New York City Hall
Challenge
A series of cracks had appeared in the brick masonry arches and walls at New York City Hall in Lower Manhattan. The client needed to repair the masonry cracks and to stabilize the structure of the landmarked building, which was completed in 1812. The renovation included addressing movement of the marble cantilevered stairs in the rotunda, built using a flat arch system without columns or other support—rarely seen outside Europe. Scheduling presented additional challenges. Much of the work took place at night, and critical inspections required precise timing.
Solution
Atkinson-Noland & Associates provided specialized repair engineering support to the engineer of record. We used surface-penetrating radar to identify wood framing in council chambers and meeting room, and we evaluated marble stairs for voids with ultrasonic pulse velocity testing. In addition, laboratory testing determined absorptive properties of the brick masonry vaults, which influenced the mix design of compatible injection fill (CIF), a specialized grout. Next, we developed crack injection protocols, from specification of CIF formulations to wall preparation and injection. We also provided quality assurance, observing CIF preparation and installation and using nondestructive testing to verify repairs.
Solutions Provided
Compatible injection fill (CIF) is customized to the material properties of the host structure, based on nondestructive evaluation and materials characteristics testing. Technicians pump a fluid, cement-based grout mixture into cracks, voids, or cavities within masonry. Low injection pressures prevent damage, while strengthening already fragile materials.
We use an in-house laboratory for testing historic and modern masonry material properties. Work performed includes characterization of brick, block, and stone properties; mortar analysis; anchor strength; and grout development, following standard test methods developed by ASTM, ACI, the State of California, and others.
Surface-penetrating radar (SPR), also referred to as ground-penetrating radar or impulse radar, provides valuable information about structural and non-structural building components without causing damage. The radar data reveals voids, construction layers, and the presence of other materials, such as metal inclusions, as well as the thickness of the element.
An ultrasonic pulser/receiver unit initiates a timing circuit as it sends an electrical signal to the source transducer, which uses an internal piezoelectric crystal to generate a low-energy, high-frequency stress wave. Transducers are attached to the masonry surface with rubber sheets or gels for maximum energy transmission. The wave travels through the section, where the receiving transducer converts wave energy back to electrical energy. Pulse transmission time is displayed in microseconds on a readout display.
A fiber-optic borescope with a video attachment is inserted in small holes drilled into mortar joints to evaluate internal wall conditions. Videoscope investigations provide detailed information about internal wall features, while eliminating the need for destructive probe openings. These devices use an internal light source to illuminate the interior space, and some have a graduated scale in the viewfinder to aid in identifying and sizing objects.
Result
We designed CIFs to match the material properties of the existing masonry and the sweeping main stair. Grout injection stabilized the masonry walls and vaults, ensuring composite action of the masonry—especially important for the rotunda stairs, in which treads must maintain intimate contact for the support system to function. Post-injection testing revealed very few areas where the CIF did not fill voids completely, and these areas were re-injected.
Project
New York City Hall
New York, New York
Client
Beyer Blinder Belle
New York, New York
Partners
Masonry Solutions International
Robert Silman Associates