Pittsburgh’s Commercial Street Bridge Replaced in Under 24 Hours

The Pittsburgh Commercial Street bridge was completed in 1951 and is now used by more than 100,000 vehicles every day. The concrete bridge went through three rehabilitations, during 1980, 2006 and 2012, but eventually needed to be replaced with a new structure that could carry permit loads.

This major infrastructure project utilized Accelerated Bridge Construction techniques. The replacement span was built beside the old bridge over a period of months, and 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, minimizing disruptions for the public.

Mammoet worked with Fay, S&B USA Construction, the prime contractor responsible for building the new bridge and the demolition of the old bridge. 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.

The low height and high load capacity of the system are the key reasons it was chosen. Most skidding systems are likely to move laterally a little when they pick up a heavy load. However, this system can resist that transverse force and minimize 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 make its foundations wider.

The skidding system also has a newly revamped control system to give operators unprecedented 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 being 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. The shoes were 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 1-inch tolerance from pier to abutment available. They conducted a test slide to pick up and move the bridge five feet forward, checking deflections and ensuring everything behaved as normal before the final slide.

The final skidding distance was 102 feet, which was performed in one continuous move. This was achieved in a single day shift.