When I learned that PennDOT was going to replace the Commercial Street Bridge, my initial reaction was that somebody had a big problem ahead of him. Located just east of the Squirrel Hill Tunnel on the Parkway East, that location is notorious for traffic jams. The prospect of partially closing down the Parkway for several years to replace this bridge would be a massive headache. Then I learned that a combination of three prominent local organizations – PennDOT; HDR; and Fay, S&B USA Construction– was planning to utilize the Accelerated Bridge Construction concept to reduce the shut-down period to less than a month, by building the replacement bridge adjacent to the existing one, demolishing the current bridge, and then sliding the new one into place. These three organizations are filled with engineers whom I know, young men and women who were our students in the Civil Engineering Department at the University of Pittsburgh.
Consequently, I have followed the project closely and was greatly relieved when the replacement was completed in seventeen days and the Parkway reopened eight days ahead of schedule. The Engineering-News-Record article that reported this feat internationally specifically mentioned three engineers as spokespersons for their respective organizations. PennDOT was represented by their District Eleven Executive, Jason Zang. He graduated from our department in 1997 and has been a significant supporter of our Senior Design Project program for many years. HDR’s spokesman was Northeastern Region Bridge Leader Nicholas Burdette (Class of 2004). Nick has been a mainstay in HDR’s Pittsburgh office for seventeen years. Senior Project Manager Michelle Olszewski spoke for Fay, S&B USA Construction. She went to work for them following her graduation from our department in 2009 and quickly became an excellent example of the ability of women to succeed in the construction world. We are proud of all three of these engineers, as well as of the numerous other alumni of our department who are major contributors to their organizations.
The original bridge was designed by Richardson, Gordon Associates and constructed as part of the Penn-Lincoln Parkway (East) in 1951 for a cost of $1,780,000 ($23,400,000 today). It consisted of a pair of parallel bridges, each of which had three 175’ long reinforced concrete open spandrel arches and two approach spans each about 165’ long. The decision to replace it was based on its general deterioration and the projected cost of future maintenance, plus the fact that it was not designed for the heavy/oversize loads that modern Interstate highways must be able to accommodate. The cost of its replacement in today’s dollars is about $95,000,000; one more example of the “sticker shock” that is prevalent in contemporary construction projects. Certainly, the portion of the cost required for the novel slide-in concept that reduced the shut-down period from years to seventeen days is justified.
According to Nick Burdette, there were two primary design considerations. They wanted to retain the general appearance of the original elegant arch design while producing a bridge stiff enough to be capable of being moved into position without damage. Arch bridges depend upon having piers that can accept massive thrust forces, a requirement that couldn’t be accommodated by a bridge that had to be moved over one hundred feet. The designers’ solution was to utilize two sets of open “delta” frames supporting a plate girder deck with abutments at each end. As a lifelong architectural critic, I must concur that the final result is quite attractive (though I still prefer the original arches). The delta frames are also quite suitable for the “slide-in” approach. The bridge deck is supported by seven deep composite plate girders; the designers chose to place a delta frame under each girder. Each frame appears to be slightly curved, just enough to echo the arch concept.
The challenge of moving the completely constructed bridge one hundred feet and matching it to the existing approaches of the demolished original bridge was subcontracted to Mammoet, a Dutch firm that specializes in moving massive loads. Their name means “mammoth” in their language; it certainly is an appropriate appellation for their specialty. This bridge, 875’ long and weighing twenty-two million pounds, was a suitable challenge for them. They solved it by positioning a pair of hydraulic pistons at the base of each delta frame plus one at the abutment end of each deck girder (a total of forty-two). Each piston had a thirty-six-inch stroke; the movement was in increments of three feet. The bases were slid on lubricated Teflon shoes on a Teflon track (Teflon has a coefficient of friction below ten percent). It took about ten hours to move the bridge the necessary one hundred feet and position it precisely relative to the permanent foundations.
Now that these folks have pulled off this remarkable feat with apparent ease, my interest has shifted toward feeling proud of Pitt’s Civil Engineering Department and my modest contributions to it during my tenure there. I suspect the three engineers mentioned in this column are all over-achievers who would have been successful regardless of where they went to school. Nonetheless I feel the apparent coincidence of their being featured in the ENR article is not accidental. Rather, I consider it vindication of a commitment by a group of my fellow faculty members in the Pitt Civil Engineering Department to focus on nurturing students who were obsessed with the goal of building major infrastructure components and on providing them with the necessary foundation to function effectively in the real world. We are proud of producing District Executives and Regional Bridge Leaders and Senior Project Managers, but we are equally proud of the dozens of our alumni and alumnae who have contributed to the success of this impressive project in lesser roles.