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The new bridge alignment improves the historic bridges roadway geometry.

The Crane Road Bridge is comprised of two bridges, both carrying the Bronx River Parkway through the Bronx River Parkway Reservation in the Village of Scarsdale and the Town of Greenburgh in Westchester County, N.Y. Nicknamed the Mushroom Bridge, the southern bridge is a multi-span concrete bridge that crosses the Bronx River, while the northern bridge, referred to as the MNR Bridge, is a single-span steel through-girder bridge that crosses the two-track Metro-North Railroad Harlem Line.

Constructed in 1925, both bridges have undergone numerous rehabilitation programs. However, in recent years, the Crane Road Bridge has become increasingly deteriorated. In fact, two separate incidents involving holes punched completely through the concrete deck resulted in emergency closures and repairs. But with its aesthetic and historic importance to the community, simply tearing down the old bridge and building a new one was not a viable option.

Aging structures, safety concerns

An in-depth bridge inspection revealed the concrete deck slab of the Mushroom Bridge was in a state of advanced deterioration: The reinforcing steel exposed through the spalled concrete had advanced section loss, and the concrete slab condition around the holes was completely pulverized. The overall condition of the deck was rated poor, and quickly approaching severe. In addition, the concrete brackets were in poor condition as seen by the numerous cracks and spalls in the brackets.

Based on field observations and inspection, the MNR Bridge was in fair condition overall. A concern with this bridge, however, was that its two main girders were non-redundant elements; the failure of one girder would lead to a complete collapse of the entire bridge. Modern bridges are designed for redundancy to reduce the likelihood of collapse, so this two-girder system is considered highly vulnerable.

In addition to the deteriorated condition of the two structures, traffic safety along the Bronx River Parkway within the project limits continued to be a major concern. Within the project limits, narrow travel lanes in conjunction with minimum shoulders, sharp curves, and limited sight distance created conditions where the rate of highway accidents was more than six times the statewide average. As a result, the Westchester County Department of Public Works and Transportation (WCDPWT) hired New York City-based firm Stantec to inspect, repair, and design replacement bridges for the aging structures.

Mueser Rutledge Consulting Engineers provided monitoring and geotechnical engineering services. Malcolm Pirnie (now Arcadis) was engaged with hydraulic analyses and environmental impacts. RK Hite & Co. provided right-of-way and land acquisition assistance, and Mary Delaney Krugman Associates was the historic preservation consultant for the project. The contractor was EE Cruz, and LKB Inc. provided construction inspection services.

The original structures uniquely shaped piers gave it its Mushroom Bridge nickname.
Respecting historic character

From the outset, WCDPWT and the other agencies involved in funding and approving this project recognized the unique historic and environmental character of the Crane Road Bridge. After an exhaustive public screening process and inter-agency coordination, Westchester County selected a replacement alternative: building a new bridge that replicates the existing Mushroom Bridge structure.

This alternative would provide wider replacement structures immediately south of the existing Crane Road Bridges. Construction would be phased to maintain traffic on the existing structures throughout construction of the replacement structures. The new alignment of the replacement structures would allow for an improvement in roadway geometry.

Proposed modifications included:

  • Replacement of the Mushroom Bridge superstructure and substructure with a substantially wider deck.
  • Replacement of the MNR Bridge with a redundant-type structure (composite prestressed concrete box beam bridge).
  • Replacement of the existing Mushroom Bridge piers to retain the form of the historic pier configuration, while relocating them along a similar alignment. The Mushroom Bridge replacement superstructure and abutments would retain a replicated architectural treatment.
  • A construction cost of approximately $39 million; construction would take approximately three years.
The partially erected Mushroom Bridge, showing steel core and steel brackets.
Constructability, redundancy, safety

Replacement structures would greatly improve the travel conditions for this stretch of roadway by increasing clearances and widths. Stopping sight distances and Level of Service ratings would also improve.

However, achieving these vast improvements was no easy task. Spanning the Bronx River, several piers are located inside of the river, complicating design, construction, and approvals. Also crossing the river is a sanitary sewer pipe, constructed in the early 1900s, which could not be rerouted. As a result, several pier footings were affected. In fact, two of the six spread footings were designed to span over the top of the sewer pipe.

While the bridge was to be widened, the number and size of the piers had to remain more or less the same due to river hydraulic concerns. This presented obstacles from a structural engineering standpoint. Each existing mushroom superstructure panel is approximately 40 feet by 40 feet, or 1,600 square feet. The proposed configuration for each panel is approximately 60 feet by 60 feet, or 3,600 square feet, so each panel has more than doubled in size. Therefore, the loads that each bracket and pier needs to carry have been increased substantially.

Design codes have also changed since the original bridge was built, requiring that the replacement structure be designed to carry much heavier traffic loads than the original design. Additionally, each bracket has been made significantly longer in order to reach the ends of the enlarged panel. Deflections at the tips of these brackets played a large role in the design of the structure.

Constructability was also a key concern. The existing bridge was made of reinforced concrete. The center of the panel, directly over the piers, was heavily reinforced as reinforcing bars from each bracket converge. Since the new brackets are longer and carry more load, using reinforced concrete brackets proved to be impossible due to rebar congestion. Eventually, an innovative steel option was used in which an octagonal core made up of steel plates is fastened to the concrete pier. Steel brackets protrude from each face of the core and are encased in concrete to maintain the aesthetic of the existing bridge.

The bridges concrete-encased brackets and core.

In order to provide for a redundant structure, the bridge was designed to remain in service even after complete failure of one bracket per panel. Additionally, the superstructure was designed so that each panel could behave independently, gaining no necessary support from an adjacent panel. Tying the panels together, while helpful in regard to gravity loads, would create temperature stresses that designers were keen to avoid. However, the differential deflection between panels must be avoided in an effort to reduce cracking between panels. This balance was achieved via stainless steel bars that protrude from one panel into sleeves cast into the adjacent panel.

Further complicating matters, the site includes a 48-inch water main and a 54-inch sanitary trunk sewer that were constructed more than 100 years ago. The south abutment and several intermediate piers were constructed over these facilities. The water main was reinforced with fiber wrap and joint clamps prior to construction, while the sewer was reinforced via a fiberglass pipe liner. The pier spread footings were designed to span over the top of the utility, via steel grillage beams embedded in the concrete footings. Realignment of the utilities and of the proposed bridge was studied but proved to be infeasible.

Overcoming these various obstacles, the project team designed a replacement structure that vastly improves the area. Geometric changes incorporated into the replacement structures make the bridge significantly safer. By increasing the overall bridge width, all four travel lanes and shoulders are wider. Acceleration/deceleration lanes, previously virtually non-existent, have been increased. The curved alignment has also been softened to improve sight distances, and a roadway that merges onto the Bronx River Parkway was raised so that drivers can easily see one another and merge safely. Pedestrians, too, can now safely use the bridges, with sidewalks and a series of pedestrian ramps added to provide access to the Bronx River Reservation and the MNR Bridge platform.

In addition to mimicking the existing structure to maintain the overall aesthetic and historic character, the existing stone facing was salvaged and reused. Supplemented with new architectural stone, the new bridge is an impressive element to the Bronx River Reservation for users to enjoy.

Christian Wiederholz, P.E., is an associate and structural engineer based in Stantecs (stantec.com) Rochelle Park, N.J., office.

Juno Garcia, P.E., is a civil engineer in Stantec’s New York City office.