Ingenuity and Technology Breathe New Life into Historic Terra-cotta Half Columns

Thornton Tomasetti, exterior, Metamorphosis Awards

1st Place, Exterior

Manhattan’s Ladies’ Mile Historic District is home to some of New York City’s finest examples of late 19th and early 20th century architecture. Among these is 114 Fifth Ave., an 18-story commercial building designed by Maynicke & Franke that was completed in 1909. The brick, limestone and granite masonry structure recently underwent an extensive restoration program after a city-mandated façade inspection deemed its signature terra-cotta half columns to be unsafe.

Thornton Tomasetti provided the critical façade inspection and evaluation, as well as structural renewal services to L&L Holding Co. for the project.

UNIQUE AND CHALLENGING PROJECT

The 350,000-square-foot tower, also known as the Merchants’ Exchange Building, sits on the corner of Fifth Avenue and 17th Street. The top three stories of the façade are clad in matte-glazed ornamental terra cotta, including seven 42-inch-diameter, fluted terra-cotta half columns. During New York City-required critical façade examinations, these terra-cotta half columns were classified as unsafe because of severe deterioration (See photos 1 and 2).

The building façade underwent a comprehensive restoration program involving a creative approach in temporary protection and structural stabilization. The team employed a unique solution to remove and restore the 3-story-tall terra-cotta half columns. This project was particularly challenging and required innovative approaches for working in unsafe conditions, time constraints and other complexities involved in restoring a historic building. Because the building is located in a historic district, all work needed to be approved by the New York City Landmarks Preservation Commission.

RESTORATION OF TERRA-COTTA HALF COLUMNS

A key aspect of the project was the removal and replacement of the fluted terra-cotta half columns. The team needed to develop a plan for protecting the unsafe terra-cotta column cladding while working on the structural and architectural design for the repair and its stabilization while the work was in progress. There also was the issue of eliminating the unsafe cladding while maintaining the integrity of the exposed elements for a prolonged period as the new terra-cotta units were being manufactured. Furthermore, the team had to reconfigure the terra-cotta cladding support scheme to overcome previous support deficiencies.

Recognizing the project’s structural and schedule challenges, the team’s goal was to quickly implement a temporary stabilization program to prevent the fragments of cracked terra-cotta units from falling to the streets below. A high-tensile strength, anti-shatter clear polyurethane coating temporarily protected the severely cracked terra-cotta units and served as an emergency repair (see Photo 3).

From a supplemental invasive probe investigation, the project team determined the original supports for the terra-cotta masonry units—stress-relieving steel brackets—only provided a sufficient bearing surface to the front-facing portion of the terra-cotta half columns (see Photo 4). The bracket extension toward the periphery of half columns did not extend far enough to provide full support. This resulted in a non-relieving condition toward the sides of the half columns. Over a prolonged period, the steel bracket corroded, resulting in large, full-height vertical shear cracks at the two mirrored sides of the half columns (refer to Figure 1). The severity of the cracked and unstable terra-cotta units made it necessary to completely rebuild the half columns. A creative restoration approach was essential to preserving the original characteristics of this landmark building façade.

The project posed additional challenges in navigating the unsafe conditions, which were reported to the New York City Department of Buildings (DOB). During the early stages of the repair construction, the column capitals were stabilized using anchors and temporarily shored using a cantilever framing hung from the roof. The project team was then able to remove the unsafe terra-cotta units.

Because of the anticipated long lead-time required to manufacture the new cladding units, it was crucial the integrity and watertightness of the exposed elements was maintained. To ensure this, the embedded steel columns, exposed when the terra-cotta cladding was removed, were cleaned and painted with a corrosion-inhibiting coating system. This was followed by an application of a lathed trowel-applied, weather-resistant cementitious fireproofing that is compatible with painted steel (see Photo 5). DOB accepted this innovative stabilization solution to bring the exposed embedded steel columns to a Safe with a Repair and Maintenance Program, or SWARMP in DOB jargon, condition, pending the completion of the full restoration.

The original terra-cotta half column construction involved open back-face units with a solid brick masonry backup. The design goal was to reduce the overall weight and provide a well-drained column cladding system. This was accomplished by eliminating the brick masonry backup. The new terra-cotta cladding units were then stacked up with three levels of steel brackets with a semi-circular plate to allow for proper bearing. Shop-fabricated stainless-steel anchors, welded to the backup steel columns, provide lateral support for the units. The team used appropriate welding electrodes to connect stainless-steel anchors to the original carbon-steel columns to avoid galvanic corrosion. At the contact points of the anchors with the terra-cotta units, custom-fabricated neoprene pads and sleeve jackets were used to mitigate localized stress.

To provide a path for the incidental moisture that entered the system, the terra-cotta cladding units were fabricated with vertical open cells as opposed to the original open back-face units. The relieving steel bracket plates were provided with holes to allow for entrapped moisture drainage for the full height of the column. Refer to Figure 2 for the bracket design concept and bracket installation in progress. The base of columns was waterproofed using a fluid-applied PMMA, or poly methyl methacrylate, waterproofing membrane and copper flashing, and the lowest terra-cotta units along with the bed joint were provided with weeps to allow for moisture egress.

With various field-condition constraints, particular importance was given to the construction sequence; the team worked closely with the contractor to identify creative solutions for the challenges that arose during the cladding assembly installation.

Exacting Standards

The project, which was completed in February 2021, successfully navigated the technical and logistical challenges of restoring large-scale terra-cotta columns while meeting the requirements of local government agencies. The team was able to repair key structural steel supporting components and breathe new life into the historic masonry façade to exacting preservation standards while improving performance and longevity.

“Scope is relatively small, but the technical expertise of the design team is immense. The restoration is indistinguishable from adjacent existing construction, demonstrating extensive attention to detail in finish color and texture alongside the technical detailing. … Wonderful demonstrations of the use of current technology in preservation.”

Ross Welch, AIA, NOMA, LEED Green Associate, associate, Trivers, Metamorphosis Awards Judge

Retrofit Team

METAMORPHOSIS AWARD WINNER, STRUCTURAL ENGINEER AND

BUILDING ENVELOPE CONSULTANT: Thornton Tomasetti

HISTORIC PRESERVATION CONSULTANT: Higgins Quasebarth & Partners LLC

GENERAL CONTRACTOR: Flag Waterproofing and Restoration

Materials

NEW TERRA-COTTA MANUFACTURER: Boston Valley Terra Cotta

ANTI-SHATTER COATING: Decothane Clearglaze from Sika Liquid Plastics

GROUT-SOCK ANCHORS: Cintec

POLYURETHANE PAINT: PerimePrime from Tnemec

WEATHER and FIREPROOFING OF EXPOSED STEEL: Monokote Z-156 from GCP

LIQUID WATERPROOFING: Parapro Fluid-applied Membrane from Siplast

TERRA-COTTA and STONE MASONRY PATCHES: Various mortars by Jahn Restoration Systems

REPLACEMENT BRICKS: Glen-Gery

MASONRY MORTAR: Spec Mix


About the Author

Kunal Badheka, P.E.
Kunal Badheka, P.E., is an associate principal in Thornton Tomasetti’s Renewal practice.

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