9. Design and Construction Considerations for a Highway Cap
This book is intended as a reference and guide for proponents of freeway caps and parks. In that context, this chapter endeavors to provide an overview of the entire process of building in highway airspace. Detailed information about freeway cap construction is beyond the scope of this book. More detailed information on cap construction are covered under other broader subjects such as bridge construction. The FHWA promotes the use of the most advanced, safest, fastest, and least expensive construction methods nationwide. The FHWA maintains an online library of detailed manuals concerning bridge construction techniques. Nevertheless, there are a few construction issues worth mentioning here with regard to highway airspace construction.
For a general idea of what freeway cap construction looks like, time lapse videos of the construction of Capitol Crossing and Klyde-Warren cap structures and support systems can be found on YouTube at:
https://www.youtube.com/watch?v=1IYUoj8UMog (Capitol Crossing)
https://www.youtube.com/watch?v=hUX0EwSPDJY (Klyde Warren)
The project management structure should be established early on, well before the entitlement process. It can be generalized as one of the following three structures: (1) Lead agency; (2) Joint powers authority; (3) Public-private partnership (P3); or (4) Private developer. Generally speaking, if the project has strong local support, it will likely move faster with the municipality as the lead agency rather than the SDOT. The municipality would then enter a cooperation agreement with the SDOT. The agreement should specify a period of time within which the SDOT responds (e.g., 2 weeks) if it has an objection or issue; and if there is no timely response, the issue is waived by the SDOT.
Design of a Highway Cap
Bridge structures, including highway caps can be broadly divided into two sections: 1) The Superstructure and 2) the Substructure. The superstructure is generally those elements that comprise the span over the highway or crossing the airspace. The substructure is generally those elements which comprise the foundation and support structure for the span. Superstructures include components such as girders, the slab, and the deck. Substructures include components such as piers, piles, footings, and abutments. Abutments are wing walls, breast walls, bridge seats, and footings. Piers include footings, stem walls, web walls, columns, and pier shafts.
The design of the cap will be determined by the geology of the site, configuration of the site, future highway and transit expansion needs, exiting and on-ramping of the highway, the planned use of the highway cap deck (including load, buildings, landscaping, and transportation), the management of existing traffic during construction, the cost of construction, and other factors. Regulatory requirements (and oddities) can also have a profound impact on design. For example, California reportedly has what is in effect a “no-touch” policy for FHWA funded highways. If the cap structure is integrated, i.e., makes contact with an existing bridge, then the lead agency or cap developer is responsible in perpetuity for the maintenance of the bridge. Hence, the design team for the Hollywood Central Park had gone to significant lengths to ensure their planned cap structure would not touch existing bridges, despite encompassing multiple existing bridges. There designed the cap sections so there would be a small gap on both sides of each bridge so that the cap structure does not make contact with the bridge. However, they subsequently learned that because the U.S. Route 101 predated the FHWA, the project was not subject to this policy.
A list or matrix of design / construction options will typically be made taking into account project objectives, environmental concerns, cost, time constraints, and the factors previously mentioned. Thereupon, the best suited design, materials, and process can be selected.
Tunnel vs. Bridge
Tunnel vs. Bridge Comparison No. 1
When a highway cap is part of a new highway project, the cap / bridge structure can essentially be built on the ground, concrete piers sunk, and then a tunnel can be excavated underneath. This can result in substantial savings in comparison to a retrofit cap project, i.e., spanning the airspace of an existing highway. Not only is it more expensive to build a cap / bridge structure in existing airspace, but building the structure while maintaining traffic flow, and building around or relocating exits and on-ramps adds significant expense.
The cap & tunnel technique for new highways can also be made to apply for capping of highways that require lowering of the highway surface. In those cases, the project essentially requires rebuilding of the road. The overall costs may be higher than capping an existing trenched highway but less expensive than lowering the road first then bridging it. It would also typically involve greater impact to traffic, necessarily requiring a total rerouting of traffic.
Tunnel vs. Bridge Comparison No. 2
In most cases where caps are being considered, it is in the context of an existing highway, i.e., a retrofit cap project. Such projects must be built essentially as bridges. In these instances, a different sort of tunnel vs bridge distinction is sometimes applied. In this context, tunnels generally refer to large freeway caps such that the cap covers a lengthy portion of the freeway underneath, requiring lighting, ventilation, fire protection, emergency call and response equipment, and evacuation exits.
The structural engineering of caps supporting landscaping or buildings is different from bridges supporting only cars and trucks. The weight caps are intended to bear is a key determinant of the type of bridge construction.
Caps can be built to support large buildings, including high-rises, as can be seen in the Hudson Yards project in Manhattan and the Capitol Crossing project in Washington D.C. Deeply drilled caissons and high-weight bearing trusses make it possible. These private development caps are some of the largest developments in the country. They are advancing the technology of large construction in airspace.
Highway cap projects supporting intensive commercial or residential development are not simply buildings constructed on top of platforms. Rather, building frames are integrated with – and are essentially one with – the cap platform. As previously noted, at least theoretically, this presents an opportunity for combining elements of “raw land” acquisition, site preparation, and building construction, thus reducing costs compared to traditional developments.
Capitol Crossing Progression:
Images courtesy of Property Group Partners.
Landscape and Open-space caps
Cap design is rapidly evolving. Caps built to support large expanses of open space and landscaping are the subject of new strategies to reduce weight and prevent water intrusion. For example, such strategies have included use of light weight fill, geofoam, support beams laid out in grid patterns to accommodate tree wells, and sloping to channel and direct run-off. The Klyde-Warren park has been a pioneer in cap park design.
Accelerated Bridge Construction using Prefabricated Building Elements and Systems
Increasingly, bridge construction is accomplished using Accelerated Bridge Construction (ABC) methods through the use of Prefabricated Building Elements and Systems (PBES). These methods and components save time, ease impact on traffic during construction, improve quality, and reduce costs. The FHWA actively promotes the use of ABC / PBES (hereinafter collectively referred to as “ABC”) and has published several manuals and pamphlets regarding ABC. The most comprehensive of these publications are the Accelerated Bridge Construction Manual and a 300 page tome on the topic entitled Accelerated Building Construction – Experience in Design, Fabrication, and Erection of Prefabricated Building Elements and Systems. It is available on-line. While freeway caps are among the most unique bridge types, there will undoubtedly be components of the cap structure that can benefit from ABC or PBES.
Maintaining traffic flow will be a high priority of any SDOT. Traffic management goals can be prioritized as “avoid-minimize-mitigate.” Minimizing any required closures and optimally scheduling construction to avoid traffic impacts is an important part of construction planning. It is also a significant augmenter of construction costs. Sometimes traffic demands, site configuration, and alternative routing options require construction of temporary routes, further increasing the expense of the project.
Column Protection Systems
Where columns are exposed to traffic, a column protection system is advisable and may be required. Systems vary. Samples of systems can be found Appendix G hereto, and are excerpted from the California Right of Way Manual.
Air Pollutants Filtration
Freeway caps are more likely to require air filtration or treatment of some sort than are traditional bridges. This is particularly true if the cap is extremely large or involve privates development or high occupancy structures. As discussed in Chapter 7, the filtration may be required either for the benefit of traffic passing underneath the freeway cap, e.g., where the cap results in a long tunnel vulnerable to emissions accumulation, or for the benefit of those who will be using the surface of the cap. However, to date, most freeway cap structures have not been required to include air filtration systems. The caps built have been mostly park caps without long term exposure risks. Nor have most freeway caps resulted in tunnels long enough to risk emissions accumulation underneath (of there are notable exceptions, e.g., Rose Kennedy Greenway in Boston aka “The Big Dig”). For more information, see Chapter 7.
 Kuchibhatla, A., Leguizamón, C., Sesma, M., “Structural Bridge Design and Construction Traffic Management Plan for the Route 24/140 Interchange.” Paper submitted by students to Worcester Polytechnic Institute. (2017).p. 2.2.1. https://web.wpi.edu/Pubs/E-project/Available/E-project-030217-125415/unrestricted/Stantec_Taunton_Interchange_Final.pdf Jan. 29, 2018 Accessed.
 Goldman, L. (Exec. Dir. Friends of Hollywood Central Park – Jan. 25, 2018). Phone interview by author.
 Id. Subsequent discussion on April 10, 2018.
 CalTrans (2017), Freeway Best Practices Guide, Final Draft, p. 3-1; Retrieved on April 4, 2018 from http://www.dot.ca.gov/hq/tpp/grant_files/final-products/11_FwyCapBestPracticesGuideFinalDraft_03122017_Watermark.pdf
 Campbell-Dollaghan, Kelsey (3/19/2014), How NYC’s Newest Neighborhood Will Float Above an Active Train Yard, Gizmodo, Retrieved on December 5, 2017 from https://gizmodo.com/how-nycs-newest-neighborhood-will-float-above-an-activ-1546127069
 CalTrans (2017), Freeway Best Practices Guide, Final Draft, pp. 3-3 – 3-4; Retrieved on April 4, 2018 from http://www.dot.ca.gov/hq/tpp/grant_files/final-products/11_FwyCapBestPracticesGuideFinalDraft_03122017_Watermark.pdf
 U.S.D.O.T, Federal Highway Administration, Accelerated Building Construction – Experience in Design, Fabrication, and Erection of Prefabricated Building Elements and Systems (Nov. 1, 2011). Retrieved on Oct. 12, 2017 from https://www.fhwa.dot.gov/bridge/abc/docs/abcmanual.pdf and U.S.D.O.T, Federal Highway Administration, Engineering Design, Fabrication, and Erection of Prefabricated Building Elements and Systems, (June 2013) Retrieved on Oct. 12, 2017 from https://www.fhwa.dot.gov/bridge/pubs/hif17019.pdf
 A cautionary note: The Florida International University pedestrian bridge that collapsed in early 2018 was erected using Accelerated Bridge Construction. As of May 2018, the investigation into the cause of the collapse continues, though it appears that cracks developed in span trusses even before the span was moved into its permanent location. See e.g., Viglucci, A., Nehamas, N., and Staletovich, J. (2018, May 7). Cracks where FIU bridge buckled may have signaled ‘imminent failure’. Miami Herald. Retrieved from www.miamiherald.com
 Dilwaria, M. Innovative Traffic Management Strategies during Bridge Replacement: Overcoming Constraints and Challenges. Paper prepared for presentation at the Road and Highway Construction – Getting You There Safely Session of the 2015 Conference of the Transportation Association of Canada. P.i. Retrieved on Jan. 25, 2018 from http://conf.tac-atc.ca/english/annualconference/tac2015/s8/dilwaria.pdf
 California Right of Way Manual, Ch. 15, Ex. 11. Retrieved on January 25, 2018 from http://www.dot.ca.gov/hq/row/rowman/manual/
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