What is Underpinning?
Underpinning is a process used to repair, strengthen, or increase the depth of an existing foundation by lowering the footing to allow it to rest on more supportive soil.
While oftentimes underpinning is associated with the remediation of deficient or failing foundations, it is also used in cases where the use of a building has changed, floors are being added to upper stories, or additional depth is desired in subsurface spaces, such as basements or cellars. In dense urban locations, such as New York City, underpinning is also a common practice during the construction of adjoining, adjacent, or nearby structures that require the removal or excavation of the soil supporting the neighboring properties.
The process of underpinning begins by removing or excavating the soil from beneath an existing foundation.
To avoid the risk of undermining the foundation, which may lead to structural failure, the removal of the soil is performed controlled stages, called ‘pins’, of limited length. The depth of the excavation is determined by a geotechnical engineer, who assesses the soil composition to identify the strata that is suitable to bear the weight of the building. The excavated soil is replaced with new material, typically concrete, which forms a new foundation beneath the existing one. Once one of the ‘pins’ is complete, and the concrete is cured, the process is repeated on the next section of the foundation until the entire length of the wall is reinforced.
There are multiple methods of underpinning including:
- Mass Pour
- Beam and Base
- and Mini-Piled
The mass pour method is the most common due to its low cost and ability to resist heavy foundation loads. In this method, a solid, continuous concrete foundation is poured beneath the existing foundation is sections. While this method uses a great deal of material, it is the simplest method to engineer, does not require heavy machinery, and can allow for continuity of use during construction.
A more technically advanced method is the beam and base method, where a reinforced concrete beam is constructed below the entire foundation to replace the existing footing. In this method, the new beam transfers the load to a mass concrete base which spreads the load evenly across the soil. While more advanced in its design, the feasibility of this methods largely depends on the structural configuration of the building above the foundation.
Where ground conditions are variable or access around the area of the foundation is limited, the mini-piled method of underpinning may be used. In this method, ‘piles’ or deep vertical structural elements are driven into the ground in drilled holes deep enough to allow the piles to rest on stable soil. The piles typically extend at least 15’ below ground, but depending on the soil condition, are capable to depths of over 50’. While this method can overcome even the most adverse soil conditions, the engineering is more involved, and the process can prove to be quite expensive due to the technical expertise and specialty equipment required.
If you are planning on performing structural work on your existing property – either elective or as part of a remediation program – it is important to hire a firm with extensive experience working closely with structural engineers, geotechnical engineers, and the NYC Department of Buildings to mitigate the risk of negatively impacting your property and those adjacent to you.
If you have any questions about performing work on your existing building, do not hesitate to contact us.Read more...
The Rehabilitation of The Tracy Mansion
The Tracy Mansion, located on 8th Avenue between President and Carroll Street in the Park Slope Historic District, is a spectacular 50’ wide mansion that is considered the single most notable example of a Neo-Classical townhouse in the district. The house was designed by renowned architect Frank J. Helme for the John Tracy family, who made their fortune in the shipping and transportation business in the late 19th and early 20th centuries.
Helmes’ was tasked with creating a mansion that would rival those on Manhattan’s Upper East and West Sides. His design features a symmetrical marble façade with a large, curved central bay with fluted Corinthian columns supporting an entablature with a decorative frieze. The bronze and lead glass arched entrance door is flanked on either side by large monumental windows with pedimented tops set within a rusticated stone base, which serves as the sills for the third-floor windows.
The magnificence of the exterior is matched on the interior, where a vaulted marble-paneled entry foyer featuring stained glass and decorative plaster work on the ceiling leads visitors to the front great rooms, themselves resplendent with intricate carved woodwork, art-glass, and colossal marble fireplaces.
The house was occupied by the Tracy family until the early 1940s and it was shortly used as a meeting house for the Knights of Columbus until it was ultimately sold and converted into the Park Slope Montessori School in 1969. After being used for classrooms for more than four decades, the house was purchased by the developers in 2012, with the intent of adding to and converting the mansion into condominium units that would retain as much of the original historic fabric as
possible. The conversion of the house from a single-family home to a multi-family residence sought to avoid many of the failings of adaptive reuse and conversion projects, which often alter or eliminate much of the historic fabric.
Recognizing the inherent value in the historic features, architect Leonard Colchamiro, and preservation architect Scott Henson, prioritized the retention of the mansion’s most notable features while minimizing the visual impact of the alterations such as the rear and penthouse additions. On the exterior, the historic façade was given new life with the restoration of the original ironwork and the bronze entry doors, the restoration of the original wood windows and the cleaning of the marble façade, among other things.
To make the project feasible, the developers determined that they would need to expand the house with rear and rooftop additions that would increase the footprint of the original house and add an additional penthouse unit on the roof. Working closely with the Landmarks Preservation Commission, Scott Henson Architect and Leonard Colchamiro Architect proposed several alterations to the original scheme that would work to make the additions more sympathetic to the historic building and reduce its visibility from President and Carroll Street. This included the incorporation of several setbacks, both front and rear, that would distinguish the volume of the original building from the addition and make the rear balconies less visible to neighbors, and refining the composition of the rear elevation to better match that of the original building.
After five years of work on the conversion and restoration of the historic Tracy Mansion in the Park Slope Historic District, the home has been returned to its original grandeur and it has (once again) been reborn as a residence worthy of its rich history and iconic presence on 8th Avenue.Read more...