces.
- Failure of the superstructure steel coating system.
- Moderate deterioration of the non-composite deck stay-in-place forms.
Due to severe steel deterioration under the sidewalk bays, elevated wood boardwalks were furnished and installed temporarily by the City’s Bridge Maintenance Unit in 2006 to bridge over weakened areas and to provide safe passage of pedestrians. Concrete median barriers were also installed at the curb lines to keep vehicles off the sidewalk. Figure 1 depicts these changes. Additionally, a truck detour was instituted around the bridge to remain in effect until full reconstruction could take place.
In 2013, the condition and section loss of the stringers, as shown in Figure 2, became so significant that another emergency repair was done. Additional adjacent “buddy beams” were furnished and installed by Buckley Construction Company to strengthen the weakened stringers. This system is shown in Figure 3. This would allow the bridge to remain in service for one more winter season without closure and impact to the commuter rail traffic below.
At this point, the structure became the primary design priority for the City’s design unit. Due to deterioration and/or section loss of up to 100% in some places, permanent bridge closure was imminent. The bridge was posted for 3 Tons/No Trucks. At the time of last inspection the structure’s sufficiency rating had reduced to 2.
The design scope included removal and replacement of the entire superstructure, substructure strengthening/adaptive reuse, roadway approach work, reconstruction of a portion of train station platform stairway, and utility work. The engineering and logistical challenges involved with replacing a severely deteriorated, weight posted, structurally deficient bridge that is also integrated into an existing SEPTA commuter rail station were apparent. Moreover, very little information by way of existing plans was available, necessitating extensive survey and substructure probing to verify project conditions. Prior to construction, Verizon, Comcast, Street Lighting, and Philadelphia Electric Company (PECO) facilities were relocated, and SEPTA Electrical Traction power cables at the west pier were detached. Work would be done in shifts, when required, and would be coordinated with SEPTA, Verizon, Philadelphia Water Department (PWD), and Philadelphia Gas Works (PGW).
Due to the uniqueness and historic setting of this bridge, the design team faced numerous challenges, apart from typical design challenges in a densely populated environment. One of the key questions that had to be answered early on was if the existing substructure could be reused for a third time. Salvaging the substructure was desirable because it would exhibit sustainability, reduce environmental impact, and lessen construction cost. Michael Baker International was contracted as part of the design team with two main substructure tasks: verify the existing substructure conditions and design an adaptive re-use.
In order to preserve the existing horizontal clearance of the rail, the existing stone masonry piers were maintained. The existing pier foundations consisted of open joint masonry stone walls founded on stepped stone footing. These existing foundations were strengthened by placing a 1’-0” thick Class C concrete collar around the perimeter to lock in the foundation and solidify the foundation for reuse, as shown in Figure 4. The existing stone masonry piers were cleaned and entirely repointed, and a new concrete cap was attached to the existing cap in order to support new HP12x84 steel columns. The concrete collar was placed over several track outages with high early strength concrete, which allowed for live rail traffic within 1’-0” of the collar placement at the end of each outage.
To stabilize the existing abutments and wing walls, existing backfill was carefully removed to the bottom of the existing walls and replaced with Class A concrete immediately behind the masonry structure to form a new gravity abutment. This concept knits the “old” structure and “new” structure together. This was done while the existing walls were monitored for excessive movement. The stone masonry of the abutments and wing walls were cleaned and new caps were provided on each abutment to serve as a beam seat. To eliminate earth pressure acting on the gravity abutment and wing walls, Class C concrete and flowable fill were placed behind the Class A concrete in lifts approximately 3’ wide and 2’ high. These abutment stabilization measures are shown in Figure 5.
After the substructure modification, stringer design began. Numerous beam design runs were performed and compared to the limited available superstructure depth envelope. The City opted against the use of plate girders in this application due to cost and fabrication issues for such shallow members. Additionally, it had committed to provide at least three more inches of vertical clearance over the electrified railroad. Still, a PUC design exception for substandard vertical railroad clearance was required.
The design scheme mimicked the existing configuration of very shallow, closely spaced stringers. The City design team would improve on the existing structure by converting the arrangement to a three span continuous structure, thereby creating a more favorable load distribution. Twelve inch deep, rolled sections were proposed, except for at the fascias, where deeper rolled sections were planned. Additional live load deflection calculations were required to gain PennDOT approval for this design scheme. The design team also had to demonstrate that cambering could be done for such a shallow rolled section, held down at four points. Cambering feasibility was verified by recognized fabricators in the region who noted that the cold cambering method could be successfully implemented in this case.
Shallow depth rolled sections could meet the existing envelope for the cartway, but left the challenge of fitting three utilities within two sidewalk bays. Further complication arose due to the fact that the north bay would feature two liquid utilities – an 8-inch PWD water main in a 16-inch casing pipe and a 4-inch PGW low-pressure gas main in an 8-inch casing pipe, as shown in Figure 6. Both utility lines would require larger diameter casings for safety and installation purposes. To satisfy the local community organizations, Section 106 consulting parties, and the Philadelphia Art Commission, the design team opted to cleverly hide the utilities within each sidewalk bay. This would require two different curb reveal depths and an atypical deck cross-section.
The deck did not extend transversely to the edge of the sidewalk, as is typical. Instead, the deck was discontinued just below the curb line. This allowed sufficient room for the utilities to sit beneath the sidewalk, while maintaining the railroad clearance below, though the north curb reveal would still have to be nearly twelve inches deep to fit the PWD and PGW mains. Though unusual, this increased curb reveal proved to be subtle and hardly noticeable. The sidewalk slab would be just six inches thick and not cast on a deck slab. This minimal thickness required special calculation to ensure that the sidewalk could support typical live loads. Two rebar mats were still required, but the position of the bars was slightly altered to accommodate all applicable covers.
To facilitate utility installation and maintenance and to avoid the issue of future sidewalk settling at the bridge corners, the approach sidewalks were designed integral with the approach slab. Unlike the deck slab, the approach slab would be full width, out to out. Strip seals would be placed on sleeper slabs at the far ends of the approach slabs to meet current PennDOT design criteria. At the east approach, the sleeper slab and strip seal would need to be adjusted to work around a new stairway foundation block designed for the SEPTA commuter train station.
A study revealed the need to improve safety at the four corners of the bridge by providing guiderail. However, guiderail is considered to be unsightly by the public, so the team searched for an appropriate design solution. All four corners featured slopes with various degrees of inclination. In order to minimize impact to the existing slope and tree root systems, the team decided to design the shallowest possible foundations to support the guiderail. This would be accomplished by utilizing moment slab with a tapered foundation. The design feature would also lower construction costs and minimize disruption to the adjacent property owners, while allowing for all work to occur within the narrow City-owned right-of-way. At the southeast corner, a thickened edge of sidewalk would be utilized to span over well-established evergreen trees. An example of this is shown in Figure 7. The design team had committed to protecting the root systems of these trees, such that all would survive during construction. Protection would come in the form of special design details and coordination with the City arborist.
The design team settled on a shallow depth, crash tested Virginia DOT barrier system that would sit atop a shallow depth parapet. This arrangement was amenable to all parties involved and would allow the bridge architect flexibility in design. The parapet would later be widened to 1’-6” to accommodate real Wissahickon stone veneer, inset on both sides. Additionally, the steel barrier was modified by the architect with decorative balusters. Neither alteration would lessen the crashworthiness of the assembly.
Several other safety improvements were incorporated into the structure, including matching the original cartway width of 28 feet, striping 10.5 foot wide lanes in lieu of 12 foot, and providing 3.5 foot-wide colored shoulders. The striping and coloring were intended as traffic calming measures to improve safety.
The project design requirements would dictate the use of a steel protective railroad barrier, in lieu of standard aluminum. Three possible designs were presented to the community and Philadelphia Art Commission, as well as the Section 106 consulting parties. The barrier chosen was modeled after the nearby railroad bridge at Springfield Avenue and would feature the name of the adjacent SEPTA station, St. Martins, on both sides. All fasteners would have decorative rivet heads, in keeping with design details of the original structure.
As mentioned previously, the moment slabs would feature real inset stone veneer, and this decorative element would be extended to the bridge parapets as well. Research was done regarding the tooling of the joints, as well as for the correct mortar mix design. The construction requirements were specified in accordance with the National Park Service Guidelines, and sample panels were provided at the community input and pre-construction stages of the project. Lastly, all future veneer and restoration of the existing stone walls would have the same mortar details.
For the approaches, extensive coordination with each property owner was required. Where tree removal was necessary, owners were given the option to have trees removed or replaced. Additionally, existing slate sidewalk could be replaced in kind or replaced with pigmented concrete. Concrete would cost less and provide greater design flexibility, especially for ADA ramps, while using slate would preserve the historic construction materials and methods.
Of course, many other historic requirements had to be met due to the bridge’s location in the Chestnut Hill Historic District. For instance, rivets were used in the existing barrier ironwork that was removed, so the barriers had to be replaced with an historic-looking bolt. The bolts chosen included rounded rivets at both ends. The actual bolt head featured a twist-off mechanism – set to go off when a certain torque was achieved by the ironworker. Historic-looking punch rivets were also used for the handrail; when hammered into place, the rivet head went smooth and expanded the bolt to secure the railing.
The project also included many interesting construction challenges which the City’ Construction Unit and the contractor, Loftus Construction Company, worked through together. One of the biggest challenges was constructing a bridge over an active operating railroad. Though only the middle span was over active tracks, the bridge’s entire footprint had overlapping Right-of-Way with SEPTA. Much of the work over the tracks could only occur during track and power outages, which could occur at night and last just three to four hours on average. Many outages and night crews would be needed to get through the demolition and deck reconstruction phases of the project.
Because the bridge was in very poor condition, contract documents included weight restrictions and equipment placement limitations. This challenged the contractor to devise creative means and methods for construction, mainly utilizing small equipment.
Abutment wall stabilization proved to be a challenge in the field due to the variability in shape of the existing stone. The doweling pattern, location, and number specified in the plans had to be modified to maintain the integrity of the existing masonry wall. Therefore, holes were placed at existing gaps between the dry-stacked masonry, trying to match the specified number and location of dowels as closely as possible.
Due to the shallow interior beams and presence of utilities in the fascia bays, standard diaphragms could not be utilized. Therefore, the utility supports functioned as braces between the girders. However, the location of the utility supports fell on the bottom portion of the fascia girder, which left the top portion of the fascia girder unbraced. This caused stability issues for the fascia girder during construction due to the exterior overhang support system.
To prevent excessive overturning force on the exterior girder, WT sections were bolted to the top of the interior girder at regular intervals. Double angles were then extended between the WT section and the exterior girder to provide sufficient bracing to allow construction operations, as shown in Figure 8.
The profile of the PWD water main would become an issue where the end bridge span transitioned to the approach slab. In particular, the steep roadway grades for the crest curve exceeded the standard limitations of standard pipe and pipe casing deflection joints. In this instance, custom length pipe sections were needed to meet the main profile.
The change in the proposed vertical profile of the bridge affected the private driveways at each end of the bridge. These already steep driveways were subject to increases of over 12 inches in some cases. Therefore, each driveway required a small sag curve for adjustment purposes. Additionally, portions of original Belgian block drainage swales were rebuilt on either side of the driveway.
Though the rehabilitation of the Willow Grove Avenue Bridge over SEPTA was complex and challenging in both design and construction, the project was a sound investment in this Chestnut Hill Philadelphia neighborhood for many reasons. The project appropriately restored a bridge, fitting it properly to the area’s historical context, and set the standard for future projects in historic areas. This exercise was lauded as an excellent example of context sensitive design by the community and critics alike. The most direct route through the neighborhood was restored, improving the flow of traffic and accessibility for EMS vehicles. The extensive use of in-house engineering excellence and consultant support services helped restore grandeur and functionality to the very deserving structure shown in its completion in Figure 9 (Track Level) and Figure 10 (Street Level).
IBC – 08-75
City of Philadelphia Streets Department Surveys, Design, and Construction Unit/Bridge Section
Darin Gatti, P.E – Chief Engineer
William Gural, P.E. – Chief Construction Engineer
Vadim Fleysh, P.E. – Chief Design Engineer
Christopher Menna, P.E. – Engineer of Design
Dhanya Jacob – Project Engineer
Timothy Dragan, E.I.T. – Project Engineer
Tyler Barile, E.I.T. – Resident Inspector
Richele Dillard – Lead Draftsperson/Designer
KSK, Philadelphia, PA – Bridge Architecture
Michael Baker International, Philadelphia, PA – Substructure
SYSTRA, Philadelphia, PA – ET Design
AD Marble, Conshohocken, PA and CH Planning, Philadelphia, PA – Cultural Resources Support
AECOM, Philadelphia, PA – Contract Management Support
Jacobs Engineering Group, Philadelphia, PA – Construction Support
Loftus Construction Co., Cinnaminson, NJ