August 31, 2016
First suspension bridges in the U. S. scheduled for much-needed facelift
In a city known for its bridges, Pittsburgh’s Three Sisters Bridges connecting the shores of the Allegheny River have special meaning for the region. When Michael Baker International was selected for preliminary engineering, final design and construction-phase services for the rehabilitation of each of the 90-year old historic bridges — owned and maintained by Allegheny County — its team of bridge engineers and designers knew it had a lot to live up to.
Pittsburgh's Three Sister Bridges
Also known as the Roberto Clemente (Sixth Street), the Andy Warhol (Seventh Street), and the Rachel Carson (Ninth Street) Bridges, the Three Sisters not only carry the monikers of three of the city’s most beloved heroes, they also reflect historic bridge design and construction feats, earning each a place in history. Specifically, the bridges make up the only trio of identical, side-by-side bridges in the world and are the first self-anchored suspension bridges constructed in the United States. They provide a vital link to downtown Pittsburgh for more than 23,000 daily commuters and pedestrians during the work week and enable the city’s ardent sports fans to park in town and walk to stadium events on the North Shore. They are part of marathon race routes, events are hosted on them, and they are featured heavily in photographs as well as in local and national commercials, and news coverage.
The bridges require rehabilitation because of accelerating age-related deterioration. The project work draws on Michael Baker’s expertise in all areas of bridge design, construction, and preservation and includes the following: bridge inspections; structural analysis and rehabilitation design; utility coordination; maintenance and protection of traffic; roadway and lighting design; paint evaluation; field surveys; and public involvement.
As part of the structural analysis, the Michael Baker team developed a series of three-dimensional, finite-element models for the Andy Warhol Bridge using MIDAS, an engineering software specifically developed for bridge modeling and analysis. This marked the first time that the bridge was modeled in 3D. The models were used to establish the existing forces on the bridge, determine the new dead loads based on the proposed rehabilitation, calculate the modern-design live loads, and evaluate proposed construction sequencing.
Bike lanes on the Roberto Clemente Bridge
During preliminary engineering, several options for incorporating bike lanes into the project were developed. After close coordination with Allegheny County officials, it was decided to permanently install one five-foot-wide bike lane in each direction along the curb line on the Roberto Clemente Bridge, reducing the travel lanes to one 11-foot-wide lane in each direction.
Once the team prepared signing and pavement marking plans for the permanent installation of bike lanes and completed the installation, the bike lanes were officially opened for public use in the spring of 2015.
“The incorporation of bike lanes on the Roberto Clemente Bridge by Allegheny County was a great example of considering all modes of transportation and improving the safety and mobility of bicyclists and pedestrians in the downtown area,” said John Tricini, P.E., PLS, Michael Baker’s project manager.
Currently, the first of the three bridges, the Andy Warhol Bridge, is under construction. It was closed to pedestrians and traffic on August 15, 2016, and is expected to reopen November 17, 2017. Construction work will involve the following: replacement of the bridge deck and joints; total repainting of the superstructure; steel repairs; replacement of the main span tie-down assemblies; replacement of the existing lighting with historic replica light poles; substructure repairs; scour protection; replacement of utility conduits and bracing; and replacement of downspouts and scuppers.
The American Bridge Company of Ambridge, Pa., and the Foundation Company of New York originally constructed the bridge in the early 1920s as a cantilever because of navigational restrictions on the Allegheny River. The builders drove temporary wooden pile bents into the river and placed struts within the panels of the suspension system, erecting the bridge essentially as a cantilevered truss. When workers used hydraulic jacks to adjust the member positions and counteract gravity deflections, the temporary supports fell away, leaving the suspension form in its anticipated dead-load position. Original design credit typically is afforded to Allegheny County Engineer, Vernon Royce Covell.
Michael Baker designed customized bike lane graphics in honor of Roberto Clemente.
Rehabilitation of the Rachel Carson Bridge is scheduled to begin in the summer of 2018 and be completed in the fall of 2019. The Roberto Clemente Bridge rehabilitation will start in the summer of 2020 and end by the fall of 2021.
“The Three Sisters Bridges have a very profound history, and we are very proud to assist Allegheny County in such a high-profile bridge rehabilitation project in downtown Pittsburgh,” said Tricini. “We look forward to continuing our work with our partners at Allegheny County, utilizing our experience and commitment to excellence in bridge design, to significantly extend the life of these iconic structures and truly make a difference in the communities we serve.”
Sub-consultants supporting Michael Baker on the project include: L.R. Kimball for inspection and design of the Rachel Carson bridge; Olszak Management for public involvement services; A&A Consultants for bridge inspection and structural analysis; Ammann & Whitney for testing the suspension system; KTA-Tator for painting assessments; and Santangelo & Lindsay for lighting design.