Segmental Construction in Urban Environments
Editorial by John Crigler, ASBI President
Our Annual Convention, held in Vancouver, British Columbia, in October, was, by all measures, a tremendous success. It is amazing to see how the quality of the program continues to improve every year. All of the speakers were well prepared and the session chairs did their part keeping the program on track. The Sunday night reception, sponsored by Buckland and Taylor, was well attended and we would like to thank them for hosting this special event to welcome us to their city with its many examples of segmental bridges. Finally, on behalf of the ASBI members, I would like to extend a special thank you to Randy Cox and Ingrid Ramsey. They have a very challenging responsibility to plan and organize this event every year and they do a wonderful job.
Recently I found myself saying to someone “building this bridge would be easy if it were in the middle of a big corn field somewhere.” While it would not be “easy” it would certainly be “easier.” The reality that we face today is that wide open sites for building bridges are becoming rare. Many of our infrastructure needs are located in and around cities with existing highways, mass transit systems, and railroads. As owners, designers, and builders we are faced with the daunting task of working in very congested areas where the existing infrastructure is, in many cases, functionally obsolete. The challenge is to upgrade and expand what we have while minimizing the impact to the traveling public. In addition, we are adding into the mix new forms of transportation such as: “hot” lanes, HOV lanes, high speed rail, and light rail. All of these are competing for space in our existing transportation corridors.
Segmental bridges have significant advantages in this environment:
• Segmental box girders with long overhangs and integral diaphragms maximize the useable structure width while minimizing the footprint and pier height requirements. This is particularity important when building in the medians of existing highways where there is not enough space available “at grade” to expand.
• Segmental bridges are easily adapted to a wide range of alignments and geometry. The inherent torsional capacity of the box cross-section is ideal for horizontal curves with tight radii and combinations of vertical and horizontal curves with corresponding super elevation can be “built-in” when the segments are cast. Additionally, segmental bridges can easily accommodate a wide range of span lengths within a project. In many situations, this flexibility is required to work around existing infrastructure.
• The construction methods used for segmental bridges are well-suited for congested urban sites. They can be built using “span-by-span” methods or “in cantilever.” The segments can be precast and erected by conventional cranes or specialized equipment. Also, they can be built “in-situ” using form travelers. All of these methods are very efficient and minimize the impact on the traveling public and surrounding neighborhoods.
The 2011 ASBI Convention will be held in Washington, DC. We plan to visit the WMATA Dulles Subway Extension Project which is being built over, around, and under the Tyson’s Corner, Virginia, area and the adjacent Washington Beltway. This region is arguably one of the most congested areas in the United States and is an excellent example of the benefits of segmental bridges in urban environment.
We hope to see you there!