By William Beining, P.E., Anthony Ream, P.E., and Patrick Hassett
In replacing the historic Greenfield Bridge, which links the Pittsburgh neighborhood of Greenfield to Schenley Park, the bridge's designers were challenged to re-create a landmark structure while minimizing the impacts to the congested interstate running under the bridge. From demolition of the existing bridge to design and erection of the new steel arch, the obstacles were overcome through careful planning, extraordinary community involvement, and innovative design.
A BRIDGE HAS CONNECTED the Pittsburgh neighborhood of Greenfield to historic Schenley Park for nearly 120 years. The first structure to span the valley was a simple 15-span wooden viaduct that crossed a neighborhood street and stream. In 1922, this utilitarian wooden viaduct was replaced with a monumental concrete arch, named the Beechwood Boulevard Bridge (locally known as the Greenfield Bridge) that reflected the city's growth and the grandeur of the time. The bridge featured decorative urns, architectural pedestals, and lighting fixtures along its length, creating a grand promenade.
Thirty years later, to accommodate the increasing amount of vehicular traffic coming into the city from the east, Interstate 376 was constructed under the bridge to connect downtown to the eastern suburbs; it is now the main artery linking downtown Pittsburgh to all points east.
By the early 2000s, the historic concrete arch that had stood as an eastern gateway for so many decades was showing its age. Rehabilitation efforts had stripped some of the bridge's grandeur; many of the original decorative elements were removed during a much-needed floor system replacement in the 1980s. Because of the deterioration of the concrete arch ribs, much of the arch was wrapped with protective netting. A "bridge under the bridge" that had been designed in the 1990s for a planned rehabilitation was also left in place. By 2012, it was clear that it was time for a new structure; however, with this decision came many challenges.
The first of these was determining how the existing concrete arch could be safely and efficiently demolished. This meant not only closing the bridge but also disrupting traffic on the underlying interstate, which is one of the busiest stretches of roadway in Western Pennsylvania.
A second challenge was building a new bridge that would reflect the historic and cultural nature of the site. This meant reinforcing the gateway nature of the structure as viewed from below while also recapturing the feel of a grand entrance to Schenley Park above.
Finally, while a design that captured the historic nature of the site was desired, it was also imperative that the new bridge incorporate state-of-the-art design so that it would last well into the 21st century and beyond. Unique design elements make the structure both context sensitive and state of the art.
Public meetings were held several times throughout the design phase to share ideas, gather feedback, and maintain an open line of communication among the community and stakeholders. The first was in January 2013, with the intent of introducing the project and project team to the community, presenting options for the typical section, and sharing other early concepts for the new bridge. The proposed deck section took into consideration the needs of vehicular and pedestrian traffic, and the bridge's footprint was widened slightly to 51 ft to include bike lanes and a 10 ft extra-wide sidewalk.
Also incorporated into the design at this stage were some of the elements of the existing structure, namely the architectural pillars and stone urns. The pillars, which acted as an entrance to the 1920s-built bridge, were reset on specially designed pedestals attached to the new bridge abutments. Likewise, the stone urns were salvaged and reset at prominent locations on top of the barrier. Similar to the previous bridge, a decorative bush hammer finish lines both faces of the bridge barrier. The pedestals, as well as portions of the new piers, have the same bush hammer finish, tying the new substructure to the decorative elements of the superstructure. In addition, a decorative fence and lighting were incorporated into the design.
Also at this initial meeting the design team presented three color options: a contextual blue or green, an infrastructure gray or black, or an iconic mix of colors such as black and gold. The community voted for contextual green.
A second public meeting was held in February 2015. With the construction of the new bridge quickly approaching, this meeting allowed attendees to see specific details of the new structure, with renderings of the arch and associated design elements on display. These elements included not just the architectural details but also greatly improved pedestrian and bicycle facilities at each end of the bridge, allowing safer and more easily recognizable routes for all types of users to access the bridge and the park. As a part of this improvement, dedicated bicycle and pedestrian paths were clearly marked at the intersection of Greenfield Road and Pocusset Street at the north approach, with bicycles having unobstructed access to the newly created bicycle path along Pocusset Street.
This meeting also focused on construction aspects of the new bridge, alerting the public to the anticipated construction schedule and the associated detours. While detours are met with displeasure on any project, sharing the schedule and potential impacts with the public helped lessen the anxiety and confusion that can occur on a project of this type and magnitude.
The city's Art Commission, which has purview over the introduction of nonstandard artistic elements into the public right of way, also had an interest in the bridge enhancements and historical accommodations. The city and project team's presentation to the commission was well received. The bridge and its design were approved.
CONTRACTORS AND CITY, county, state, and federal agencies met to assess safety considerations as well as the means and methods for disposing of the existing bridge, primarily whether it should be removed by full deconstruction or explosive demolition. The two-day meeting also included a visit to the site and a review of the structural condition of the existing bridge and its urban context.
Full deconstruction of the bridge was complicated by the presence of the interstate passing underneath it and the related costs and limitations of closing the highway for any extended period of time. Thus, given the structure type, the convenience of access below the bridge for debris removal, the somewhat sparse and topographical landscape at either end of the bridge, and the efficiency of imploding, the decision to implode the bridge was made. However, the cost of closing the interstate and the desire to reinstitute the remaining architectural features from the existing bridge led to the consideration of partial deconstruction coupled with implosion. Mosites Construction Company in Pittsburgh was awarded the contract, and its first order of business was to detail how best to proceed with a combination of deconstruction and implosion.
The Pennsylvania Department of Transportation (PennDOT) set a 5-day closure period within a 10-day window, beginning December 25, 2015, and ending at 6 a.m. January 4, 2016, for the implosion. Within the 10 days, the contractor could select any five consecutive days to close the interstate, depending on weather and other factors. Additionally, within these five days, the contractor had to finalize bridge prepping over the travel lanes, including weakening structural members, setting charges, and wrapping points of detonation; place a 5 to 8 ft thick mat of timber, gravel, and dirt on the interstate to "cushion" the falling debris; implode the bridge; dispose, recycle, and/or store the imploded mass; and reopen the interstate to traffic. If the interstate remained closed beyond the five days, the contractor would be assessed significant roadway-user liquidated damages. Two additional weekend interstate closures in advance of the December implosion were included in the schedule. The purpose of these closures was to strip off as much of the bridge over the interstate as possible, thereby expediting explosive charge placement and reducing the risk of damage to the roadway below from falling concrete and steel. By the end of the second closure, only a skeleton of the bridge remained over I-376.
Closing the interstate also inconvenienced local commuters who had to detour around the closed highway. Adjacent city neighborhoods and business districts were directly affected by detoured traffic. In addition, public safety was a concern with the implosion itself, as 5,000-8,000 residents live near the bridge and 2,000-3,000 spectators were coming to witness the implosion. Thus, an extensive public awareness and community engagement campaign was developed to ensure a safe implosion—including preventing any harm to private property and people—and to provide safe and clear detour provisions for the traveling public and a safe and attractive overlook for spectators.
The implosion required the establishment and enforcement of a 1,000 ft exclusion zone. Residents within the zone had to remain indoors, and spectators had to refrain from entering the zone. Drones were deployed to assist in monitoring the exclusion zone for intruders, providing the demolition contractor a higher level of confidence in detonating the implosion.
The interstate was shut down on December 26, and implosion, which was witnessed by thousands of people, occurred two days later, on December 28. Cleanup began immediately with an assortment of heavy equipment and a caravan of tri-axles hauling the concrete and steel around the clock to distant landfills, local recycling facilities, and scrapyards. The interstate was reopened well within the five-day window, and the process of building a new structure began.
THE STAKEHOLDERS involved in the replacement of the Greenfield Bridge and a design team from HDR strove to create a context-sensitive design that reflected the unusual nature of the site and the wishes of the community. The design team determined early in the design process that an arch would meet the structural needs of the site and maintain the gateway appearance of the previous bridge.
Throughout the design of the steel superstructure, careful consideration was given to elements that would reduce the impacts on the traveling public during construction as well as eliminate the need for costly fracture-critical inspections in the future. Once an arch was selected for the structure type, steel was chosen to achieve the former, and detailed design and analysis were necessary to achieve the latter.
Any disruption to the bridge's operation would require extensive coordination and have potential ripple effects on traffic throughout the region. Minimizing the required closure time mitigated these impacts. Furthermore, using a steel arch minimized the number of elements to lift and connect over a closed interstate. Both arch ribs were composed of three field pieces. The contractor connected two northern pieces on the ground and lifted them into place using falsework towers that supported the free ends. This portion was accomplished adjacent to I-376 under live traffic. As a result, only one piece per arch rib needed to be lifted during the weekend road closure.
The original design assumed two full weekend closures of I-376 to complete the steel erection. Modifications to the erection method, above, reduced the number of full weekend closures to one. One modification was the method for maintaining the steel dead load camber of the arch during erection. The original design plans specified grouting and posttensioning of the fixed ends of the arch ribs after their closure while still under temporary support to maintain the proper arch geometry and reaction forces at the skewbacks, the substructure elements that support the arch ribs.
This necessitated performing the grouting during the period between consecutive weekend closures. To remove the grouting and posttensioning from the critical path, the contractor used base leveling nuts to adjust the geometry and induce temporary forces into the fixed connection. This method enabled the contractor to erect the entire steel structure during one weekend closure before grouting and still obtain the cambered shape and fixed-base reactions specified in the design plans.
Additional detailing choices were made during design to simplify and accelerate the erection. These included wider spacing of spandrel columns along the structure to eliminate the number of elements and using a vierendeel system to brace the arch ribs. The bracing system consisted solely of transverse struts connecting adjacent ribs. While this system increased the size of transverse struts, it eliminated all diagonal bracing and end connections associated with a conventional truss-style bracing system. This design choice significantly reduced the number of crane picks required over the interstate. Additionally, the transverse struts, which typically occur at the spandrel locations, were offset 4 ft along the rib centerline from the spandrel connection. This allowed for simple splice-plate connections for flanges and clip-angle connections for the web.
Connecting the struts at the spandrel locations would have required complex welded connection shapes and/or multiple splice-plate plies that would have complicated the erection and reduced flexibility for making the connections. The eccentricity of the strut connection from the spandrel column was considered in the design forces for the arch ribs.
Finally, steel floor beams spaced widely apart are generally considered to be fracture-critical elements requiring special material testing and fabrication in addition to costly periodic hands-on inspections of the elements. While a hybrid floor system consisting of concrete floor beams and potentially stringers would have eliminated the fracture-critical elements, a steel floor system is lightweight and reduces erection times. The steel floor system did not require time-consuming closure pours or posttensioning.
While the erection method shown in the design plans called for a stick-built floor system (installing each floor beam followed by single stringer picks), the contractor proposed building modular sections of the floor system on the ground, composed of multiple floor beams and stringer spans, and lifting them into place in a single pick. As an additional benefit, this method allowed the contractor to safely install the overhang brackets on the exterior stringers before lifting, reducing the number of future overnight closures to complete these operations.
To eliminate the fracture-critical concerns associated with the steel floor beams, redundancy design of the floor system was performed in tandem with detailed 3-D finite-element analyses (FEAs) of the overall structure, including dynamic fracture effects to ensure that the structure could resist load and continue to perform if a fracture-critical element (floor beam) did indeed fracture. These calculations required a minimal upsizing of the floor system elements in comparison to the overall steel weight of the entire structure. A benefit of this upsizing was the increased capacity that allowed the contractor to lift the floor system in modular units. While the floor beams were still required to be fabricated as fracture-critical members (FCMs), the analysis method allowed them to be termed system redundant members (SRMs) per Federal Highway Administration (FHWA) guidelines, eliminating the need for any in-depth inspections of FCMs over the life of the structure.
The floor beams are typically nonredundant members, or FCMs, requiring special fabrication and inspection. A June 20, 2012, memo from the FHWA, using the definition provided in the American Association of State Highway and Transportation Officials' LRFD Bridge Design Specifications, sixth edition, defines an FCM as a "component in tension whose failure is expected to result in the collapse of the bridge or the inability of the bridge to perform its function." The memo also defines an SRM as one "which is a non-load-path-redundant member that gains its redundancy by system behavior." Or stated differently, a member is an SRM if the remaining structure has the ability to transfer and carry the forces typically carried by the FCM. Per the FHWA memo, SRMs are not considered to be FCMs for in-service inspection protocols.
To label a traditional FCM as an SRM, the FHWA memo requires a refined analysis. For the Greenfield Bridge, this analysis was performed in accordance with the FHWA memo and the specifications in PennDOT Design Manual Part 4, April 2015 edition. There are six floor beams located over the steel arch with seven stringers that frame between the floor beams. Four floor beams are rigidly bolted to the spandrel columns while the two floor beams at the crown of the arch are attached to the spandrel columns with bearings. There are some variations in the structure about its longitudinal and transverse centerlines, but for analysis purposes, the structure is essentially symmetric in both directions, requiring separate redundancy analyses for the three distinct floor beam members.
During the initial sizing of the floor system for strength and service loads, the depths of the stringers and floor beams were matched to allow for robust simple connections, which could carry significant forces after the loss of a floor beam. This detailing required a modest amount of extra steel, but the amount was minimal compared with the savings in simplified detailing and the elimination of future FCM inspections. The top figure on page 71 shows the connection with a section of the floor beam and stiffeners in yellow, the stringer elevation in orange, and splice plates in blue. The strength and service designs of these connections were based on 2-D line-girder analyses required by PennDOT using its proprietary software.
Based on the geometry and initial design of the FCM elements (floor beams), the redundancy analysis took a two-step approach. The first step was a series of 2-D line-girder analyses for the stringers and floor beams assuming the complete loss of one floor beam with similar methods as the standard design. The second was a series of 3-D FEAs with full loss of the steel floor beam section at both the maximum negative and positive moment locations for each of the three distinct floor beam types (yielding six total analyses). The fracture locations are shown in the middle figure on page 71.
Knowing that the 2-D floor system results were generally much higher than the 3-D results, no increase for the dynamic effects of the fracture event was included. Member capacities were the same as those used for the strength design, which resulted in an elastic response during the fractured state.
With the 2-D analyses complete and floor system and connections sized appropriately, the second step of 3-D FEAs was used to verify the floor system forces from the 2-D analysis and check the effects of a floor beam fracture on the diaphragms, spandrel columns, arch ribs, and struts. The analysis model was based on the 3-D geometric, nonlinear, staged-construction model used for the strength and service design. A conservative dynamic allowance value was used to account for inertial effects during a fracture event.
The diaphragm forces exceeded the strength forces in some locations near the fracture, but they were still below capacity. For the arch members, the 3-D results were generally below the strength design forces. Where they were not, they were still below member capacity. The only exception was the transverse bending of the spandrel columns at the location of a floor beam that fractured in negative moment, specifically at its bolted connection to the floor beam. This is because the fracture requires the spandrel column to resist a large portion of the overhang load without continuity to the opposite spandrel column, as shown in the bottom figure on page 71, where the spandrel column is in blue and the resultant of the overhang load is the green arrow. As a result, using a conservative dynamic allowance factor would cause the forces from the fracture event to control the design. Therefore, a full dynamic time history analysis was performed for these controlling cases to determine a more realistic dynamic allowance factor. The resulting refined dynamic allowance factor lowered the connection forces below the strength design capacity.
The method of redundancy analysis described for this structure is not applicable to all structure types. For this type of structure, the fracture-critical floor beam elements are primary members that affect the ability of the bridge to perform its function; however, the floor beams are not the main supporting members of the overall structure. As such, the fracture involves a minimal amount of structure dead load and inertia. Additionally, there is a clear, definable load path of how the load in the fractured member is bypassed and supported. Finally, the floor system and entire structure can be detailed to remain elastic with relative ease, reducing the required intensity of 3-D modeling. However, for structures such as twin girders or tied arches, where the FCM elements are major supporting members, whose fracture involves a significant amount of energy, load redistribution, and inelastic behavior, models must be of a higher order with more refined meshing, including explicit modeling of secondary elements and complex material properties.
Construction of the new Greenfield Bridge began in January 2016, and the bridge was reopened to the public in the fall of 2017. The new bridge embodies the grandeur reminiscent of the preceding structure, while incorporating state-of-the-art elements that ensure the bridge will last for many years to come.
William Beining, P.E., is a project manager and Anthony Ream, P.E., is a senior professional associate at HDR , both working in the firm's Weirton, West Virginia, office. Patrick Hassett is assistant director for the city of Pittsburgh's Department of Mobility and Infrastructure. This article is based on a paper originally presented at the 34th annual International Bridge Conference, sponsored by the Engineers' Society of Western Pennsylvania and held at National Harbor in Oxon Hill, Maryland, in June 2017. This year's conference will be held June 11-14 at the same venue.
Owner: City of Pittsburgh
Designer: HDR, Omaha, Nebraska
Contractor: Mosites Construction Company, Pittsburgh
Erector: Amelie Construction & Supply, Pittsburgh
Fabricator: High Steel Structures LLC, Lancaster, Pennsylvania
Construction inspection: Michael Baker International, Pittsburgh