This is part 3 of 6
Survey and investigation
Through most of the design process, Corbin remained occupied by its various tenants. Even access to spaces owned by part of the client body, New York City Transit (NYCT), was far from straightforward, as the subway system typically operates 24/7 and most of the areas owned by NYCT were egress corridors or staircases that couldn’t be closed without significant advance warning. Only the basement and sub-basement could be freely accessed with little nuisance or disruption to tenants — which was fortunate as these spaces, which had remained without finishes or decoration, provided much useful information on the original construction.
Fortunately some of the original structural drawings had survived since 1898 (copies obtained from the NYC Department of Buildings), and these showed rudimentary framing plans and beam loading data, together with some details of cast iron column sizes. Some discrete field surveying and testing were also allowed — essential for preparing the design documentation — and the design team used these windows of opportunity for a series of focused studies. These started with visual inspections and then focused on specific areas of interest, or later, as the design progressed, on areas that showed an important need for information. Nevertheless, as with all existing building projects, there were information gaps that could only be closed out once construction began, requiring the design to react and evolve rapidly in response to any unforeseen conditions.
The following summarizes the investigation work:
• structural visual survey inside and outside
• architectural spaces survey determining size and configuration of internal walls (many were later partitions masking the original structure)
• levels survey within the historic stair core, agreed to be the location taken as the datum for the building
• 3-D topographical surveying, including a limited survey of façade alignment
• photogrammetric survey of the façade for HABS documentation
• structural and architectural survey of the external façade by hoist (requiring a 24-hour closure of the street in front of the building)
• structural probes:
– through partitions to determine walls behind
– at beam and column locations
– at critical connections/interfaces (eg column brackets)
– to take samples for materials testing
– through the Guastavino floor vaults and cinder fill above
– to help determine critical masonry wall thicknesses
– to establish sizes and thickness of beams to match against published historical data and information in the NYC Department of Buildings drawings.
• materials testing:
– existing masonry and mortar (compression)
– suspected cast iron columns (tensile, chemical and weldability)
– suspected wrought iron beams (tensile, chemical and weldability).
The measured size and strength of the wrought iron beams gave the designers an immediate problem: based on historic values of design strength for the period, the floor beams could not carry the design floor loads. Testing showed the wrought iron to have a tensile yield strength close to 30ksi (207Mpa), but contemporary design would have limited the typical flexural design capacity to 12ksi (83Mpa) (working loads) due to variability in quality of manufacture. This lower value was not enough to justify even the existing floor condition and loading under current design codes.
Pursuing a solution, the team managed to access an area of floor slab in the basement for an in situ load test of the existing floor system, fully monitored by strain gauges. This was enough to demonstrate its capacity, given typical office live loadings plus a 50% factor of safety consistent with current codes and standards. However, this also placed a design constraint that existing office loadings should be maintained, as well as requiring that existing floor construction weights should be mimicked in the new design to avoid reduction in allowable live loads.
Originally published in the Arup Journal, November 11, 2014. Authors include: Ian Buckley, Craig Covil & Ricardo Pittella.
Coming up next: The Corbin Building, Fulton Center: Rediscovering and Renewing An Architectural Gem – Part 4