When replacing an old bridge with a new one, time is always a critical factor, especially if the crossing serves as a main route in and out of a busy city.
The original Pittsburgh Commercial Street bridge went into construction in 1948 and was completed in 1951. Today it is used by more than 100 000 vehicles daily. The concrete bridge went through three rehabilitations in its life, in 1980, 2006 and 2012, but eventually needed to be replaced with a new structure capable of carrying permit loads.
This major infrastructure project used accelerated bridge construction techniques. The replacement span was built beside the old bridge over a period of months, then laterally slid into position immediately after the old bridge was demolished. This methodology reduced the time of traffic impacts on the public, from nearly four years of intermittent lane restrictions and closures via traditional bridge construction methods to just 17 days of a continuous, around-the-clock closure. It also meant the old bridge could remain open until the very last moment, minimising disruptions for the public.
Mammoet played a significant part in making all this possible, combining its heavy-lift experience with specialist skidding equipment previously used to install the Chernobyl New Safe Containment structure in 2016.
Appropriate shoes for the occasion
The new steel arched delta frame bridge was assembled just metres away from the old one with only intermittent lane closures required for site preparations.
Mammoet worked with Fay, S&B; USA Construction, the prime contractor responsible for building the new bridge and demolishing the old one, which included conventional demolition methods and a controlled implosion. Mammoet’s scope was to plan, manage and perform the skidding installation of the new 10 000 ton bridge once the old structure and all debris were removed.
“We moved the entire bridge structure whole, the deck and the main support structures,” explains Mark Lane, engineering team lead at Mammoet. “Our skidding system allowed for this type of solution to be feasible, and provided the quickest means of replacing the bridge.”
The low height and high load capacity of the system are the key reasons it was chosen. Most skidding systems tend to move laterally when they pick up a heavy load. However, this system can resist transverse force and minimise stresses on the lifted load.
To provide additional protection against twisting forces, a temporary frame was erected between the bridge and the skid shoes, securing everything in place. The low height of the skid shoes gave the engineering team more room to make this frame strong enough without needing to widen its foundations.
The skidding system also has a revamped control system giving operators precision and oversight, from the stroke of the main cylinder to the skidding force. Each skid shoe can be operated independently with the integrated hydraulic jack used to lift the load from its temporary construction supports and set it down on its permanent bearings.
A total of 42 skid shoes were used for the installation, spread over four primary support runs: two at the bridge piers and one at each of the abutments.
The team needed to ensure every section of the bridge moved at the exact same time with only a 25 mm tolerance from pier to abutment available. They conducted a test slide to pick up and move the bridge 1,5 m forward, checking deflections and ensuring everything behaved as expected before the final slide.
The final skidding distance was 31,1 m, performed in one continuous move and achieved in a single day shift.
Instant bridge installs
While other skidding solutions could have been considered, they would have presented different challenges with shoe height and system control. Mammoet self-propelled modular transporters have also been used for similar jobs, but the limited space on the construction site would have been a challenge.
To date, this marks the heaviest bridge that Mammoet has moved in the United States of America. The operation was made possible in such a short time using a skidding system unlike any other in the world.
“It has been a true global effort from our side to make this project happen. We’ve involved multiple offices and departments,” says Lane. “The scale of bringing a system of this capability and size to bear on a USA project is significant. We are now looking at utilising this system on several other projects now that it is in the USA.”
For more information contact Mammoet Global,
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