February 5, 2019
by Taylor Aikin, AIA
For architects and building teams working in historic contexts, structural glass systems can offer compelling design solutions that can improve functionality while simultaneously highlighting and contrasting with the spaces’ original fabric and character. However, challenges with the integration of glass systems that relate to design development, specification, fabrication, and installation do raise a host of considerations that may not be immediately apparent. Examining a recent renovation program at a New York City landmark, the St. Patrick’s Cathedral, provides a useful case study of effective design strategies for melding modern glass expression with historic preservation and also renovation scopes. Exploring these solutions would offer insight into the benefits and the challenges that are inherent in this approach.
Led by Murphy Burnham & Buttrick Architects (MBB), the conservation and renovation of the St. Patrick’s Cathedral was a multifaceted undertaking spanning 10 years and $175 million of construction and improvements on a campus occupying an entire city block. Designed by American architect James Renwick, Jr., in the Gothic Revival style and constructed between 1858 and 1888, the cathedral is a designated city, state, and federal landmark and one of New York’s most-visited destinations. The architecture and preservation team at MBB helped develop a comprehensive project approach focused on stabilizing the cathedral structure and also providing a better experience for clergy, visitors, and worshippers—all while preserving the character of this sacred and iconic building.
In addition to completing exacting preservation work—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. Along with Silman and Eckersley O’Callaghan Engineers, construction manager Structure Tone, and two glass fabricators, the design team developed two major insertions:
Contemporary approach to preservation and facility goals
When designing for historic preservation work, why would an architect consider the use of modern glass systems? In the case of the St. Patrick’s Cathedral renovation, glass structures presented a means of turning several of the cathedral’s facility operations challenges into opportunities for improving functionality, increasing community connections, and enhancing appreciation of an iconic landmark. Just as importantly, glass both maximizes transparency and minimizes the impact of interventions on the original historic fabric. For the Archdiocese of New York’s leadership, using glass in the St. Patrick’s Cathedral renovation presented an opportunity to develop architectural elements that also improve the building’s expression of the congregation’s core values: openness, a welcoming atmosphere, and a sense of sanctuary and refuge.
The cathedral entrance, for example, is symbolic of those principles. Throwing open the large, 4082-kg (9000-lb) bronze doors serves as a welcoming gesture, signifying to visitors and passersby alike this centrally located complex is open and available as a physical and spiritual resource. However, keeping the doors open would compromise energy efficiency. The archdiocese leaders sought a solution with a level of functionality that was also in keeping with how the building is used.
Since the scope of restoration work included significant upgrades to St. Patrick Cathedral’s mechanical plant, MBB suggested creating sliding glass pocket doors that could be hidden, but would also create a transparent barrier affording views down the nave when in the closed position. This minimal glass entry creates a sustainable and sensitive solution, allowing the original doors to remain open year round while minimizing heat and air-conditioning loss. The discreet glass panels also work to emphasize the original architecture.
Given the historical importance and complexity of the project, a sensitive touch was necessary. The motorized glass pocket doors sit on a series of bearings allowing them to slide into the wall with the push of a button. This sliding-door hardware system is engineered for large structural glass panels. To create a pocket for the doors to slide into the existing structure, the project team shifted the interior wall by 101 mm (4 in.) and removed some of the stone entirely.
This design solution, which could be considered for a range of historic settings, required careful planning and significant consideration of preservation logistics. In this case, the doorframe’s limestone surround was removed, cut down 127 mm (5 in.), and reinstalled to accommodate the channels.
The work of setting the glass entry doors into place was made even more complicated by the need to keep the cathedral open to visitors and worshippers throughout the process. Intelligent and precise coordination and scheduling was instrumental to success. The project team conducted preparation work during the day by simply cordoning off the area around the entry doorframe, maintaining public access to the cathedral’s interior. Installation work took place at night and in the early morning. Working with an architectural metal and glass fabricator and installer, Structure Tone orchestrated closing off a portion of Fifth Avenue so the glass assemblies could be delivered and set into their pockets.
A minimalist enclosure and technical challenges encountered
In addition to improving the cathedral entry, archdiocese leaders sought to enhance the functionality of interior worship spaces. Over time, the popularity of St. Patrick’s Cathedral and the sheer number of daily visitors had acutely impacted the ability of its Lady Chapel, an addition on the east side of the cathedral built in the early 1900s, to serve as a quiet, intimate space. Responding to the changing needs of worshippers and the visiting public required creating an enclosure that could provide acoustic separation between the main cathedral areas and the chapel itself. Any intervention also had to preserve the original character of the spaces and allow them to remain quiet while still also feeling contiguous and consistent with the rest of the structure’s architecture.
As with the glass entry doors, for the Lady Chapel enclosure, the project team was committed to a design approach that complemented and worked
in concert with the existing aesthetic. A carefully detailed glass structure again proved to be the most sensitive solution. Employing an iterative design process in collaboration with the glass fabricator, the architecture team gradually reduced this separation to its essentials. Early schemes featured substantial framing with steel beams and wood tracery responding to the adjacent cathedral space. As the design and engineering developed, the team formulated a solution that would eliminate excess framing material while creating an all-glass assembly.
A high level of technological and engineering sophistication was required to achieve the resulting liminal gesture and its required level of performance, from the design of the installations to the suitable materials and detailing. 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—now fill the Lady Chapel’s 14-m (48-ft) high arch. The installation presents a light and effortless sensibility, yet is a significant presence. Most importantly, it improves facility performance without negatively impacting the surrounding architecture or creating any visual barrier around the Lady Chapel.
While St. Patrick’s Cathedral is an iconic landmark and destination for visitors, it is also a thriving house of worship. There are up to 18 said masses performed each day and 150 weddings each year. With an acoustically separated chapel, the cathedral can accommodate simultaneous ceremonies (e.g. worship, weddings, funerals) in the chapel and the nave. Perhaps more importantly, the Lady Chapel was conceived as a sanctuary within a sanctuary—a place for quiet prayer and devotion. In a building that now sees more than 5 million visitors each year, creating a space of quiet refuge is more vital than ever to the cathedral’s mission as a spiritual home.
With transparency as a crucial element, this glass structure sits in a minimal and carefully detailed bronze frame. Aiming for the highest possible level of visual integration with the cathedral, the project team treated the bronze to match the historic architecture. Acid treatment brought the metal from a bright gloss to a darker finish, calibrated to match the historic bronze doors at the front of the cathedral. The frame itself is entirely machine fabricated, but the acid treatment had to be hand applied. A traditional crafts specialist sponged the acid on from a bucket, then washed it back off after a prescribed time period to achieve the desired finish.
The skill of the project team’s various tradespeople and craftspeople allowed the design team to go above and beyond, adding small touches that would bring outsized value. For example, using custom door hardware allowed MBB’s team to design a quatrefoil seal—the symbol of St. Patrick’s Cathedral—for the inner side of the bronze door pulls on
the glass doors into the Lady Chapel enclosure. This attention to detail in deference to the historic architecture was important for cathedral leaders. A key moment in the design and planning process took place on a trip to Germany, where archdiocese representatives became convinced of the appropriateness of an all-glass intervention after seeing a mockup of the assembly in the glass manufacturer’s workshop.
Skilled collaborators, advanced technology for complex site conditions
Before any glass even came into the cathedral, the project team had to design and install the bronze frame to fit perfectly into the Lady Chapel’s historic plaster arch, marble columns, and floor. This was an exacting process, as all the existing building components are irregularly shaped. To document the structure, the project team engaged digital surveyor Hypsometric to undertake a full 3D scan of all interfaces. Using a terrestrial light detection and ranging (LIDAR) scanner to create a virtual model of the building components enabled the project team to form and shape the structure to follow the irregularities of the historic architecture, but remain regularized enough to create clean visual lines. The historic columns presented a particular challenge to the scanning tools, as subsurface refraction within the marble returned inaccurate data. The team compensated by covering the columns with painter’s tape for the scan, then adjusting for the thickness of the tape in the final point-cloud model.
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.
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.
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.
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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