Architectural Precast Concrete Facade

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The post-modern architecture of the new Studio Museum in Harlem is both imposing and grand. Its modulated precast concrete facades are interspersed with sleek, curtainwall glazing. The intriguing exterior is designed to pique the interest of visitors, beckoning them to enter and discover the museum’s collection, as well as its artists in residence (Figs. 1 and 2).

The building is a six-story steel structure supported on a concrete mat foundation and clad with architectural precast concrete panels of different sizes and shapes. The building was designed by Adjaye Associates in collaboration with Cooper Robertson. Simpson Gumpertz and Heger Associates Inc., P.C. (SGH) was the Structural Engineer of Record, and they collaborated with consulting engineer Guy Nordenson and Associates (GNA). Sciame Construction was the general contractor, and Beton Prefabrique du Lac (BPDL) was the fabricator and specialty structural engineer for architectural precast panels. The design and construction of the intricate precast concrete facade required significant attention and extensive collaboration among the project team members.

This article focuses on the challenge of connecting the large precast panels of varying geometries to the steel superstructure, while controlling both vertical and lateral deflections, so that the size of the panel joints are minimized.

A typical precast concrete facade includes several components, and their design is usually shared between the design team (owner, engineer of record (EOR), and architect) and the construction team (general contractor, specialty structural engineer (SSE), and precast fabricator). In addition, the design of architectural precast facades is typically iterative and often includes the following steps:

While facade connections are a crucial component of a building’s load path, they are often delegated to specialty structural engineers, leading to potential coordination challenges. Industry resources, including the Architectural Precast Concrete Manual (MNL-122) from the Precast/Prestressed Concrete Institute (PCI), PCI Design Handbook (MNL-120), and American Institute of Steel Construction (AISC) Design Guide 22: Facade Attachments to Steel-Framed Buildings, emphasize the paramount importance of coordination between the design team and the construction team. These documents stress the responsibility of the design team to provide clear guidance, delineate design responsibilities, and review subcontractor submissions carefully.

For example, AISC Code of Standard Practice recognizes that the delineation of responsibilities between EOR and SSE remains a challenge, particularly at connection points where facade components interface with the primary structure. Furthermore, because of myriad options for design delegation, the coordination and communication between design and construction teams is essential. By addressing potential areas of contention, design and construction teams can mitigate risks and ensure alignment with project requirements.

Cognizant of the coordination challenges of a complicated precast facade, the entire project team of the Studio Museum in Harlem began design coordination early in the project. Specifically, the owner engaged the precast manufacturer to participate in a design-assist exercise right after the completion of the construction documents. During this design-assist phase, the design team and the precast facade manufacturer selected a portion of the north elevation to develop typical connection details to be used throughout the facade. The intent was to complete the collaborative and iterative design process on a limited number of connections and identify the “typical” issues that would be expected during the coordination for the remainder of the connections. This exercise also included the fabrication of a full-scale mockup of a section of the facade. This mockup was invaluable, as it allowed the project team to evaluate the facade design and to finalize decisions related to color, aggregate size, panel geometry, and panel joints. From the mockup, we refined the joint analysis and designed portions of the connections bridging the panels and the base building structure. After the mockup, the project team realized that the project would not feature “typical” connections because of limitations on the size of the panels, complicated geometry of the facade, and interior architectural constraints. The final design would include more than 200 different connections of the panels to the base building structure.

Precast facades usually consist of individual panel elements connected to the building's structural framing. The gaps between the panels (joints) are often filled with sealant to provide a weatherproof barrier and to accommodate the anticipated movement of the structure without panels bearing on each other and creating unintended load paths. For the Studio Museum in Harlem, the design objective was to maintain joint sizes no larger than 3/4 inch and to align the precast joints with the curtain wall glazing joints and with joints in interior finishes. This alignment aimed for uniformity and harmony between the facade lines and interior spaces. Conventional sealants typically allow for a maximum movement of 50% from the average joint size. Therefore, the typical sealant in a 3/4 inch joint can expand up to 1 1/8 inch and compress down to 3/8 inch without failing.

Despite the original target for joint size appearing manageable, achieving this target posed significant challenges in the design of the base-building structure. The architecture required large open gallery spaces, resulting in spans of 50 feet for several composite steel beams supporting the facade panels. While building codes allow for a maximum live load deflection of L/360 for beams (which exceeds 1 5/8 inch over a span of 50 feet), this large vertical deflection significantly impacted the joint size. Therefore, the team engaged in precise modeling of individual panels and their connections along the spandrel beams to focus on the expected gravity deflections of the base building structure, facade movement, and ultimately joint size (Fig. 3). After the completion of the gravity analysis for the preliminary sizing of the joints, the same modeling approach also allowed the design team to confirm the adequacy of the preliminary joint size for the building’s expected lateral drifts.

Construction sequencing also played a pivotal role in the joint size analysis. Depending on the construction sequence, deflections from the self-weight of the structure , the self-weight of the facade, superimposed loads from components other than the facade, live loads, snow loads, wind loads, seismic loads, and rain loads needed careful consideration. Given the ability to shim, level, and plumb the facade panels relatively independently of the supporting structure, the joint analysis could solely focus on the expected deflections occurring after the installation and shimming of the panels. As such, the joint analysis did not need to account for the deflections associated with the self-weight of the structure and the facade panels. Finally, the joints were sealed after all the other permanent deflections had taken place (e.g., associated with superimposed dead loads of other building components). In the end, after extensive work and several design iterations, the project team met the general architectural vision of 3/4-inch-sized joints at most locations, except a few panels featuring extreme geometry.

Typically, precast panels are designed to be statically determinate, with few connections per panel. Common types of precast connections include gravity and lateral connections. Gravity connections usually consist of concrete shear keys or embedded steel angles/plates protruding from the back of the precast panel. These protrusions normally bear on and are connected to steel spandrel beams or concrete slabs. As precast connections often introduce eccentricities, the overturning from gravity loads is typically resisted by lateral connections.

At the Studio Museum in Harlem, the gravity connections are constructed from hollow structural section (HSS) steel tube framing connected to the spandrel beams. The lateral connections are adjustable in two directions and constructed from threaded rods in vertically slotted tracks that are embedded into the precast panel and connected to steel channels or angles with horizontally slotted holes. The channel or angles are then welded to the spandrel beams, which are, in turn, connected to the concrete diaphragm by shear studs (Figs. 4 and 5). During the joint analysis, the project team spent significant time locating precast connection points to also avoid interference with curtain wall connections, mechanical, electrical, and plumbing (MEP) systems, and architectural features.

The Studio Museum's articulated facade required unconventional approaches to connection design to support the intricate reveals and soffits in the limited space available. In specific scenarios, substantial box-shaped precast panels positioned below floor levels prompted the implementation of underslung outriggers to support the gravity connections. These outriggers are designed to attach beneath the structural steel framing while seamlessly integrating into the architectural ceiling space, preserving programming integrity (Fig. 6).

In other scenarios, where spatial constraints are imposed both above the slab (e.g., for architectural programming) and below the slab (e.g., for facade soffits), the project team needed to conceive alternative connection designs. This design includes HSS tubes strategically positioned within the slab's depth, connected to the top of the floor framing, and horizontally extending out to support the panels (Fig. 7). Given the geometrical constraints and the strength and stiffness requirements, the project team considered the novel solution of HSS in-slab supports.

Where spatial constraints due to panel geometry were such that the direct extension of outriggers was not feasible, the project team needed another creative connection. This time, the approach includes “kinked” HSS tubes, custom-designed to fit within the slab depth and curb width (Fig. 8).

The coordination between design and construction professionals stands as a cornerstone of a successful facade design and construction, as it did for the Studio Museum in Harlem. By adhering to industry standards, clarifying design responsibilities, and fostering effective communication, design and construction teams can navigate the complexities of facade construction with confidence and precision. After countless hours, extensive coordination, and exceptional dedication, the Studio Museum in Harlem’s team achieved the vision of creating an iconic precast concrete facade worthy of the newly constructed architectural marvel. ■

About the Authors

Alexander Stephani, PE, is a Structural Engineer with Simpson Gumpertz & Heger (SGH), specializing in new design, repair and rehabilitation, and flood resiliency design.

Filippo Masetti, PE, is an Associate Principal at Simpson Gumpertz & Heger Inc. (SGH), specializing in the assessment, repair, and rehabilitation of existing structures, as well as in the evaluation of unusual detailing in ground-up construction.

Kevin Poulin, PE, is a Principal at Simpson Gumpertz & Heger Inc. (SGH), specializing in the restoration and the adaptive reuse of existing buildings, as well as the design of new spaces for arts and culture.