75 Rockefeller Plaza is a landmarked 34-story steel moment frame building constructed in 1947 for Rockefeller’s Standard Oil Company. It is situated in the heart of Rockefeller Center on 51st Street between Fifth and Sixth Avenues in Midtown Manhattan. It totals 623,000 square feet, with typical floor size ranging from 14,000 – 30,000 square feet.
RXR is undertaking a major repositioning which includes restoration of the building façade limestone and base metalwork, retail and lobby enhancements, and upgrades to mechanical systems and infrastructure. The redevelopment of the building began in 2014. This work included a reconfiguration of the lobby and the removal of four columns at the ground floor, three of which supported existing transfer girders.
Multi-story transfer trusses and removal of each column from the top down were among the schemes considered before the final design of a composite box girder was selected. The multi-story truss would have blocked too much leasable space. Column removal and transfer floor by floor, on the other hand, would have provided a large column free area across all floors, but this method would have been prohibitively expensive. The unique box girder solution (as opposed to a pair of regular or built-up wide flange beams) was also driven by the planned sculpted lobby ceiling,requiring the transfer to be as narrow and shallow as possible. The composite box girder had to be carefully specified, as it is typically applied to bridges rather than buildings. Both AISC 360 (which provides specifications for structural steel buildings) and AASHTO (which includes steel bridge specifications) were consulted.
Other design challenges included eccentrically reinforcing existing columns, and modifying the existing partially restrained wind frame. Of particular note were the constructability challenges of erecting and preloading a new steel box girder around an existing transfer girder to effectively extend that transfer to the next column line. A scaled 3D model was printed to help communicate the design concept to the steel fabricator and the owner.
To preload the girders, a solution was developed that maintained redundancy throughout the entire loading procedure and did not require any temporary structure or shoring. This involved a yolk system with 500-ton jacks. The method pulls the column up and loads the girder in flexure without any significant displacement. At that point the final connections are completed and the existing column removed.
Effects of column shortening, elongation, and resistance from the steel moment frame above were all considered during the loading process. There was a big focus on maintaining lateral stiffness. Since the building was originally designed under the 1938 code, which required a small amount of lateral stiffness compared to modern buildings, careful attention was paid to reinforcing connections and keeping the relative increase in the load in the surrounding members to a minimum.