John F. Kennedy International Airport (JFK) is a primary gateway to densely populated New York City for people from all over the world. To improve access to the airport and relieve traffic congestion in the city, the airport’s operator, the Port Authority of New York and New Jersey (Port Authority), undertook the design and construction of Airtrain JFK, a nearly 13-km (8-mile) long elevated, dual-track light-rail system that ties the airport’s central terminal area to major transportation systems, remote parking lots and car rental facilities.

The Port Authority retained PB as its general engineering consultant for the design-build-operate- maintain (DBOM) program used to procure Airtrain JFK. We provided a wide range of planning, preliminary design, environmental analysis, construction management support, and procurement support activities; and engineering support to the Port Authority staff in reviewing the DBOM contractor’s design. PB continues to support Port Authority staff members at a program office.

Figure 1: AirTrain Alignment

Principal Features of the Elevated Guideway

Airtrain JFK comprises three branches (Figure 1):

• A 5.0 km (3 mile) -long line extending west from JFK to a car rental area at Federal Circle then south to remote parking and on to the Howard Beach Station of New York City’s subway system.
• A 4.8-km (2.9-mile) line extending west from the airport property to Jamaica Station, Queens, a major transfer station to New York City’s subway system, and the Long Island Rail Road, a heavily traveled regional commuter rail line.
• A 2.9-km (1.8-mile) loop within JFK’s central terminal area.

Performance Specifications. The light rail system envisioned during preliminary design was a continuous aerial guideway utilizing direct fixation track work supported on concrete or composite steel box girders. When the performance specifications were being prepared, it was determined that no single existing code or standard applied to such a structure, so the American Association of State Transportation Officials (AASHTO) Specifications for Highway Bridges and its applicable guide specifications were used with modifications to incorporate light rail loads and effects, including:

• Light rail vehicle weight and impact factors
• Centrifugal force
• Rolling force
• Longitudinal braking and traction force
• Rail/structure interaction force

Such a system required analysis for interaction between the rail and structure for the effects of:

• The structure expanding and contracting beneath the rail
• One rail breaking
• The structure restraining the rail from displacing radially on horizontal curves.

The standard AASHTO loading combinations were used with rolling force added to the loading combinations with live load, and rail/structure interaction forces added to the loading combinations with thermal forces. Loading combinations for service load design and load factor design were included in the project design criteria.

Seismic Considerations. New structures at JFK have been designed for seismic forces since 1987, and seismic provisions were an important aspect of the design criteria. Due to the presence of deep loose sands at JFK, soil borings indicated the potential for liquefaction up to a depth of 6 m (20 feet) under a design seismic event that had a peak rock acceleration of 0.15 g.

In addition to calling for the conventional design for forces and displacements, the seismic design criteria also required that additional limitations be met by the foundations and superstructure in order to allow the system to return to operation shortly after a seismic event. These limitations avoided misalignment of the guideway and limited repairs to only the track work.

Design criteria for the foundations required the DBOM contractor to take additional borings, prepare a geotechnical report and select a foundation system. Sufficient borings were given in the request for proposal (RFP) to allow preparation of bids; however, a contingency fund was established to address unknown soil condition or utility interferences risks.

Figure 2: Type 1 Box Section

Figure 3: Type II Box Section

Figure 4: Multiple Span Continuous Units

Figure 5: Completed Structure

Figure 6: Traffic was maintained during construction

Figure 7: Tight curvature called for balanced cantilever construction

Preliminary Design

To ensure that Airtrain JFK was procured under a competitive process and the successful bid was based on an awareness of key site constraints, the preliminary design of the elevated guideway was based on a modularized twin- or single-box concrete type girder that demonstrated the need to erect the structure with limited disturbance to the surrounding existing infrastructure facilities. The RFP required that the final design be selected and completed by the successful bidder. A composite steel box with a composite reinforced concrete deck slab was included as an alternative in the RFP.

For the concrete option, overhead launching trusses in the form of erection gantries were envisioned for installing individual full-length girder sections. The trusses would use gantry supports placed on top of the previously-constructed pier caps to launch the girders onto their proper positions, and they would be moved along with the gantry supports hopping from pier to pier.

Features of the DBOM Final Design

The two main differences between the DBOM final design and the preliminary design in the RFP were that the guideway superstructure was precast segmental construction that utilized seismic isolation. The superstructure is comprised of two typical cross-sections-Type I and II Box Sections that support a single or a dual-track configuration respectively (Figures 2 and 3). Typically, individual spans were longer in the DBOM design and post-tensioning tendons were applied across span closures to create multiple-span continuous units (Figure 4). The longer spans and continuous units between expansion joints made it more challenging to control the possible gaps that could result from a rail break. The DBOM car supplier had to accept the anticipated rail break gap or, as an alternative, the contractor had to provide a rail-break detection system. Figure 5 shows a general view of the completed structure and the design vehicles.

The majority of the guideway was built span by span with cranes and erection trusses so that traffic on adjacent streets and highways could be maintained (Figure 6). Some portions that are on tight curvature or have longer spans were built in balanced cantilever (Figure 7). The substructure design was often required prior to the superstructure design, so prudent design assumptions had to be made by the substructure designers and subsequently verified as the superstructure design was finalized. For example, on highly curved structures, creep and shrinkage redistribution of support reactions on the piers occurred transversely between two bearings at the same line of support, as well as longitudinally between piers.

Seismic isolation was achieved by using lead-rubber bearings that allow the superstructure to “float” during a seismic event. For non-seismic loading, however, the bearing must be fixedlaterally relative to the track centerline with movement limited to a 3-mm (1/8-inch) range. The DBOM contractor developed an elastic restraint system that would withstand non-seismic loads with an appropriate factor of safety, but would fail at design level seismic loads, thus freeing the structure to float and avoid potential damaging seismic forces. For this reason, correct modeling of the guideway structures and an especially high degree of accuracy in the structural analyses were important.

Technical Review Issues

The technical review of the DBOM contractor’s structural design revealed several areas of concern, including those discussed here.

External Tendons. In the proposed use of external tendons in the span-by-span constructed portions of the guideway, it was understood that large deflections and joint openings were expected before the ultimate strength of any girder section was reached; however, the ultimate strength of the superstructure was based upon a crushing failure of the concrete at the segmental joint. This is a non-ductile failure mode. Given the fact that the width of the compression flanges is significantly less for the guideway superstructure than that of a regular highway superstructure, the Port Authority team felt that it was prudent to require the DBOM contractor’s designers to perform a strain compatibility analysis.

The Effects of Post-tensioning and Secondary Forces on a Highly Curved Structure. An independent three-dimensional analysis of stresses and bearing reactions was conducted for highly curved portions of the aerial guideway that were to be constructed using the span-by-span and balanced cantilever methods. The analyses, which accounted for locked-in construction forces and reactions, creep, shrinkage and elastic shortening, confirmed that highly unbalanced and even uplift forces could result at those bearings supporting highly curved superstructure if construction procedures were not properly planned and executed.

The Application of the AASHTO Guide Specifications for Seismic Isolation Design. The DBOM contractor selected a complex multi-mode analysis including combined bearing-pier- foundation system damping values for the guideway system. The independent verification of this was a significant issue. The Port Authority team developed an independent seismic analysis for selected critical spans and piers that used the more simplified single mode spectral method to check the damping values and parameters. Additional complexity was added to all of the models because the continuous rail joined several multi-span units together.

Conclusions

The DBOM contractor’s approach was a valid option for a complex urban project such as Airtrain JFK. Lessons learned from the Port Authority’s perspective may be summarized as follows:

• The owner’s role is vital in overall project management, construction management oversight, and project coordination with all affected agencies, tenants and communities.

• The submittal approval process customary to U.S. public works projects must be modified for DBOM contracts to accommodate the sequence and pace of the design and construction process.
  – The owner’s engineers and the DBOM contractor’s designers should be prepared to meet frequently to resolve questions.
  – Independent analyses can be an expeditious way to identify and resolve technical differences between the DBOM contractor’s designers and the owner’s engineers.
  – Up-keep and maintenance of design documentation for reference must be well planned.
• The obligation to conform to project criteria and the prerogative to elect means and methods in design and construction must be left with the DBOM contractor and its designers.
• The DBOM contractor must be required to coordinate the work performed by its designers and their subconsultants and subcontractors.

Acknowledgements

A number of consulting firms and individuals were involved in the planning, design and construction of this project. The focus of this article is on the structural aspects of the guideway; therefore, so too are the acknowledgements. The preliminary design team was led by Zuss Izakson, a senior supervising structural engineer for PB. Joseph Kelly, senior consulting engineer for the Port Authority, supervised the preparation of structural design criteria.