Boston high-rise finds perfect fit in unitized curtain wall system

by arslan_ahmed | November 4, 2022 1:34 pm

Photos courtesy Binyan Studios

By Alexander Donovan, LEED AP

When designing and building high-profile high-rises, a unitized curtain wall system is often the favored facade solution, for more reasons than just highlighting the skyline. The benefits of this approach range from manufacturing, quality control, onsite installation speed, and enhanced water resistance.

When the colonial-style Singaporean brand selected a prominent corner site in Boston as the location for its first North American mixed-use property, it was no surprise the project team chose a unitized curtain wall facade system for the building. Raffles Boston Back Bay Hotel & Residences, which is well under construction and slated to be open in 2023, will be a 35-story tower, set to hold 147 guestrooms, and 146 residential units.

With a complex set of environmental and contextual conditions, the project’s specification and design approach offers valuable takeaways for building teams engaged in urban high-rise design and construction with curtain wall systems. The case history also sheds light on key considerations and best practices in such applications.

One immediate challenge for the project team was site context: the building lot is set just 20 m (65 ft) from the largest all-glass building in Boston. 200 Clarendon is an iconic structure, colloquially known as the John Hancock Tower, or simply the Hancock. Designing in proximity to a local landmark is always complex. A primary design goal for the Raffles tower—which includes many elements of glass specification—was how to create a new skyline statement of subordinate scale and make it visually stand apart from the taller Hancock, while respecting the tower’s importance to the city’s urban fabric. How the team resolved this is explored further in this article as well.

The construction process involved the simultaneous use of both creeper cranes and a tower crane, allowing the building team to enclose the tower on lower levels while forming and laying concrete slabs on upper levels. Photos courtesy The Architectural Team Inc. (TAT)

Initial considerations

Initially, the project team explored several potential materials and dozens of design iterations. For instance, they investigated combinations of glass areas, mixed with opaque materials, such as precast concrete and composite aluminum panels.

The owner-developer group preferred a primarily glass facade, and the building team, along with structural engineers, focused on a unitized system for its benefits, which allowed for a faster construction process and earlier occupation.

The glass selection process then began with the client making an informed decision based on their aesthetic and performance goals for the structure. For the Raffles tower, the project team shared several precedents, and referenced high-rise buildings around the world to consider many options. Questions began with aesthetic and visual performance parameters. For example, is this building’s facade too reflective or not enough? Is this color in line with the vision for how the tower will look on the skyline? Will the tinted glass be clear enough when viewed from inside? These questions set basic expectations to help drive downstream the selection criteria.

Subsequently, the project team approached manufacturers to request glass panel samples used in the specific buildings, which were seen as benchmarks. From there, discussions refined the list to four or five specific glass assemblies. Then, they provided samples from other buildings, manufacturers, and fabricators to their chosen manufacturer in Colombia. This helped establish what the manufacturer could deliver based on the provided precedents, performance, and aesthetics, such as specific percentages of reflectivity and translucence.

Building program and glass specification

The Raffles planned program is a mix of residential and hospitality uses, alongside significant amenity spaces. Each of these three spaces are typically served with a different type of vision glass. Residential glass tends to be clear to render exterior colors as true as possible; whereas in a hotel or in public areas, there is a higher tolerance and even a demand for tinting or other glass treatment. To combine these different properties into a single building, it is best to avoid a facade which looks striated because of glass panels with different levels of reflectivity or transparency on a portion of the building.

In addition to the tower’s vision glass, the size of the building called for a significant amount of spandrel glass, and the opaque glazing covering the edges of the floor decks and components such as building columns, heat pumps, and other elements had to be obscured from the view. To make the spandrel glass fully opaque, the inside surface was back painted, which creates a perceptible clash with the appearance of the vision glass, particularly if the contrast is not intended as a deliberate design feature.

When designing a visually coherent glass facade for a mixed-use building, the structure itself can be an obstacle. Columns on the Raffles tower’s hotel level are at a different location than those on the residential levels or in some of the larger public areas. Even within residential units, varying layouts dictate the location of heat pumps and other elements along the building perimeter. All these considerations meant the pattern of opaque spandrel glass would vary from on every floor. Selecting the right glass, for any project, becomes a crucial design choice for the resulting patterns to not appear as checkerboards across the building’s facade.

Table summarizing key factors for the assembly and why the team chose them.

Choosing the right vision glass is also critical in making a coherent visual statement. In most lighting conditions, the spandrel glass can blend in with the vision glass when the latter has either a higher reflectivity or darker color. By increasing reflectivity, it is possible to adjust the average for every piece to appear closer to the designed values of hue, reflectivity, and the like. Since the Raffles client group desired a lower reflectivity, the project
team had to strike a delicate balance.

The solution was to slightly increase the reflectivity and then compensate by moving to a darker vision glass color to blend with the spandrel glass; it would not be fully black or brown, rather, a warm tone with hints of bronze which picks up the tone of the composite metal panel trim. Further, the deeper hue and reflectivity level of this glazing offers the benefit of differentiating the Raffles tower’s appearance from the bright blue, highly reflective curtain wall of the Hancock—providing a respectful and complementary foil to this iconic neighbor. Another advantage of these choices was its improved solar heat gain performance, which helped the tower meet local code-mandated requirements for a Leadership in Energy and Environmental Design (LEED) gold certification.

Acoustic solutions

Designing a curtain wall system for hospitality brings interesting and unexpected constraints beyond aesthetics. To maintain a comfortable and luxurious interior environment, the Raffles Hotels & Resorts brand requires strict limits on the amount of outside noise transferring through the exterior wall into the building interiors. Controlling the Outdoor/Indoor Transmission Class (OITC), which is essentially a Sound Transmission Class (STC) rating system for exterior wall, is complicated by a curtain wall design, as it cannot receive added insulation or denser substrates to increase acoustical performance. The same is the case with many opaque assemblies.

For this project, the design team found another creative solution. A standard glazing unit with a 25.4 mm (1 in.) sandwich of glass and argon gas, comprising 12.7 mm (0.5 in.) of airspace, with a 6.35 mm (0.25 in.) glass pane on either side. An acoustical consultant assisted in devising a glass assembly to mitigate as much sound as possible within these dimensions.

The result was an asymmetric assembly in which the outside face of glass is 7.94 mm (0.3125 in.), and the inside pane is slightly thinner than 6.35 mm (0.25 in.). This very small increase over the standard exterior thickness proved to have a significant positive impact on the level of noise transferring from the outside, and the balance of the asymmetry notably mitigated external sounds without affecting the depth of the glazing pocket within the extruded aluminum mullions.

Instead of using curved glass to form the building’s rounded profile, smaller individual facade modules were created with gently faceted mullions to create the same profile when assembled.

This solution, driven more by hotel brand guidelines rather than the local jurisdiction’s ordinances or building codes, brought visual benefits and acoustic improvements. Standard glass panes can pillow or deform, a phenomenon in which the pane is thin enough to move under pressures, such as wind. The effect increases as the glass lites increase in size and the center of the lite moves further away from the aluminum support frames, causing the center of the glass to move in and out to a much greater degree than at the edges. At a thickness of 6.35 mm (0.25 in.), there would have been a significant amount of movement on the Raffles facade’s glass panels.

By increasing the lite thickness to 7.94 mm (0.3125 in.) on the exterior pane, the glass was better stabilized, and the pillowing effect was noticeably reduced. This also made it look flatter and more consistent across the facade than it otherwise would have, particularly for a building with large pieces of glass at the various spaces spread throughout the project.

Wind considerations

A consideration for all tall buildings is wind performance. Many building professionals will recall the adjacent Hancock Building’s early wind-induced glass panel failures, which led to more conservative design standards in the industry. Today, Boston’s Article 80 development review process requires projects of certain sizes and at certain locations
to undergo types of wind studies, each measuring a different impact on the project.

For example, this project required a pedestrian comfort study, which analyzes wind movement at sidewalk level. A lateral load study, which explores how wind affects the base building structure, especially the columns, lateral bracing systems, and the foundation. The final study, which is a cladding load study measuring the extent of the wind forces that are either pushing or pulling on the exterior surface of the building.

These tests involved a physical scale model constructed from project drawings and it was then placed on a large-format model of Boston and subjected to wind tunnel testing. With an array of highly tuned sensor points, the wind tunnel team was able measure how much wind load exists at different points on the building’s exterior. All these tests and studies helped set parameters for the facade system. In addition, the information attained from the wind studies was given to the general contractor and fabricator to ensure the engineering of the curtain wall system matched the design requirements to resist anticipated wind loads.

The Raffles tower faces a challenging set of wind considerations. It is sited on one of the windiest blocks in Boston, due to its orientation and the proximity to the Hancock, which shifts the prevailing winds. Due of these environmental factors, the wind studies played a much larger role in the Raffles tower’s design and development than on a typical urban project.

Located in Boston, the Raffles tower is 20 m (65 ft) from 200 Clarendon, also known colloquially as the Hancock, therefore a curtain wall facade solution was used to differentiate between the two structures. Photo courtesy Binyan Studios

For example, the tests showed prevailing winds come across the face of the Hancock Building and hit the Raffles tower directly perpendicular on its northern facade, the Stuart Street side, before pushing downward. The design decision to create a curved profile to the building aided the wind performance by forming an airfoil effect, deflecting wind pressures around its perimeter and reducing some of the high-pressure critical zones. In conjunction with the color and reflectivity of the glass itself, the curvilinear profile also offered a strong point of formal differentiation from the adjacent Hancock. Another realization from the studies was the Raffles tower needing an entrance canopy to project over the sidewalk for a significant distance to prevent the wind energy from moving straight downwards and onto pedestrians.

Instead of using curved glass to form the building’s rounded profile, the project team decided to create smaller individual facade modules with gently faceted mullions, which created the same profile when assembled. This generated a narrow, tall shape for the modules—one bay wide but three lites high—to accommodate the spandrel glass covering the deck edge, the vision glass, and a band of operable windows at the residences. For those, the project team selected awning-type windows, which have certain benefits from a performance and weather perspective. Since the units are hinged at the top and the bottom pushes out as the windows open, the top is sealed and the air comes in from the bottom, acting as a roof and shedding any water over the face of the window.

Curtain wall-building structure interface

Wind and related environmental conditions, including potential seismic forces, also influenced the curtain wall design, the connections between the facade system, and the building structure.

For this as well, structural wind load modeling and other environmental testing, including seismic racking tests, revealed to the design team how much and what types of movement they should expect from the building, including forward, backward, and twisting or torsion forces. An especially critical measure and indicator is the interstory drift ratio (IDR), which describes the relative translational displacement between two consecutive building floors, divided by the height of a given story. Having an accurate IDR is critical for designing the right amount of movement into the curtain wall and anchoring system, and for understanding the range of conditions in which the curtain wall and building structure will need to perform.

For the Raffles project and other sites with similar conditions, a worst-case scenario might call for 25.4 mm (1 in.) of movement, whereas a typical condition would be closer to 12.7 mm (0.5 in.). In both extreme and normal conditions, facade flexibility is key, and this is another advantage of a unitized curtain wall approach, which offers inherent flexibility from panel to panel because it is a system of individual units attached to the building and they overlap at gasketed connection points. Additionally, there are horizontal “stack joints” designed to accommodate differential movement between the building stories.

As a result of extensive wind studies, the design team created a curved profile for the Raffles tower, because the airfoil effect can deflect wind pressures around the building’s perimeter.

At the Raffles tower, the gaskets are primarily made of silicone, reflecting the flexible but inert nature of the material, which is non-reactive to materials. As with all other elements touching the glass, most gaskets are shop-applied in a controlled factory environment to ensure proper installation conditions. A benefit of contemporary module fabrication systems is how the gaskets can also
be replaced before the units are erected if they
are damaged, without the need for any structural changes to the module. Project teams can easily slide an old gasket out if it is cut or ripped and replace
it with a new piece. However, in general, the only elements which are field modified or modifiable are the interfaces between the panels.

In a related process, to effectively install and secure the curtain wall panels, the team had to choose which types of structural anchors would connect the curtain wall to the building structure. This is another element which warranted careful consideration due to the tower’s curved profile. On a typical building with
a more rectilinear shape, the contractor will cast
or weld anchors into the exterior face of the concrete or steel floor deck and attach the curtain wall to those anchors. However, the Raffles tower’s structure is ovoid and faceted, therefore anchors resembling embedded plates on the horizontal top surface were installed on each floor deck, offering the ability
to adjust them to match the curve of the building.

This system requires a series of specifically engineered clips and hangers designed to parameters and set by the envelope consultant as a result of the wind studies. The anchor assembly begins with the embedded piece itself, essentially a metal box, with reinforcing rebar projecting from its form. Since this is a cast-in-place concrete building, the contractor was required to install the pieces into the formwork before they placed the concrete, allowing the rebar
to be tied into the other floor and become reinforced.

The embed plates themselves are a very thin steel, which helps to shape and form a void in the concrete. Once the concrete is poured and the glass is mounted, heavy-duty aluminum clips hold the curtain wall back to the structure. The T-anchor shape of the clips enable a degree of adjustability. Once they are attached and tested, the void is grouted to be flush with the adjacent concrete floor deck.

Mock-up testing

After establishing the basic facade design of the Raffles tower, there was a rigorous mock-up process to test its viability and performance, with two initial mock-up modules in total. The primary module was built at a testing laboratory in New Hampshire, as a full two-story section of the building, including the terrace doors, an inside and outside corner. Construction of the 12.2 m (40 ft) wide and 7.62 m (25 ft) tall module took a week and it only fit inside the lab facility. The experts, who would eventually handle the final installation at the project site, also built the mock-up module to identify any immediate erection or fabrication issues.

The mix of uses within the Raffles tower, including hotel rooms, made the glass selection process a very important balancing act, as each use traditionally brings different requirements for reflectivity and clarity.

The module was placed under pressure, with air and water being spayed to simulate winds and rain at specific velocities, determined by the earlier wind studies, varying water, and humidity conditions. Once this initial baseline testing was performed, the entire mock-up was subjected to structural tests meant to mimic the real-life forces exerted on the building. This determined the limits of allowable movement within the system. After the module was stressed, the same air and water tests were performed to ensure the final system could withstand the forces placed on the building. Lastly, a glass unit was removed and reinstalled to simulate a field repair on the building. This reglazed section was then tested to ensure the field-glazed unit would perform identically to the factory-built units. The cycle testing process lasted for a week to fully determine how the curtain wall modules would stand up under the specific conditions facing a high-rise at the project site.

The project team, including architects, engineers, and curtain wall consultants and installers, attended the testing sessions, observing the erection and overseeing the tests themselves. The curtain wall consultant then generated a report recommending several fabrication changes to ease the field installation. This process resulted in several sequence changes for the assembly of parts and identified two areas where the waterproofing membrane, specified between the unitized curtain wall, and the adjacent composite metal panel rainscreen system needed to be modified.

The second mock-up for visual performance was built locally by the general contractor in Boston for visual review and approval by city agencies. Staff from the Boston Planning and Development Authority (BPDA) came onsite to review the vision glass, the spandrel glass, the proportions of the horizontal bands and fins. The final set of drawings was then reviewed and approved.

Project teams continue the testing process after the mock-up stage as well. As the facade installation continues at the Raffles tower, for example, there is an ongoing test schedule involving the architect, contractor, and curtain wall consultant. Currently, the project team is conducting active air and water testing across segments of the facade to ensure it performs up to the anticipated water and airtightness levels. This kind of testing also extends to specific facade elements, including terrace doors and building corners.

Curtain wall systems vary in terms of carbon footprint, depending on factors including the type and size of mullions and other metal elements, as well as the size of the panels themselves. In terms of performance, curtain wall systems can reach a high level of efficiency when the balance between opaque insulated areas and vision glass is correctly balanced. During the design process for the Raffles tower, the project team actively tracked and monitored this ratio down to the square inch to meet the building’s thermal performance and carbon goals.

Installation best practices

While the Raffles project does have a tower crane onsite, in fact, nearly all the facade glass is installed by creeper cranes, which are small, portable cranes that sit within the building structure two or three levels above the floor, and using those, the modules are being installed. There are several functional advantages to this approach. For one, because the modules can be loaded directly onto the crane from within the building, this method eliminates the need to hoist large-format glass modules from ground level to high elevations, which reduces risk and minimizes complex and overlapping logistics on urban streets.

In a related vein, the creeper crane approach also frees up the project’s tower crane for other functions, including work on the superstructure. This creates
a much higher level of efficiency because it allows for simultaneous work, where concrete slabs can be formed and placed on upper levels at the same time the curtain wall is being installed on floors below. At the Raffles project site, taking this approach meant the building was almost fully enclosed by the time the structure topped out, with skilled trades conducting work inside the building at
the same time.

As Raffles Boston Back Bay Hotel & Residences rapidly nears completion, it is evident the unitized curtain wall facade system played a significant role in realizing the client’s desire for a memorable skyline statement and an efficient, comfortable, and high-performing hotel and residential destination. While every curtain wall project is unique and requires an individualized set of solutions to overcome its own challenges, the Raffles tower should help building teams across the globe feel confident about this robust process and how it is possible to achieve successful results with the collaboration of trusted project partners.


Alexander Donovan is a senior project manager with The Architectural Team, Inc. (TAT), an award-winning architecture, interiors, and planning firm based in Chelsea, Massachusetts. With more than two decades of experience in design and construction for multifamily, mixed-use commercial, and hospitality buildings, Donovan is regarded as an expert source on advanced facade solutions.

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