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Custom Bridge Scaffold Case Study

Engineering Project: New NY Bridge
The New NY Bridge Project, NY
Those who cross the Tappan Zee Bridge, 25 miles north of Downtown Manhattan, NY, will notice a new structure rising from the Hudson River water just next to it. In 2013, the New York State Thruway Authority (NYSTA) awarded the contract to replace the existing Tappan Zee bridge to Tappan Zee Constructors, LLC (TZC), a design-build consortium of some of the world’s best-known and highly-regarded design, engineering and bridge construction companies including Fluor Enterprises, Inc., American Bridge Company, Granite Construction Northeast, Inc., Traylor Bros., Inc. and designers HDR and Buckland & Taylor.

The replacement effort, called the "New NY Bridge" project, will create is the longest bridge in the state of New York, crossing the Hudson River at one of its widest points. The old cantilever bridge, which currently accommodates 140,000 vehicles per day, is being replaced with a state-of-the-art, 3.1-mile long twin-span bridge. Essentially there are two new bridges, one traveling eastbound and one westbound. The new bridge is designed and constructed to stand for the next 100 years without requiring major maintenance efforts.

This massive, bridge-building undertaking requires today’s most modern technology, highly skilled labor and specialized teams performing some very creative engineering feats. This includes the most efficient way to access the construction of the bridge safely from start to finish. UMC, located in Zelienople, PA, was contracted by TZC to help achieve these goals through innovative bridge scaffolding engineering, design and fabrication of the custom bridge access products to meet the project’s unique specifications.

UMC worked with two different bridge engineering teams on the project. One team for the approach span bridge piers and one for the main span bridge piers. Each bridge consists of 43 concrete piers, 41 approach span piers and two double and much taller main span piers. Fourteen of the concrete piers were less than 45’ in height and did not require bridge scaffold access. The remaining 30 piers are varied from 45’ to 409’ in height to allow for bridge stability and an increase in clearance for ships to pass beneath the structure. Every pier required bridge scaffold access stair towers engineered and designed to match the contractor’s specifications. TZC would specify each location of the scaffold stair tower on the bridge pier base in order to match up with the openings in formwork on each bridge pier cap. UMC then provided detailed bridge stair tower drawings for each pier noting location on the pier base, tie-in specifications to the columns and exit height of each bridge stair tower. This design and layout process was repeated for all bridge piers.

Limited in area to stage and assemble the stair components at the project site, the custom bridge stair towers for each specified pier were assembled by crews up river matching the numbered engineered drawings provided by UMC. The stair towers were then craned onto a barge using custom lifting rigs provided by UMC then transported by barge to the construction site and lifted in place by crane using the same lifting rigs. Each lifting rig was a steel-beam, single-lift, bridle unit that attached to each coupling pin to allow for consistent, safe lifting.

The focal points of the iconic cable-stay main-span structure are the four pairs of outwardly leaning pylon towers located at the two main-span bridge piers. The overall height of each concrete pylon tower is 409 feet and each splays outwards at a constant angle of 5 degrees, presenting unique engineering challenges for providing a safe and reliable means of access. TZC’s access requirement for the 409 feet was to build a 290-foot stair tower, which would lead to a crossover truss connecting the two pylon towers and two additional 120-foot stair towers supported on a platform anchored to the concrete pylon at the crossover elevation. Challenged with engineering a single system scaffold stair tower 290-feet tall that would support its own weight and meet the capacity requirements of the customer, UMC developed a hybrid system scaffold/shoring frame. The hybrid frame worked with all system scaffold components and was used from the base of the 290-foot tower to approximately 119 feet at which point standard system scaffold stair tower construction was used for the remaining 171’. All scaffolding for the main-span piers was designed to follow climbing concrete jump forms – as the forms climbed up the tower facilitating rebar operations and placing concrete, additional scaffold access was barged to the piers and craned into place.

Scaffold tie-in plans and product were provided for all stair towers with the 290-foot towers presenting the largest challenge. Due to the fact that the main span pylon towers were outwardly leaning, the 290-foot stair towers had to be positioned at a diagonal from a corner of the pylon tower and far enough from the pylon to allow for access to the climbing concrete jump forms. Placement of the scaffold stair towers at this location had stairs at some locations as far as 10 feet from the pylon corner with 20-foot-long multiple tie configurations at each point. The construction stair towers had to be engineered to withstand wind at 115 mph.

“UMC collaborated with our engineering team to find the safest, most efficient and most economical means of access for our construction crews to get to work everyday”, says TZC Professional Engineer, Andre Markarian. “There are many scaffolding companies we could have purchased scaffolding from. UMC is different because their Engineering team provided the continuous support we needed.”

In addition to the engineering custom bridge stair towers, UMC designed and fabricated various custom bridge staging platforms for this project. Pre-Cast Staging was designed and installed over the top of the pier caps to allow for bridge access around the perimeter of all pre-cast bridge pier caps. Grout Platforms were also designed in the same manner to allow access to the pre-cast tubs where additional grout needed to be placed. Each of these one-piece platforms provided a safe work bridge platform, that were craned into place using custom-fabricated lifting units.

“The Cap and Grout Platforms were very useful and efficient to install and remove throughout the construction of the bridge, “says TZC Professional Engineer, Joseph Rynn. “Universal did a great job designing these for us.”

UMC has been pioneering the highest quality steel scaffolding shoring and custom fabricated access equipment for more than 85 years. Their free scaffold design services and engineering teams have revolutionized the way they service their customers and provide the safest, highest-quality and most innovative custom access solutions available today.

“This project required many hours of innovative engineering work because of the complexity of the structure, says UMC Sales and Marketing Manager, Mike Bredl. “It was great to be challenged by Tappan Zee Constructors so we could truly showcase our scaffold engineering and custom fabricating capabilities.”

About the Author
Mike Bredl is the Sales and Marketing Manager at Universal Manufacturing Corp.; He can be reached at mbredl@universalscaffold.com. Other contributors to this article include Tony Fleming, UMC Project Lead/Design; John Zbiegien and Bill Munsch UMC Engineering Team; and Stephen Fleming and Shawn Hineman, UMC CAD Team.

New NY Bridge Photos courtesy Andre Markarian and New York State Thruway Authority
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