Preservation pathologists

A building investigation need not be prompted by a design or construction misstep or a catastrophic failure. The renovation of historic structures, for example, often involves an investigation and evaluation phase. Thornton Tomasetti is performing such an evaluation in its role as building envelope consultant for the owner of the Williamsburgh Savings Bank in Brooklyn, New York. Developer Dermot Company is converting the late 1920s former office and bank building into condominiums.

The 35-story "early skyscraper" is supported by a steel frame but is clad in foot-thick brick-masonry as solid and substantial as though it were load-bearing, but without provision for thermal movement. "There were no expansion joints in this era," explains Thornton Tomasetti's Hammarberg. As a result, the skin has suffered extensive cracking.

A facade evaluation and restoration is part of a project to convert the Williamsburgh Savings Bank in Brooklyn, New York (left), into condominiums. Image: Courtesy Thornton Tomasetti

In order to document these cracks and find the most appropriate locations for new expansion joints, the envelope consultants needed access to the entire exterior of the 512-foot-tall tower. On this project, as with many similar facade evaluations, Thornton Tomasetti used rope access rather than the more common swing stage or frame scaffolding. Rope access is typically less expensive and provides investigators with more flexibility because there are no platforms, outriggers, or counterweights, explains Kent Diebolt, president of Vertical Access, the building inspection subcontractor. The system, which relies on two static ropes, one for fall protection and one for descent control, also intrudes less on the building fabric, he adds.

In an "almost real time" process, the Vertical Access surveyors documented their finds using hand-held computers, and sent the files to Thornton Tomasetti each night. The surveyors noted the cracks' locations, width, and amount of displacement. "Then we identified areas where we wanted probes," says Hammarberg.

Deteriorated window heads (top) were one of many conditions revealed by the envelope-consultant's survey. Repair (above) will include replacement of specialized brick with cast stone. Image: Courtesy Thornton Tomasetti

The investigation also revealed a unique window-head condition. What looks like a soldier course of brick is instead a course of custom brick notched to hide a steel lintel. The shape is extremely vulnerable to thermal expansion and contraction, and many have cracked and broken. The consultants have specified cast stone as a replacement, primarily because of specialized brick's long lead time and the tight construction schedule, which requires that facade work be completed by next summer.

Thornton Tomasetti's recommendations include replacing elements that are broken, while salvaging those that are still serviceable. For the building's terra-cotta decoration, the firm is recommending using epoxy to repair broken units wherever possible, and replacing those beyond repair with the original material. Terra-cotta is lighter than many substitutes, and its thermal performance is akin to the adjacent materials, explains Hammarberg. He is confident that this "surgical" approach will serve the building well, and that the work will be a "50- to 100-year repair."