Glass structures in historic milieux

As St. Patrick’s Cathedral needed to remain open to visitors during the day, a skilled installation team worked at night and into the early morning to successfully fix the glass wall.
As St. Patrick’s Cathedral needed to remain open to visitors during the day, a skilled installation team worked at night and into the early morning to successfully fix the glass wall.

Where lines of sight were restricted and a higher degree of precision was required, the glass manufacturer utilized a hand-held digital scribing tool linked to a total station-surveying device to document the profile of the column capitals.

The advanced digital surveying technology allowed the design team to determine all these parameters in the design phase, before any fabrication occurred. All architectural irregularities were scanned, so the components could be fabricated overseas to close tolerances and precise specifications. Fitting the new structure into the stone and plaster arch required complicated detailing work to accommodate movement of the glass. The 3D scan allowed the team to set precise inside and outside tolerance limits, guaranteeing components machine fabricated overseas would fit when they arrived onsite.

Since the project team’s fundamental design approach was any intervention could be installed— and, if necessary, removed—without significant damage to the historic structure, working around the arch’s columns presented a particular challenge. On the column capitals, for example, the frame’s support brackets are machined with notches so they can be slotted precisely into and around the capitals without causing any damage to the stone. Additionally, the bronze frame itself is scribed and cut to fit precisely around the capitals and also connected by a silicone joint providing acoustic separation.

The Lady Chapel enclosure consists of three 38-mm (1 ½-in.) thick, triple-ply laminated glass panels—each weighing up to 2267 kg (5000 lb) and entirely supported by a horizontal, 101-mm (4-in.) wide, 7-m (24-ft) long laminated glass beam.
The Lady Chapel enclosure consists of three 38-mm (1 ½-in.) thick, triple-ply laminated glass panels—each weighing up to 2267 kg (5000 lb) and entirely supported by a horizontal, 101-mm (4-in.) wide, 7-m (24-ft) long laminated glass beam.

While precise measurement and fabrication were critical to the process, the interface between the new intervention and the historic elements also required a series of carefully considered design decisions about alignments, priorities, and the preservation of the existing architecture. The sum of these decisions produced a contemporary installation that feels entirely integrated into the historic building.

Structural installation and team reinforcements

In order to ensure successful results for the installation, the glass fabrication team included specialists in the skilled orchestration required for this kind of work. During a six-week process, the work was staged on a side street by the southern side of the cathedral to protect the structure and interior spaces.

The project team first moved all the pews and built a wooden ramp platform allowing the components to be delivered to the Lady Chapel. Next, a gantry was constructed to lift and manipulate the glass panels via a series of counter-weighted suction fittings. Attached to a scaffolding assembly and sliding back and forth on rails, this gantry was operated by a team of installers from the glass fabricator via a chain-and-pulley system. Supporting the work was a temporary floor structure made of steel and plywood.

Given the limited space in which the team could operate, the surrounding historic architecture was susceptible to damage. Minimal clearances from the gantry to the plaster ceiling above, for example, meant the largest panel could only be lifted to 25 mm (1 in.) above its final installation height. As a result, the panels were placed through the arch diagonally, then manipulated into a vertical position from either side of the enclosure until they sat properly within the frame.

Installing the Lady Chapel enclosure’s glass panels required constructing a hydraulic gantry to lift and manipulate the glass panels via a series of counter-weighted suction fittings.
Installing the Lady Chapel enclosure’s glass panels required constructing a hydraulic gantry to lift and manipulate the glass panels via a series of counter-weighted suction fittings.

Working with an experienced project team was critical to success. Despite the enclosure’s near-weightless appearance, the structural gymnastics that support the glass within, below, and above the space are remarkable. For example, the beam supporting the upper glass is susceptible to bowing and twisting under the material’s weight, but is stabilized by the lid of the vestibule, which acts as a diaphragm and stabilizes it along its length. However, because the installation sequence required vestibule assembly to come last—after the horizontal beam and the three vertical panels—an intermediary bracing structure was necessary to steady the beam until the vestibule lid was in place.

To address this challenge, the team engineered a series of C-shaped aluminum knife plates supported by a wood frame, which slotted into the joints between the glass panels to support and brace the beam until the vestibule was installed. The rails and frame for the vestibule itself were engineered to be as minimal as possible while still supporting the glass. Shaving down every possible millimeter, the team arrived at a 40-mm2 (1 9/16-si) solid bronze profile.

Floor reinforcements were also necessary. The existing steel-framed floor (which sits above a thin, Guastavino tile ceiling) did not have the bearing capacity to support the weight of the new glass wall, so Silman engineered a series of wide-flange steel members to span between and reinforce it. The new steel had to be installed in sections and limited to 152 mm (6 in.) in depth to preserve the decorative ceiling below. The portions of ceiling that were removed were rebuilt by Deerpath Construction, an expert in historic masonry, using a combination of reclaimed tiles and new ones made to match. Though the ceiling is currently painted, forensic analysis showed the original tile had a decorative glaze. The new tiles were glazed to match this, then painted to coordinate with the current ceiling. This will allow the reconstructed tiles to blend with the originals if the paint is ever removed in a future restoration project.

The project team also incorporated significant reinforcing measures into the attic space above the Lady Chapel ceiling. Another set of steel beams span wall to wall here, with diagonal steel members carrying the lateral load from the glass wall. All these new steel support structures had to be brought up into the attic in smaller pieces, then assembled and installed within this tight space.

In addition to completing exacting preservation work at St. Patrick’s Cathedral—including conservation of exterior and interior stone, wood, and plaster, as well as stabilization and conservation of stained glass—MBB worked with a multidisciplinary team to incorporate several new glass elements entirely integrated into the historic building. Photo © Floto+Warner
In addition to completing exacting preservation work at St. Patrick’s Cathedral—including conservation of exterior and interior stone, wood, and plaster, as well as stabilization and conservation of stained glass—MBB worked with a multidisciplinary team to incorporate several new glass elements entirely integrated into the historic building.
Photo © Floto+Warner

Beyond the engineering of the supporting structure, successfully enclosing the Lady Chapel also necessitated modifying the mechanical systems. Sealing off this space required a new return air duct path to be routed through the attic above the glass wall. MBB worked with custom craftspeople to create HVAC grilles allowing return air to flow back into the main cathedral space. Shaped, in these experts’ words, as a “torqued paraboloid,” the highly complex and elegant design of these grilles allows them to slot seamlessly into the ceiling above the archway, fully integrated into the architecture. This small element of a much-larger undertaking encapsulates the project team’s successful approach to St. Patrick’s Cathedral, seeking at all times a level of design functionality that works in service of both historic context and contemporary need.

Respecting the past while looking forward

Exploring design solutions that use contemporary architectural practices and materials, such as architectural glass, in the service of historic buildings shows a minimalist approach is sometimes the most sensitive, contextual answer. It can also be the most effective means of meeting multifaceted facility needs. The successful incorporation of glass structures into the iconic St. Patrick’s Cathedral underlines how smart choices and a strategic approach can help architects and building teams turn preservation and renovation challenges into opportunities to improve functionality, increase community connections, and enhance historic fabric.

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