e-Mail
Print
Single-family townhouses for sale in the stylish West Village neighborhood of New York City are a rarity, and the townhouse at 112 Washington Place was a special case. During our initial site visit in spring of 2007, it was apparent that the building, built in 1832, had seen little to no maintenance for decades. The four-story, Federal-style townhouse had been solely occupied by a elderly man who led an austere lifestyle, living on the top floor of the structure and surviving without heating, air-conditioning, or hot water. For over four decades, he had lived alone in the building after inheriting the property from his grandmother, who purchased it in 1878.
|
Like most townhouses built in the early 19th century, the layout has the traditional levels of garden, parlor, and first and second floors, with a half-story unfinished basement. The parlor is located a half story above-grade, accessed from the sidewalk via a flight of stairs, while the garden level is located a half story below-grade, with high windows to an areaway at the front and access to the garden in the rear. Located mid-block, the building has 40-foot-long party walls and a street frontage of 21 feet. The structure consists of wood joists floors spanning to brick-masonry bearing walls, which are typically two-wythe thick, except at the garden level where the party walls thicken to three-wythes. These brick masonry walls then transition to a rubble stone foundation approximately at the garden level.
When the property was put up for sale in summer of 2007, brokers touted the townhouse as a “rare opportunity to own the best of bones!” before somberly concluding that it was “in need of a full renovation” and a major “fixer-upper.” The property garnered much interest. An article written about the townhouse appeared in the June 2007 edition of The New Yorker magazine. Potential buyers included celebrities, but most were dissuaded by the asking price and by the time and effort needed to renovate. Blesso Properties, a real estate, investment, and development corporation, bought the property with the intent to renovate it into a single-family luxury residence in the Modernist style.
Simpson Gumpertz & Heger Inc. (SGH) initially acted as a value-engineering consultant, but eventually became the engineer-of-record. To make the townhouse more appealing and sellable, several structural modifications were undertaken, such as underpinning to increase the height of the basement space, removal of long-term sag in the floor joists, and the creation of a double-height space. Pattress plates and liner walls were used to remediate cracks between party walls and adjacent facades.
At first, the developer asked SGH to evaluate the condition of the wood floor joists and to determine if they could be salvaged. The team found deterioration at some joists on the upper floors, which had been long exposed to water from the leaking roof and skylights, but the majority of the joists were adequate. Nevertheless, the contractor and the owner wanted to eliminate the long-term sag in the floor and asked us to design wood “sisters” to render the floor level. This work became moot, though, when the interior finishes were removed, exposing the interior face of the masonry walls.
At the four corners of the building, the team observed wide vertical cracks at the stack-bond joint between the facades and the party walls. In the worst case, they noted cracks up to 5/8 inch wide, rotated sections of the masonry, and exposed wood studs supporting the interior finishes of the neighbor’s townhouse. At the second floor, cracks extended from the floor line up the party walls diagonally to the roofline, revealing an original A-frame roof construction. Together the vertical and diagonal cracks eliminated the structural connection between the facades and the party walls, rendering the facades as free-standing walls. Positive anchorage was required to prevent the wall from “peeling away” and potentially falling to the street.
|
SGH restored the connection between the facades and the party walls using an anchor-rod-and-pattress-plate system, and rehabilitated the building using a concrete masonry unit (CMU) liner wall inside the brick masonry walls. The team designed the 6-inch-thick CMU liner wall to be installed at the inside perimeter of all four exterior walls for the full height of the building, while maintaining the masonry openings at the doors and windows, creating a “box-within-a-box” structural system. The liner wall supports the new floors and resists code-prescribed wind loads (spanning between floor diaphragms out-of-plane and acting as a shear wall in-plane). The existing masonry facades are connected to the liner wall via a 1 inch grouted collar joint and stainless steel helical ties at 2 feet on-center vertically and horizontally; the project used Simpson Strong-Tie’s Simpson Heli-Ties. In addition, SGH designed an anchor-rod-and-pattress-plate system consisting of custom-fabricated star-shaped pattress plates at the exterior brick facade that are connected to 5/8 inch diameter threaded, stainless-steel rods, which are grouted into a bond beam of the liner wall. New York City’s Landmarks Preservation Commission mandated the star shape of the pattress plates.
The new liner wall presented the team the opportunity to use light-gauge steel C-joists rather than requiring that the existing wood floor joists be salvaged. This was a distinct advantage since the contractor could install the C-joists more quickly than he could sister the wood joists. Furthermore, the C-joists could be installed level, which was one of the developer’s requirements. The new floor structure consists of 10-inch-deep C-joists with double headers at the chimneys and stair openings and 3/4-inch plywood sheathing. The plywood panels are attached to the C-joists, forming an un-blocked diaphragm and perimeter light-gage straps to resist the chord forces of the diaphragm.
To support the liner wall, we designed underpinning of the existing foundation to spread the increased load over a larger area. The underpinning design was also an opportunity to increase the headroom of the original half-height basement, which could then house finished guest bedrooms and a mechanical room. Underpinning is common in New York City where space is scarce, and owners can gain additional square footage by lowering the basement. Underpinning is commonly achieved by the “pit method,” in which new concrete piers are placed under the existing rubble foundations. Typically, the foundation contractor excavates 4 foot wide pits under the existing foundations in a four-stage process such that the minimum distance between simultaneous cuts is 12 feet, allowing the existing foundation to arch between pits. After a concrete pier has reached 50 percent of its 28-day strength, steel wedges are driven between the existing foundation and the underpinning pier to transfer the gravity load. This process is repeated sequentially along the length of the foundations until it is fully underpinned. In addition, building movement and structural cracks are monitored during underpinning.
Often the underpinning and the existing foundation must be temporarily braced to support the lateral thrust from additional unbalanced earth pressures that result from the increased height of the foundation system, as was our case. SGH designed a bi-level raker system that temporarily braced the existing rubble foundation wall until the CMU liner wall was installed. In the end, the engineers designed a 2-foot-deep reinforced concrete underpinning and a 5 inch slab-on-grade, which increased the floor-to-floor height from about 5 feet to about 8 feet.
|
To create a stronger visual connection to the backyard garden and to increase daylight into the parlor and garden floors, the architect’s design showed the removal of a portion of the rear, brick-masonry wall, and the adjacent parlor floor to create a dramatic double-height space and a large picture window. Removing the floor diaphragm and the masonry to such a large extent left only a 2 foot, 7 inch pier between the large picture window and the adjacent door across the whole rear elevation. Instead of CMU, the team designed an 8 inch thick reinforced concrete wall over the bottom three stories at the rear elevation. The reinforced concrete provided the necessary strength, but also required that the existing rear wall be braced during the concrete placement to ensure that the fluid pressure of the fresh concrete did not overstress the existing masonry wall.
At the top of the building, a partial floor-plate penthouse with an entertainment room, wet bar, and roof deck was added. To frame the penthouse, we extended the CMU liner wall above the existing masonry party walls to form the side walls of the penthouse, and used light-gauge steel covered with plywood sheathing at the windowed front and rear walls and at the roof. Each of the light-gauge steel exterior walls sits on a 10-inch-deep HSS section that spans between the CMU-lined party walls.
The renovation of 112 Washington Place transformed the neglected building into a modern and historically inspired luxury townhouse, but it was no easy feat; the design and detailing for the strengthening and improvements of this 3,500-square-foot building was similar to that of a much larger project. The structural improvements complimented each other in that the addition of the liner wall simplified the installation of the new flooring, the penthouse addition, the enlargement of the rear window, and underpinning of the foundations. All together, the structural renovations helped to achieve the vision of the developer and architect, and to restore this major “fixer-upper” to its former grandeur.
Rebecca A. Melton, P.E., LEED AP, is senior staff I – structures and Kevin C. Poulin, Ph.D., P.E., is an associate principal, both at Simpson Gumpertz & Heger Inc. They can be reached at ramelton@sgh.com and kcpoulin@sgh.com,respectively.
