- CHI Consulting Engineers
“Telescope” Rocker Links for The Benjamin Franklin Bridge
The Benjamin Franklin Bridge (BFB) was opened to traffic in 1926 and is owned and operated by Delaware River Port Authority (DRPA). The suspended spans of BFB consist of a 1,750-ft main span and two 752-ft side spans. The bridge carries 7 lanes of vehicular traffic and two PATCO tracks. After being in service for over ninety years, the rocker links at both sides of the two towers required replacements, a total of 8 links. The construction project was awarded to Skanska USA Civil, and CHI Consulting Engineers was retained by Skanska to design jacking frames and supports.
To replace the tower rocker links and maintain the normal operation of both vehicular traffic and PATCO trains, robust temporary jacking frames and supports are needed, especially since each of these tower rocker links needs to resist about 2,200 kips of compression and 600 kips of tension at the strength limits. CHI designed robust jacking frames and temporary supports, as well as construction sequence, that can resist full dead and live loads, greatly reducing the impact to the traffic. In addition, the lengths of temporary rocker links can easily be adjusted through a unique “telescope” mechanism, in which an upper link is inserted into a lower link and are engaged by four (4) 400-ton capacity self-locking pancake jacks and four (4) 1 7/8” diameter high strength rods in-between.
Common temporary rocker links would have splice plates between the upper and lower links, and slotted holes are usually used in these splice plates to allow adjustments of their lengths, resulting in slip-critical connections and the need of many high strength bolts. Compared to the common temporary rocker links, the benefits of these “telescope” rocker links are as follows:
More robust since the load resisting mechanism remain the same during the jacking operation, as compared to the common temporary rocker links with bolted splices, in which the bolts need to remain loose during jacking, resulting in weaker links.
Faster erection operation since there is no need to tighten a large number of bolts.
Easier and faster adjustments of lengths of temporary rocker links during the installation of new permanent rocker links since there is no need of unbolting of any temporary splices and the lengths can be easier elongated or shortened as soon as the pancake jacks or the bolt tensioners are reconnected.
The “telescope” rocker links have been installed on the North side of the New Jersey Tower in both the main and side spans. The jacking operation to transfer loads from the existing tower links to the temporary “telescope” rocker links was successfully carried out around the midnight on 12/10/2021. Once the jacks were connected, it took less than half an hour to unload one existing tower link, lock off the jacks, and demobilize crew and equipment – a very smooth and fast operation.
The “telescope” rocker links have been supporting the trusses at the main and side span side since 12/10/2021 without any issues. By 12/16/2021, the existing tower link at the main span side was completely freed by wire-sawing the upper and lower pins and was removed shortly afterwards.