Few buildings are as iconic as Willis Tower. Generations of Chicagoans have a collective memory of this building playing a role in their entire lives. We mark time with Willis Tower, but time has caught up with this aging supertall. The way the building engaged with the city and its occupants needed a fresh approach. Understanding how Willis Tower is being reimagined by its new owners is crucial to the success of old and new supertall towers around the globe. A new city block-sized podium structure and substantial infrastructure improvements are part of this work, and the results have a dramatic effect on a piece of civic history while transforming the building into a destination for tenants and visitors alike.
It is only fitting that since Chicago is the birthplace of the skyscraper and the supertall building typology, the solutions to how we ensure a vital urban center, which includes legacy tall buildings from the late mid-century modern movement, is explored with this project. The design team was conscious of supporting a people-centric planning approach and understanding how people move in the city and how the new podium building is part of their daily routines.
This article will describe how the Gensler design team along with Thornton Tomasetti Façade and Structural Engineering selected several high performing glass systems for this project on behalf of the building owner’s EQ Office and The Blackstone Group. Crucial to the success of the glass systems was the involvement of Glass Solutions, Inc. and Novum Structures who performed design-assist façade contractor roles under the General Contractor, Turner-Clayco Joint Venture. Other critical partners on this project include RL Edward Partners who served as program managers and Environmental Systems Design who performed as MEP engineers and energy modeling consultants.
Project Design Drivers
In 2015, The Blackstone Group purchased Willis Tower with the goal of taking this signature property into the future with substantial building and infrastructure upgrades. Gensler was engaged to study and research possibilities for how this iconic building could meet the needs of current and future tenants and the surrounding urban environment. The design team and ownership identified many drivers for the success of the project. We will look at a few of the drivers which directly relate to the selection of façade glass systems. The goal was to create an inviting and scale appropriate destination for building tenants and the city at large. The result had to be a new icon that merged the legacy Sears/Willis iconography with both old and new Chicago building iconography.
Porosity, Transparency and an Active Streetscape
An important lesson from earlier street level designs of Willis Tower was the need for a porous and transparent grade level façade. The Gensler team knew this approach would help activate the streetscape, particularly at night when the building could become a beacon of sorts, letting people know that there are food and beverage businesses open after the workday ends. This was important to the owners as they looked to extend the time that the building was generating revenue and to create a living urban streetscape. The original tower plinth design at street level featured large expanses of stone-clad walls and storefront systems with dark tinted glass which did not create an environment that welcomed human activity. Success for this new project meant an active streetscape and an inviting presence.
The design team therefore proposed street elevations that incorporated oversized glass units with no tint and a visible light percentage of 65% and 79%. Gensler set the façade rhythm to the existing tower grid and worked with our Landscape Architect partners, OLIN, to carefully align the building grid with the paving and streetscape design. The result feels orderly and has a degree of openness that welcomes views into and out of the new building. Retail canopies made of steel and glass mitigate the scale of the new building at street level while secondarily afford opportunities for signage and lighting.
Main Lobby Façade System 1 (FS-1) and Exterior Glass 1 (EGL-1)
The Main Lobby of Willis Tower is located on Wacker Drive and is enclosed by Façade System 1 (FS-1). This façade is composed of custom solid steel mullion members which are 3 ½” wide by 9” and 18” deep. Solid steel mullion members were incorporated into the design in order to facilitate oversize glass units (5’ wide by 12’-6” high) and to achieve the 35’ vertical span for this elevation. The major members of this façade system also carry the load of the lobby skylight and pedestrian bridge above. A pre-finished aluminum carrier frame secures the IGUs to the solid steel members. The carrier frame system was beneficial for setting large glass units due to the lack of construction staging areas in this dense part of downtown Chicago.
Façade System 1 uses glass type EGL-1 which is a Viracon IGU composed of all Pilkinton Optiwhite low-iron extra clear substrates with a laminated inner light. The outer light is 5/16” (8mm) thick with Viracon VE-85 low emissivity coating on the #2 surface. A 1/2” (13.2mm) argon gas-filled space separates the laminated inner light composed of two pieces of 1/4” (6mm) Optiwhite and a .06” (1.52mm) clear PVB interlayer. This overall composition yields a Visible Light Transmittance of 79%, a U-Value Winter of .26 Btu/(hr x sqft x degF), a Shading Coefficient of .69 and an Exterior Reflectance Value of 13%.
This coating was specifically chosen for the high visible light factor which is a key element in connecting the lobby to the streetscape and urban fabric. The lobby elevation is inset from the main building line and receives shading from the canopy and side walls which makes the high visible light factor work with the adjacent building massing.
Podium Façade System 3 (FS-3) and Exterior Glass 3 (EGL-3)
The new Podium Building portion of Willis Tower houses retail, food and beverage tenants at Levels 1, 1.5 and 2. On Level 3 are spaces devoted to meeting rooms and large gatherings. Façade System 3 (FS-3) is composed of unitized curtain wall panels with custom extrusions by Glass Solutions, Inc. The unitized façade panels are typically 5’ wide and range from 25’ to 30’ feet high with glass units spanning up to 11’-4” high. The lack of construction staging areas drove the decision to unitize the curtain wall and the large panel sizes enabled the contractor to efficiently install their work while also benefiting from the ability to shop glaze the panels.
Façade System 3 uses glass type EGL-3 which is a Viracon IGU composed of all Pilkington Optiwhite low-iron extra clear substrates with a laminated inner light. The outer light is 5/16” (8mm) thick with Viracon VNE-63 low emissivity coating on the #2 surface. A 1/2” (13.2mm) argon gas-filled space separates the laminated inner light composed of two pieces of 1/4” (6mm) Optiwhite and a .06” (1.52mm) clear PVB interlayer. This overall composition yields a Visible Light Transmittance of 65%, a U-Value Winter of .24 Btu/(hr x sqft x degF), a Shading Coefficient of .33, and an Exterior Reflectance Value of 10%.
This coating was specifically chosen for the need to respond to two different programmatic uses behind the glass. The grade-level retail and food and beverage tenants need low exterior reflectance values. These tenants want people walking by to be able to see into the spaces instead of seeing their own reflection. The upper-level meeting room areas, however, need more solar control even with the use of blackout shades when projectors or television monitors are in use for presentations. Since this is the majority of glazing for the new building, the energy performance has to be very good. The design team felt this coating also worked well at night when internal illumination can be seen in the immediate neighborhood.
Main Lobby Skylight Façade System 2 (FS-2) and Exterior Glass 2 (EGL-2)
Transparency extends to views of the iconic tower itself. The amount of programmatic square footage required for the project and the footprint this would occupy on the site created a challenge for the goal of exposing views of the tower. Gensler incorporated several design moves in order to provide a visual connection for building occupants and visitors. The first is a 75’ wide by 30’ long skylight at the Wacker lobby. This skylight joins to the tower itself and affords views straight up to the top. It also greets tenants and visitors as they enter the building and provides a visual connection to where they are headed. The skylight is supported by custom solid steel beams which are 3 1/2” wide by 18” deep and solid steel 3 ½” wide by 6 ½” deep purlins. The IGUs are secured onto the solid steel purlins and beams and sloped away from the existing tower.
Skylight Façade System 2 uses glass type EGL-2 which is a Viracon IGU composed of all Pilkinton Optiwhite low-iron extra clear substrates with a laminated inner light. The outer light is 3/8” (10mm) thick with Viracon VRE-38 low emissivity coating on the #2 surface. A frit pattern composed of 1/8” diameter high opacity white dots in an offset pattern with 20% coverage was utilized to aid in solar control for the lobby space below. A 1/2” (13.2mm) argon gas-filled space separates the laminated inner light composed of two pieces of 1/4” (6mm) Optiwhite and a .06” (1.52mm) clear PVB interlayer. This overall composition yields a Visible Light Transmittance of 32%, a U-Value Winter of .24 Btu/(hr x sqft x degF), a Shading Coefficient of .24 and an Exterior Reflectance Value of 45%. This glass assembly composition was specifically chosen for its ability to control internal glare in the lobby without the use of tinted glass substrates. Thornton Tomasetti Façade Engineering performed a reflection analysis of this skylight to determine if the reflected daylight would cause issues with adjacent neighboring properties. They concluded that there was a very low risk of incidence.
Skylight glass selection is extremely challenging. A careful balance needs to be struck between designing for worst-case sunlight days, overcast days, and energy performance. Tinted glass substrates are often used but these can alter the quality of light in the space below with less than desirable results. The design team felt that the EGL-2 glass type found a proper equilibrium by using a somewhat more reflective coating combined with a frit pattern in lieu of relying on tinted glass.
Atrium Skylight Façade System 4 (FS4) and Exterior Glass 4 (EGL-4)
The other skylight occurs over the Jackson Boulevard atrium known as Catalog. This is a custom, double curved 75’ by 85’ skylight which frames spectacular views of the tower, serving to remind visitors of the iconic power of the SOM designed building. The skylight framing is composed of 4” wide by 8” deep hollow steel sections with steel connection nodes by Novum Structures. The toggle glazed IGUs are set off the HSS members with stainless steel posts and a proprietary system of clamp plates.
The double-curved geometry of the skylight is rotated 45 degrees from the tower grid with a system of faceted 6’-8” by 6’-8” IGUs. The skylight of two pieces of 5/16” (8mm) low iron and a .06” (1.52mm) clear SGP interlayer. This overall compostion yields a Visible Light Transmittance of 32%, a U-Value Winter of .28 Btu/(hr x sqft x degF), a Shading Coefficient of .30 and an Exterior Reflectance Value of 35%. This glass assembly composition was specifically chosen for its ability to control internal glare in the atrium without the use of tinted glass substrates.
Thornton Tomasetti Façade Engineering performed a reflection analysis of this skylight to determine if the reflected daylight would cause issues with adjacent neighboring properties. They again concluded that there was a very low risk of incidence. The design team again faced the challenge of balancing the desire for abundant natural light in both sunny and overcast conditions but still being able to meet intense energy savings goals. This atrium space had the added issue of being less of a transition space than the lobby as people will be resting here or having a meal during the day. The frit pattern coverage was therefore increased to 30%, lowering the exterior reflective value, a benefit at the occupied green roof level above the atrium.
Glass selection for buildings with mixed-use programs is inherently challenging. Each program will have its own target levels of acceptable visible light, shading coefficients, and exterior reflectance values to name a few. Some of these spaces will be transitory while others will have occupants spending greater amounts of time. Energy codes effectively place limits on glazing system performance as the mechanical system of a building can only offset a limited amount of lower-performing glass. Glass must, therefore, perform at a high level and from an aesthetic design perspective, a uniform or at least complementary appearance is desired.
Finding a single glass for vertical or horizontal applications is therefore not an easy task. Cost can also drive many decisions. Our experience on this project led us to aim for a middle ground of somewhat higher visible light transmittance and very low exterior reflectance for the vertical glass. For horizontal glass, we chose a much lower visible light transmittance by using a more reflective coating with a frit pattern instead of relying on tinted substrates. These two methods supply a degree of uniformity over the façade while also being flexible when the orientation of the surface required a different result.
This article was originally published in IGS Magazines Winter 2019 Issue: Read the full Magazine here for more thought-leadership from those spearheading the industry
Author: Stephen Katz, AIA, LEED AP BD+C. Senior Associate and Technical Director at Gensler
Stephen Katz is a Senior Associate, Technical Director and Regional Office Buildings Practice Area Leader at Gensler. Stephen has worked and lectured in the United States, Asia and Europe and has authored papers about façade design, intelligent building technology and sustainability. Stephen is the founder of Gensler Enclosures; a group dedicated to innovation and research for building enclosure design.
Stephen has played strategic positions on award winning projects including the Johnson Controls Asia-Pacific Headquarters, the Willis Tower Repositioning Project and the Kohler Global Communications Headquarters to name a few. He favors an inclusive design process which recognizes the importance of teamwork, listening and the power of imagination. Stephen is a member of the American Institute of Architects, the AIA National Building Performance Advisory Group and is a LEED Accredited Professional. He holds a Bachelor of Arts degree from Hobart College and a Master of Architecture from Washington University in St. Louis.