The project: The Noble (200 Spring Garden), is a 315,000-sf, 13-story, mixed-use residential development that is part of a multi-phased initiative to connect Philadelphia’s Northern Liberties and Old City neighborhoods. The L-shaped tower with a radiused corner features 360 residential units, 5,400-sf of retail, a fitness club anchor tenant, and extensive amenities, including 10,000-sf of public green space and a pool. The building also offers a 1½-level below-grade parking garage for 83 vehicles.

Challenge: Design a “tree column” at the northwest corner to branch out and support the area above the building’s entrance driveway.

Solution: Eight upper-floor columns sit on a series of transfer elements at the third floor. These transfer elements are supported by six sloping columns that converge down to one main concrete and steel composite column approximately 10 feet above the ground floor. This design creates space for a drop-off area and access to a public plaza. The main column sits on a thick 7-foot mat foundation supported by 36 auger cast piles. Because of the heavy loads and connection geometry required to slope six columns to a single node, the final column had to be 76 inches in diameter. A steel column with headed studs was placed inside the concrete column and welded to a central node made up of a series of fifteen 2-inch-thick plates welded together. This steel column provided temporary support during construction and was designed to support, on its own, steel and precast planks levels up to the fifth floor until the final concrete column was cast around it. The six sloping columns support four interior and four exterior columns, creating unbalanced horizontal trust forces. To handle these trust forces at the third floor, a diaphragm system consisting of a 12-inch-thick concrete slab on a metal deck in combination with a horizontal steel framed truss was used instead of hollow-core planks. This diaphragm system provides a load path for these trust forces to three braced frames, which carry the remaining force down to the foundation.