Structural Loading in Facade Performance Mock-Up Testing

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Exterior cladding for mid-size and high-rise buildings is an important aspect of whole building design. Architects, clients, and engineers spend a significant amount of time discussing the unity of architectural expression, structural and thermal performance, and efficiency of design. Structural engineers in the curtain wall/facade engineering field help to collaboratively design exterior cladding that meets stringent project specifications and performance requirements, while also working to maintain the integrity of a building’s cladding system for the building’s service life. Performance mock-up testing is frequently completed on many projects to assess cladding system performance and provide the project team with an opportunity to refine the design, fabrication, and installation of the system.

A common type of cladding system used in a mid to high-rise structure’s facade system is unitized curtain wall: modular units (typically 5 feet wide by 15 feet tall) that are factory assembled and then shipped to site and installed with a small installation crew of 3-5 ironworkers rather than the complete assembly of facade systems on site (referred to as stick-built curtain wall). Using a unitized approach offers enhanced quality assurance and control (as units are assembled in a factory environment with controlled processes), as well as increased cost effectiveness with the reduction of field labor. Units are shipped to site in crates and then “de-crated” on each floor of a building and individually installed via proper mating of unit framing elements and gaskets with those of adjacent units. This approach helps form a fully enclosed, air and water-tight building cladding system. Figure 1 shows a standard curtain wall section view and an example of curtain wall installed on a building.

For custom unitized curtain walls, the design is unique for every project. Curtain wall manufacturers such as New Hudson Facades (NHF) will commonly use custom aluminum extrusion profiles, custom gaskets and seals of varying materials, and proprietary strategies to achieve optimal system performance. On larger towers (where over 500,000 square feet of curtain wall may be required), there may be a substantial cost and disruption associated with modifying the system once fabrication or installation is underway; the performance mock-up phase provides the project team with an opportunity to identify and resolve necessary design, fabrication, and installation refinements before the system enters full production. Testing is conducted according to industry standards developed by ASTM International alongside the American Architectural Manufacturers Association (AAMA), which is now part of the Fenestration and Glazing Industry Alliance (FGIA). The project-specific curtain wall system is evaluated through a performance mock-up unit (PMU) testing program at an independent, third-party facility. The testing involves participation from the broader design and construction team—not solely the curtain wall contractor—to assess whether the system meets project-specific performance criteria.

Although the overall goal is to pass all ASTM and AAMA tests to substantiate adequate air, water, and thermal performance for each unique curtain wall system, a PMU test offers practical insight on how every new system works; the mock-up demonstrates how the units will be installed most efficiently in the field, provides new ways for improving air/water/thermal performance at key locations on the wall, and gives an opportunity to conduct additional testing scopes for special project requirements or research purposes.

A PMU is a full-scale assembly of actual curtain wall units selected to represent the predominant cladding system on a given project. It often combines the most recurring unit types along with atypical design features. The architect, facade consultant, general contractor, and clients collaborate to define the PMU scope and overall system performance. The arrangement of units is representative in nature and intended to incorporate unit sizes representing governing design conditions, such as taller and/or wider units, where loading on framing members and infill (e.g. glass or flat metal panels) is highest. The layout also mimics the juxtaposition of varying unit types and sizes on the building; the result is a full-scale prototypical representation of the overall facade. Test wall units are installed with anchorage to a base steel structure (intended to mimic a typical tower’s structural system) and enclosed from behind by a test chamber wall.
Figures 2 and 3 show elevations of two mock-ups: PMU-01 (tower system) and PMU-02 (lower level podium system). These are separate test walls for an 800-foot-tall skyscraper project located in the Hudson Yards neighborhood of Manhattan.

Although curtain walls do not aid in the superstructure support system, they can be subject to transient and extreme loading from both natural and man-made sources. Structural loading on curtain walls takes multiple forms. The predominant structural loading for a typical mid- to high-rise building is wind loading. Commonly assumed to be an equivalent static load, wind loading is transient and variable over a building’s cladding surface. Other natural loads include snow, ice, and impact from wind-borne debris from hurricanes and tornadoes. On certain projects, artificial, or “man-made” live loads such as blast, ballistics, and forced entry can also be considered and tested to meet client needs. Curtain wall systems are also required to withstand building movements from wind, seismic, and thermal loading. On projects without an elevated risk of hazards, PMU tests are typically limited to wind, thermal, and building movement.

Wind loads are transient and represented in curtain wall by positive inward pressures and negative outward (i.e. suction) pressures. For tall, geometrically complex buildings, project-specific cladding pressures are commonly developed through wind tunnel testing studies. ASCE 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) Chapter 30 provides analytical procedures for components and cladding, while Chapter 31 addresses the wind tunnel procedure. ASCE 49 (Wind Tunnel Testing for Buildings and Other Structures) details the requirements for conducting wind tunnel studies and interpreting results.

Boundary layer wind tunnel studies are typically sought by the design team early on and involve creating scaled models of the building and surrounding structures on a rotating surface in a wind tunnel to simulate upwind terrain and surface roughness local to a project site. Local climate data used to derive wind tunnel air flow speeds includes the basic wind speed for 3-second wind gusts and probabilistic wind speeds based on random simulations of hurricanes/cyclones. This data is combined to create a statistical climate model which, when combined with empirical data from pressure sensor data installed on the scale model of the building during wind tunnel tests, is used to derive topographic maps of recommended wind pressures for each area of each elevation of a building as shown in Figure 4.

Static pressures for PMU testing are typically equivalent to the “prevalent” negative and positive wind loads on the cladding. In the PMU test sequence, the “Uniform Structural Design Load Test” and the “Uniform Structural Overload Test” (where 150% of the test chamber pressures are applied for testing system integrity) reference these static pressures and are implemented per ASTM E330 (Standard Test Method for Structural Performance of Exterior Building Elements). Other static pressure differentials of lower magnitude are used in the test sequence to measure static air infiltration/exfiltration in the test chamber construction and for simulating wind driven rain in the “Static Water Penetration Test.”

Dynamic wind pressures are also used in testing, specifically for water intrusion testing according to AAMA 501.1. Unlike the static pressure which is simulated with modifying the internal pressure of the test chamber with a blower, loads are directly applied on the PMU from the exterior in an unusual yet exciting approach. Curtain wall test facilities typically employ the use of a large fan—and even sometimes, a turboprop aircraft engine—that can produce a wind stream with velocities adequate to produce a dynamic wind pressure high enough to create an accurate representation of a wind-driven rain event on the facade; an example test setup is shown in Figure 5. Recalling the equation for velocity pressure from ASCE 7-22:

q_z=0.00256*K_z*K_zt*K_c*V² (lbf⁄ft² )
q_z=0.613*K_z*K_zt*K_e*V² (N⁄m² )

where equivalent velocity pressure is proportional to velocity squared, dynamic wind pressures of up to 15 psf can be generated with windspeeds of at least 77 mph from the large fan for at least 15 continuous minutes. This informs the design team of a curtain wall’s performance under a simulated severe storm.

All wind pressure testing results can be used to inform the design team about any additional upgrades to the system before mass production of units. They are also used to verify calculations for system performance, including stresses in critical load-carrying elements and connections, expected deflections for insulating glass units (IGUs), and rigidity of all exposed cladding elements. Further post-processing of test results can even be used in a performance-based design approach for envelope design.

Curtain wall units are installed on anchors located at floor slabs, shear walls, and columns; building structural movements such as interstory drift from wind and seismic loading, slab deflections, thermal expansion/contraction, and column shortening must be considered at the curtain wall joints. These gasketed joints between units are referred to as stacks. Curtain wall systems accommodate building movements by allowing for stack longitudinal slip, opening, and closing as shown in Figure 6.

To simulate base structure movements, hydraulic pistons are built into the mock-up steel frame. Horizontal building movement tests are conducted per AAMA 501.4 and require one level of the mockup to move a specified lateral displacement (typically provided by the engineer of record). The displacement represents the maximum elastic interstory drift from wind loading (typically under 0.5 inch, or H/400) or inelastic interstory drift from seismic loading (sometimes surpassing 3 inches). To pass these tests, glass breakage, torn silicone joints, dislodged cladding elements, and compromised connections cannot occur after a minimum of three cycles of movement. After the elastic movements, all air and water tests are repeated to ensure system performance has not been compromised.

Relative vertical movement tests are conducted per AAMA 501.7 and simulate building movements that cause relative slip along vertical stacks, as well as horizontal stack opening and closure. A level of the test chamber is displaced upward and downward for three full cycles; pass/fail criteria are the same as those of the elastic horizontal movement tests, but the repeated air and water tests are conducted with the horizontal stacks in the open position, which represents a worst-case project condition.

Thermal cycling tests per AAMA 501.5 are also performed on the whole PMU test chamber to understand system integrity under extreme climatic temperature fluctuations and to verify the design’s condensation resistance. Thermal cycling is performed from the outside with insulating enclosures and can range from temperatures below 0F and above 180F.

Additional climatic testing on mock-ups can include snow/ice accumulation tests, hurricane missile/projectile testing (for product approval in certain jurisdictions), and even harmonic testing of cladding components that may be subject to dynamic instabilities and excess vibration under project wind loads. In fact, an additional testing scope for PMU-02 focused on the dynamic stability of full-height vertical fins (shown in Figure 3). Accelerometers were strategically placed along the height of the fin features to measure precise flexural and torsional movements of the fin during a dynamic air test. A spectral analysis of natural frequency was then completed to aid the design team in calibrating a parallel finite element model, quantifying any potential for dynamic instabilities, and preventing unwarranted noise or vibrations.

Point load testing can also be completed. Tests can simulate the impact of falling objects on cladding appurtenances and exposed features, or even the maximum expected loading in all cardinal directions at building maintenance unit tiebacks and guide tracks. Other tests can focus on the security design of curtain walls under threat of manmade hazards.

Performance mock-up testing evaluates how a custom, project-specific curtain wall system performs under the loads and movements it will experience in service and allows the project team to identify and resolve design issues before full production. Although the results apply directly to the tested assembly, a well-selected PMU provides valuable confirmation that the system can meet a project’s performance requirements. ■

The authors would like to acknowledge NHF Project Manager Don Tice for his mock-up photos at the test facility.

David Fratamico, Ph.D, PE, is a senior structural engineer with New Hudson Facades in Linwood, PA, with experience in testing and new product development.

Kyle Berg, PE, is a structural engineering manager with New Hudson Facades in Mendota Heights, MN, with over 14 years of curtain wall design experience. He leads engineering teams in Minnesota and Pennsylvania.