The Illinois Tollway replaced structural bearings and patched concrete without impeding traffic on a busy suburban Chicago bridge, thanks to a solution devised by engineering firm Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.
The goal was to lift the bridge deck about 1/8 inch so crews could install new, 18x12-inch Type 1 elastomeric bearings under the steel girders.
Usually, hydraulic cylinders are installed between the shoring tower and the item being lifted, forcing crews to operate the cylinders at the top of the tower, where access is limited. But for this project, the towers’ lifting column was jacked using a 100-ton “lock-nut” cylinder that placed the cylinders at ground level.
Once the ram's hydraulic piston is extended, the mechanical lock nut is screwed down tight. This prevents the piston from retracting into the ram body and mechanically holds the load so hydraulic pressure in the system can be released. The cylinders have swivel caps at the top end of the piston to adjust to load contours and ensure cylinder stability. A built-in stop-ring prevents piston over-extension, keeping the cylinder centered and captive as the ram operates.
An electric pump pressurized the cylinders, and crews monitored girder elevation with lasers. When at the proper height, the hydraulic ram locking collars were screwed down and engaged, ensuring constant girder elevation. Locking collars allowed the crew to release hydraulic pressure and disconnect the pump.