In the dense urban environment of Manhattan, available land is finite, but demand for additional space is insatiable. Consequently, owners devise ambitious and inventive strategies to eke out more space from their land. Some tear down their buildings and rebuild to the heights limited by zoning restrictions. Yet, in Landmarked Districts, owners cannot tear down. They can, however, expand in two directions: up and down. Expanding up can be constrained by either sightline requirements or a contextual design. Expanding down can be a logistical nightmare. Both are expensive and they both require significant structural engineering to redirect load paths and to understand the affect of the expansion on adjacent properties. The existing building at 23 Cornelia Street in Manhattan's historic Greenwich Village is a two-story townhouse built in the late 19th century. The unreinforced masonry and wood-framed structure, with a footprint of 20 ft by 100 ft, has been landmarked by The New York City Landmarks Preservation Commission, who ruled that the building's sightline must remain unchanged. The owner decided to increase his living space by adding an additional story below with a swimming pool and an additional story above with a gabled roof. This article presents the structural design considerations required to enlarge this townhouse. To seismically retrofit the building, we inserted new concrete shear walls and steel-moment frames and tied the existing unreinforced masonry walls to the new lateral system. To meet the challenges of excavating in an urban environment, we specified construction sequences, underpinning procedures, and designed temporary bracing for the substantial lateral soil pressures and surcharges from adjacent buildings.
Expanding a New Landmark: The Stable at 23 Cornelia Street
June 29, 2011
Publication: Structural Engineering & Design p 46
Markets: Residential - Condominium