IIBEC 2020 Virtual International Conve ntion & Trade Show | June 12-14, 2020 O ’Brie n and Kaske l | 45 Air Force Academy Cadet Chapel: Mocking-Up the Reclad Design of an Icon William O’Brien, Jr., REWC, PE; and Bruce Kaskel Wiss, Janney, Elstner Associates, Inc. 10 South LaSalle Street, Suite 2600, Chicago, IL 60603 312-372-0555 • wobrien@wje.com 46 | O’Brie n and Kaske l IIBEC 2020 Virtual International Conve ntion & Trade Show | June 12-14, 2020 William “Bill” O’Brien has contributed to numerous building enclosures projects, primarily involving glass curtainwalls, window assemblies, metal panels, masonry walls, precast concrete, cast-in-place concrete, waterproofing, and roofing systems. Prior to joining Wiss, Janney, Elstner (WJE) in 2012, O’Brien was part of the building sciences consulting group of Architectural Testing, Inc., at their York, PA, headquarters. As a graduate research assistant at Penn State University, his research was dedicated to advancing the understanding of the seismic performance of glass curtainwall and storefront systems. As a consultant for the Applied Technology Council (ATC), he developed analytical models to predict the seismic capacity of glazing systems as part of the developing performance-based design method. Since joining WJE in 1985, Bruce Kaskel has investigated and designed repairs for distress conditions in existing buildings. He has authored papers on exterior façade materials, glass and façade testing, the history of glazing systems, and structural failures of walls. He has presented seminars on aluminum and glass curtainwalls, exterior wall systems design and repairs, and lessons learned from cladding failures. Kaskel has guest lectured at Purdue University, Illinois Institute of Technology, and the School of the Art Institute of Chicago. He led WJE’s design effort on the recladding of the U.S. Air Force Academy Chapel as part of the AECOM team starting in 2015. The project is now under construction with anticipated completion in 2021. ABSTRACT SPEAKER The Air Force Academy Cadet Chapel—an iconic and landmarked structure—has experienced water leakage since it was built in 1963. The unique skin on the repeating tetrahedrons is sloped, with exterior water drainage and management behavior somewhere between a wall and a roof. This study will review the existing detailing conditions resulting in water leakage. A reclad design was developed to incorporate a new weather-resistant panel backup wall and inboard metal cladding panels matching the existing historical appearance. The building science principles governing the design development are examined. To evaluate the design, a laboratory mock-up was constructed and performance-tested. Challenges related to acquiring and constructing the custom repair design skin assemblies for a full-scale laboratory mock-up will be reviewed. Performance testing of the design included rounds of air, water, thermal, and structural load testing. Lessons learned from the performance testing and the resulting design modifications will be presented. IIBEC 2020 Virtual International Conve ntion & Trade Show | June 12-14, 2020 O ’Brie n and Kaske l | 47 AIR FORCE ACADEMY CADET CHAPEL Along the foothills of the Rocky Mountains, the 17 spires of the Cadet Chapel rise from the west edge of the United States Air Force Academy campus near Colorado Springs, Colorado. The campus is the design of Chicago-based Skidmore, Owings & Merrill (SOM), and Walter Netsch of SOM was the lead architect of the chapel. Construction began in 1959 and was completed in 1963. With spaces for Protestant, Catholic, Jewish, and all other faiths, the chapel is a much-used icon of this landmark campus and the most visited man-made tourist attraction in Colorado (Figure 1). Supported and hinged from a series of concrete buttresses, the structural steel tubes form a series of geometric tetrahedrons. There are over 100 tetrahedrons that stack in threes and join to meet at the spire. Leading and trailing edges of the tetrahedrons are 6-in. steel pipe. The truss chords between edges are 4-in. steel pipe. Continuous steel built-up box beams tie the tetrahedrons together at three levels of connection called “working points.” Consistent with the 7-ft. module used by SOM throughout the campus, the tetrahedrons are spaced on 14-ft. centerlines (Figure 2). The steel tetrahedron structure is the backbone for the distinctive anodized aluminum cladding. The aluminum trim highlights the hidden structure, while triangular and trapezoidal aluminum striated plank panels fill the space between trim. The striated plank panels are also anodized aluminum, but each 9-in.-wide plank has a slight alternating anodized finish that contributes to the beauty of the cladding. A 2-ft. zone between the edges of the tetrahedron structure is glazed Air Force Academy Cadet Chapel: Mocking-Up the Reclad Design of an Icon Figures 1A and 1B – The chapel as viewed from the south and west. Figure 2 – Partial image of chapel work lines from repair drawings with tetrahedrons and other façade areas highlighted. with strip windows made of an aluminum frame grid infilled with 1-in.-thick “dalles de verre” colored glass blocks ranging in color from cool blues and greens to warm yellow, orange, and red. The visual effects of the colored glass strips provide a stunning appearance both outside and inside the chapel (Figure 3). Opening ceremonies for the chapel commenced on September 22, 1963 (Figure 4). The chapel received the American Institute of Architects’ (AIA’s) National Twenty-Five Year Award in 1996. The entire SOM campus was designated as a National Register Historic District in 2004. THE WATER LEAKAGE PROBLEM As constructed, the chapel enclosure is essentially a barrier system, relying on sealant or elastomeric gaskets to resist water at joints at the face of the cladding. Any water that bypasses this barrier is no longer contained and can result in a water leak to the interior. These seals and gaskets with deficiencies have proven to be ineffective in stopping rainwater. A series of repair attempts to stop water intrusion that began early in the life of the chapel have unfortunately made an 48 | O’BrieIEn and KaSKEl IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 Figures 3A and 3B – The chapel interior and dalles de verre glass. Figures 4A and 4B – The chapel opening ceremonies commenced in September 1963. Left photo courtesy of USAFA, Special Collections; right photo by Stewarts Commercial Photographers, © Pikes Peak Library District, 013-9068. aesthetic impact to the building (Figure 5). After numerous attempts to repair, the Air Force Academy determined that a 21st-century solution would be required to adequately restore the cladding and to address the persistent leaks. In 2014, AECOM was retained and partnered with WJE and Hartman-Cox to design this cladding solution. CHAPEL RESTORATION DESIGN The goal of the restoration was to solve the water leakage problem while maintaining the aesthetic of the 1963 original chapel design. WJE provided the AECOM team with structure and enclosure consulting to investigate the condition of the existing structure, document the existing enclosure, and develop the enclosure repair design. Since the building is part of the Air Force Academy district on the National Register of Historic Places, Hartman-Cox took the AECOM team lead in ensuring that the enclosure repair design satisfied Section 106 requirements of the Preservation Act. Investigation Original construction documents were crucial for understanding the existing chapel conditions and developing a repair that would meet the historical aesthetic requirement. Available archive documents included original architectural drawings and construction documents. Of great interest to the design team were the structural steel and cladding shop drawings illustrating the intricate design details, which were found in the Air Force Academy Special Collections library. Unfortunately, due to the microfiche process on original large format drawings, the quality was often challenging to read (Figure 6). Following a careful review of these documents, the team made close-up inspections to confirm design details. These inspections were performed within the confined interstitial spaces of the tetrahedrons, fortunately aided by originally installed internal ladders and catwalks. The structural steel and welded steel connections were inspected in spot locations and found to be in serviceable condition despite the potential for corrosion due to the water leaks. Inspections of representative aluminum cladding connections to the structure confirmed the unique original shop drawing connection designs, which allow cladding thermal movements while resisting wind loads. Instrumentation was installed at several locations within tetrahedrons to collect seasonal data. Strain gauges, pressure gauges, and other monitors gathered data on structural movement and steel strains, which were calibrated against temperature and wind data from a nearby rooftop weather station. To confirm design-level wind forces, a wind tunnel study was per- IIBEC 2020 Virtual International Conve ntion & Trade Show | June 12-14, 2020 O ’Brie n and Kaske l | 49 Figure 5 – Storm windows (yellow arrows) and trim (blue arrow) repairs added to original construction. Figure 6 – Image at the left: original shop drawing enclosure detailing (courtesy of USAFA, Special Collections). Right image: inspection in existing interstitial tetrahedron space. formed on a scale model of the chapel, including the local Air Force campus. Since winds on the front range of the Rockies are highly local, another wind tunnel model included the nearby topography. Once these data were collected, the structural behavior of the building and the cladding effects were analyzed with computer software (SAP2000) to model design-level structure behavior under the loads determined from the wind tunnel study. The team had considered the possible need to augment the structure early in the design. However, following this analysis, no structural augmentation was deemed necessary. Enclosure Repair Design To solve the ongoing water leakage problem and ensure a long-term and durable repair, the chapel recladding was designed as a “rainscreen.” The rainscreen design resists most of the rainwater at the cladding face (the aesthetic face panel), while incidental water that gets past this cladding is stopped at the air and water barrier plane and then drained to the exterior. This approach provides a “belt-and-suspenders” 21st-century cladding solution for the chapel. It is critical that the weather barrier be continuous, as it is the primary air barrier and the last line of water infiltration defense. In the proposed design, the air and water barrier were integrated into insulated aluminum weather-resistant (weather) panels located between the tetrahedron tubular structural steel framing inboard of the cladding. Aluminum pipe clamps and saddle anchors were engineered to attach these weather panels to the steel frame. To provide continuity, aluminum covers were designed over the outstanding profiles of steel tubes and seals provided at all panel-to-cover joints. Aluminum “hubs” were designed over the complicated steel structure intersections. Covering the dalles de verre strip windows is a laminated clear-glass panel to replace the aesthetically unsympathetic storm windows that were installed years ago. The laminated glass is designed to minimize its appearance and to regain visibility of the colored glass from the exterior, while shielding the dalles de verre from exposure to rain, snow, and ice. A detail of the cladding and strip window design is shown in Figure 7, and an aluminum hub in Figure 8. DESIGN PHASE MOCK-UP Considering the custom and complex nature of the repair, a physical mock-up was desired to evaluate the design’s constructability, performance, and historical aesthetic prior to contract bidding. A mock-up would allow the opportunity to learn more about the performance of this design through a series of tests. Adjustments to the design could be made before the final design was completed for contractor bidding. 50 | O’BrieIEn and KaSKEl IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 Considering the custom and complex nature of the repair, a physical mock-up was desired to evaluate the design’s constructability, performance, and historical aesthetic prior to contract bidding. Figure 7 – Cladding and strip window trailing edge detail of repair drawings with weather-resistant plane highlighted in red and bulk water resistance highlighted in orange. Figure 8 – Aluminum “hub” detail at ridgeline of repair drawings. The Air Force agreed to this strategy and requested that the AECOM team proceed with the design and construction of this mock-up. At AECOM’s request, WJE led the mock-up design, construction, and testing effort. WJE obtained the vendors, materials, and installers to build two separate mock-ups to capture key design conditions (Figure 9). Both mock-ups included faithful replicas of portions of the steel structure based on the original structural steel shop drawings. Mock-up A was approximately 29 ft. high by 33 ft. wide. Mock-up B was approximately 29 ft. high and 23 ft. wide. The steel frame was manufactured off site, shipped to the testing laboratory, and then erected and attached to the laboratory chamber structure necessary for testing. To construct the aluminum cladding, custom extrusions were needed. Fabrication and extrusion die drawings were prepared for this process. Aluminum extrusions were produced from these dies. Quantities were based on assembly fabrication drawings. Extrusion samples were reviewed to verify proper profiles. Following extruding, the parts were anodized as specified. In total, 22 weather panels, 18 cladding panels, the strip window frames, pipe anchors, and all the other necessary parts were fabricated. For the weather panels, a weathertight face panel was fabricated by welding 1/8-in.-thick aluminum plates together. A perimeter aluminum extrusion was mechanically anchored and structurally sealed to the face panel edges. The cavity space of the weather panel was stiffened and filled with thermal mineral wool insulation before a metal backpan was fastened and sealed with foil-scrim kraft tape to the interior face of the panel. The weather covers were formed from bent aluminum sheets, and the hubs fabricated by welding sections of profiled aluminum sheets into various shapes. During fabrication, it was learned that hand-welding of the weather face panel was causing “oil-canning.” Since relatively flat panels were necessary for proper panel securement and weathertightness, a revised manufacturing process to automate seam welding of the face panel metal sheets was introduced to solve the issue. With the steel structure erected, pipe clamps and saddle anchors were installed using care to locate them correctly on the steel tubes; otherwise, the weather panels would not fit properly. While the anchors were designed to allow for field adjustment, their design was found to be challenging for the installers. The anchor design was simplified with input from the installers to ease the installation process. Revised IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 O o’BrIEn and KaSKEl | 51 Figure 9 – Locations of Mock-up A and B highlighted on a photo of the chapel. Figures 10A and 10B – Pipe anchor installation at mock-up (left); simplified pipe anchor installed (above). anchors were installed to verify efficacy (Figure 10). Weather panels were next installed and attached to these anchors (Figure 11). Once the weather panels were set, the aluminum covers and hubs were installed (Figure 12). Silicone sealant and pre-formed silicone sheet seals were used to seal the joints between the panels, covers, and hubs. Strip windows were assembled and glazed on site. Aluminum extrusions were fastened together to form a window frame in a stick-built fashion and attached to the structure with steel clips. The laminated glass was weather-sealed around the perimeter and mechanically secured on top with silicone rubber glazing seals. Joint sealant installed between the frame and the aluminum covers provided weather barrier continuity. Following weather panel and strip window installation, aluminum cladding panels were installed over half the final mock-up surface (Figure 13). The 9-in.-wide striated planks were assembled on site into their triangular and trapezoidal panels. The cladding panels were lifted and attached to the weather panels with anchor points welded on the weather panel faces to avoid penetrations. At the leading edge, a replica of the aluminum track for cleaning and maintenance scaffolding was added and bolted to the steel structure. Aluminum cladding trim was installed as one of the final elements. PERFORMANCE TESTING Water testing began during installation to check key components critical to performance, such as the directly exposed weather panels and sealant joints. Once the whole mock-up was completed, a rigorous 52 | O’BrieIEn and KaSKEl IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 Figure 12 – Installation of weather-resistant covers (yellow arrow) and hub (pink arrow). Figure 11 – Installation of weather panels. Figures 13A and 13B – View of Mock-up A on the left (with rainscreen cladding at right side) and Mock-up B on the right (with rainscreen cladding at left side). test program was performed to evaluate air- and watertightness, structural movement, wind loading, and thermal cycling. Testing guidance came from American Architectural Manufacturers Association (AAMA) and ASTM International standards. The first tests were of air infiltration and exfiltration, using ASTM E2831 as guidance, and performance was compared to project specifications. During this testing, smoke was used to identify sources of moving air to look for vulnerabilities (Figure 14). This indicated some deficient sealant conditions, including at strip window weather seals and weather panel joints, which were repaired and rendered airtight. It also allowed another opportunity to simplify the design (e.g., at sealant joints and connection locations) and lessen the risk of workmanship errors. Following air testing, water testing was performed with ASTM E3312 as guidance. A water spray rack created a large volume of water on the mock-up, while an air pressure difference was exerted across the mock-up wall. Another form of water testing was performed by conducting dynamic water testing with guidance from AAMA 501.1.3 The water is sprayed like the test reported above; however, an airplane engine fan is used to generate a dynamic wind on the exterior. This test is capable of forcing water in different directions on the façade, mimicking wind-driven rain. Leaks attributed to minor construction deficiencies were found during both water tests and easily corrected. A curiosity about the amount of water that can actually get behind the rainscreen cladding during dynamic testing led to the creation of two viewing ports through the weather panels to allow direct view of water on the weather panel (Figure 15). As anticipated, this revealed some water circumventing the cladding and flowing down the weather panels. Hence, it aided in the appreciation that incidental water needs to be controlled at the weather panels and drained harmlessly to the exterior. IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 O o’BrIEn and KaSKEl | 53 Figures 15A and 15B – Dynamic water test (left) with viewing portal (above). Figures 14A and 14B – Smoke testing revealed air leaks at gantry track penetrations (left, yellow arrows) and at a cladding anchor point conflict with perimeter sealant joint (right, pink arrow). Structural load testing followed air and water tests, using guidance from ASTM E330.4 The testing was a series of positive and negative load cycles at full design load and overload conditions. Deflections were measured during these tests to check the response of key structural components of the cladding. Finally, a thermal cycle test with guidance from AAMA 501.55 was performed, to observe the mock-up during low and high temperature extremes. Following these tests, air and water tests were repeated to re-evaluate performance. LESSONS LEARNED The process of designing, constructing, and testing the chapel mock-up resulted in valuable lessons learned. Coordination among many parties—including designers, extruders, fabricators, installers, and the test laboratory—was essential to getting the mock-up accurately built and tested. Shop-drawing-level details were needed for steel structure and cladding fabrication. Reviewing extrusion samples helped to ensure that correct profiles were being made. Miscellaneous materials were sometimes needed on short notice to ensure it all came together as planned. The exercise of fabricating and erecting the mock-up installation was useful in evaluating the constructability of the custom design. Challenges in fabrication techniques, such as hand-welding, led to specifying an automated machine-welding technique with more quality control. When elements were found to be difficult to install, another approach was considered to simplify and ease the installation. Testing allowed the design team to see how minor revisions would reduce the likelihood of workmanship errors. By relocating some metal elements, sealant joints were easier to install. Changing from a stick-built strip window assembly to a design that uses a shop-fabricated unit concept (i.e., unitized) would aid in reducing field glazing deficiencies experienced in the mock-up. All changes were added to the contractor’s bid package so these lessons learned would be incorporated in the final repair design. Testing also confirmed the underlying essentials of the design: the integrity of the air/water barrier to ensure proper performance, quality sealant joints, and weathertight panels. The structural testing portion of the mock-up test program confirmed that under wind loads and thermal movements, the design performed as anticipated. CONCLUSIONS The Air Force Academy Cadet Chapel, a landmarked icon made of steel, aluminum cladding, and glass, has leaked since its construction in 1963. A custom reclad repair design was developed that incorporates new weather panels inboard of the aluminum cladding rainscreen panels that match, or return to, the original historical appearance. The 21st-century drainage-type enclosure system provides a long-term repair solution. The constructability and efficacy of the custom recladding of the chapel, with the intent to create a weathertight cladding while honoring the original aesthetic, was evaluated and verified by building and testing a design-phase mock-up. The mock-up allowed the designers to learn valuable lessons applied to the final repair design, prior to the contractor bidding process. Although not a simple undertaking for a building as iconic as the Air Force Academy Cadet Chapel, the benefits to construction gained from building and testing such a mock-up in the design phase yielded invaluable information to aid the designers, and it ultimately improved the final design. If the necessary resources and commitment are available, a mock-up is a useful tool to the design team, and need not wait until the contractor is selected, as illustrated by the United States Air Force Academy Cadet Chapel repair project. REFERENCES 1. ASTM E283 Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen 2. ASTM E331 Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference 3. AAMA 501.1 Standard Test Method for Water Penetration of Windows, Curtain Walls and Doors Using Dynamic Pressure 4. ASTM E330 Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air Pressure Difference 5. AAMA 501.5 Test Method for Thermal Cycling of Exterior Walls 54 | O’BrieIEn and KaSKEl IIBEC 2020 Virtual International ConveVEntion & Trade Show | June 12-14, 2020 The exercise of fabricating and erecting the mock-up installation was useful in evaluating the constructability of the custom design.
