Walking columns: The structural solution for the tallest residential building in Philadelphia

By D. Kirk Harman  

The Laurel Rittenhouse Square – a 50-story, 583,000-sf, ultra-luxury mixed-use tower – is the tallest all-residential building in the City of Philadelphia and required the use of a particularly unique structural strategy: walking columns.  

Designed by architectural firm Solomon Corwell Buenz of Chicago, the building houses 65 condominiums and 184 apartments atop a three-story podium with 44,000 square feet of retail, dining and amenity space. The Laurel’s 50 stories are constructed of a cast-in-place concrete flat plate system with concrete shear walls, and it sits next to one of the project’s biggest challenges: a three-story historic building that needed to be preserved. 

The Harman Group, now IMEG provided structural design for the project. In order to maximize the floor-to-site-area ratio permitted by the City of Philadelphia Zoning Code per the developer’s wishes, we had to find a way to expand the new tower floors above the historic structure to capture additional floor area where the edge columns would be located outboard of the columns below. Our solution was walking columns, which are used when a column is not located directly above the column below, creating an offset and eccentric load on both columns and resulting in horizontal forces at each end of the column that must be resolved into the floors above and below – in this case, in four successive floors to achieve the transferred column location in the 40-plus floors above.  

We resolved these horizontal forces in the design of the floor diaphragms with mechanically terminated tension drag struts to provide a load path to the concrete core. In areas outboard of the core, similar drag struts were designed, and then supplemental diaphragm reinforcing transferred the forces horizontally into the core. 

Walking columns solved just one of the unique challenges our team encountered on this project. Fellow IMEG engineer Todd Cambell and I recently published an article in Structure Magazine detailing all the challenges and successes we found in designing the building, including the foundations, a 12-hour continuous concrete poor, and more. Read the full article here.

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