New York City Landmark Structures - Reconstructing from the Inside Out-Paper

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The paper describes the reconstruction of three historic Greenwich Village townhouses, joined into one property while preserving their landmark façades. The project involved completely gutting the buildings, removing a shared load-bearing party wall and parts of the rear wall, and excavating two full-footprint sub-cellars up to 24 feet below grade. The neighboring owner’s refusal to permit underpinning added significant geotechnical and construction challenges.<br /><br />Before demolition, engineers installed temporary, multi-level steel diaphragms to stabilize the perimeter brick walls and transfer wind loads. A braced vertical shear frame, supported by rock-socketed micropiles, provided an alternative load path when the party wall was removed. A separate frame of needle beams and clamps supported the rear wall while portions were opened for construction access. Excavation support included braced underpinning and an offset micropile soldier-pile wall beside the neighbor’s property.<br /><br />An automated monitoring system tracked prisms on the project and adjacent buildings hourly, supplemented by liquid-level sensors. Monitoring continued around the clock during excavation and foundation work for 18 months. Movement was generally negligible, with a localized maximum settlement reported.<br /><br />Key lessons included the need for close, frequent coordination among design teams, contractors, and the steel fabricator. Severe site constraints required steel components to be short, lightweight, and carefully field-measured. The authors recommend allowing for construction loads on temporary platforms, preparing alternative connections to address field conditions, and designing for reasonable misalignment. They also highlight the potential value of 3D/BIM coordination to identify conflicts between temporary and permanent systems earlier.

Keywords

Greenwich Village

townhouse reconstruction

historic preservation

temporary steel diaphragms

load-bearing party wall

micropiles

excavation support

geotechnical engineering

structural monitoring

BIM coordination