By Todd Campbell, PE

IMEG provided structural engineering design for The Noble, a complex project that needed practical but creative structural solutions.

The Noble is a 13-story apartment building with 360 units, ground-floor retail, and one underground parking level that connects Philadelphia’s Northern Liberties and Old City neighborhoods. The L-shaped building has hand-laid brick on the courtyard and street sides, with a curved metal trim at the top. A unique “tree column” at the northwest corner branches out to support the area above the building’s entrance driveway. Next to SEPTA’s Spring Garden Station, the building acts as a gateway connecting Northern Liberties to Old City.

The building sits on 18-inch-diameter auger cast piles with 116-ton compression capacity. Most columns are supported by standard pile caps, while the braced frame areas use thicker mat foundations up to 8 feet deep. Because of the poor soil, even the retaining walls needed pile support to resist sliding and tipping forces. The upper structure is made of a steel frame. Floors below the third level use concrete slabs on metal decks, while floors above use precast hollow-core planks. Braced frames are used to resist wind, earthquake, and thrust forces from sloping columns.

Fitting a loading dock on the ground floor was challenging because of the column layout needed to integrate the residential layout above. IMEG transferred out five full height building columns and one braced frame at the second floor, using beams up to 44 inches deep weighing up to 529 pounds per foot.

The most complex and unique part of the project was the “tree column” at the building’s northwest corner. 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.

To aid in the erection of the “tree column,” 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 thrust forces. To handle these thrust 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 thrust forces to three braced frames, which carry the remaining force down to the foundation.

The Noble’s structural engineering design shows how innovative solutions can be practical solutions.