47 East 91st Street

A lot of people would have looked at the site at 47 E. 91st St. and thought it wasn't worth the trouble. Besides keeping a one-story Citibank branch in operation, project team members had to address school, traffic and community issues.

"Teamwork is what made it happen," said one jury panel member. "They took a site that wasn't really viable, thought out of the box and made it work."

Now, the site is home to a new 10-story luxury condominium building over an existing Citibank branch. The building footprint is 70 by 70 ft. with seven one-story residences and a luxury duplex occupying the ninth floor and penthouse.

The $13.5 million project also included renovation of an adjacent landmark four-story townhouse to be used as a superintendent's apartment and a new heating and cooling plant that serves the condominium building.

The structural engineer proposed a system comprised of a conventional reinforced concrete building supported on a structural steel transfer plate that would deliver building loads to a new residential core and just five other steel columns.

Working closely with the architect and Citibank, the engineer chose column locations that would minimize disruption of bank operations because aside from 14 prearranged closure dates, the bank operated during its regular hours Monday through Saturday.

Over one weekend, the five steel columns - which would support about three quarters of the building's gravity load - were "needled" through holes in Citibank's roof and ground floor and placed upon concrete footings in the bank basement. Over two subsequent weekends, the steel transfer plate on the second floor was erected.

Once completed, the steel transfer plate and existing bank roof provided enough protection so that construction could proceed without closing the bank.

The project team faced another hurdle with installation of the mechanical, electrical and plumbing lines, which would have to enter the new building in the shallow space between the bank roof and the steel transfer plate on the second floor.

Access to this space was limited by construction of a slab-on-deck on the second floor. The slab-on-deck needed to be almost complete before construction of the building over it could proceed.

The structural engineer solved the problem by identifying zones where construction of the slab-on-deck could be delayed. The architect and mechanical engineer devised a layout that allowed the MEP lines to be accessed from these zones.

The building's heating and cooling plant had to be located in the adjacent townhouse basement to maximize the amount of salable space in the new condominiums. The fire rating on the floor above the basement was inadequate and needed to be replaced, but an existing storefront was deemed a landmark and could not be disturbed.

The structural engineer and construction manager found a solution by deciding to "hang" the storefront construction from the second floor, so that the ground floor could be demolished and reconstructed. The storefront was successfully preserved and now rests upon the new ground floor.