Modern Steel Construction • June 2021 • I Spy

THE INTERNATIONAL SPY MUSEUM has nothing to hide—at least when it comes to its framing system.

The 141,000-sq.-ft, eight-story, $162 million facility in Washington, D.C., includes three floors of museum exhibits resting on a five-story base of retail, education, and lobby spaces—and plenty of prominent architecturally exposed structural steel (AESS). Perched above the museum floors and cantilevering more than 20 ft beyond them to the north, the events space box provides architectural contrast to the inverted-pyramid museum structure below and contains offices, additional educational space, a dramatic events facility, and a green rooftop gathering space offering sweeping views of the city.

Having outgrown its original home in D.C.’s Penn Quarter, the museum wanted a new iconic location where it could continue its mission of educating the public on and showcasing the history of espionage. The overarching goal was to create a world-class museum with Smithsonian-level thermal and humidity controls in an architecturally impactful building.

Creativity and collaboration were critical to the success of this project, which was to be built above an existing operational subterranean shopping mall and garage that support a major Metro station and surrounding office building, just south of the National Mall on L’Enfant Plaza. In addition, the design and construction teams were also faced with a strict budget and a 48-month schedule. By applying for a building permit that specifically allowed the team to begin below grade, general contractor Clark Construction was able to expedite the procurement of design-assist trade partners and begin the first phase of construction well before the full building design was complete.

The final concept is a play on the business of espionage, hidden in plain sight. One of the main architectural features is a five-story glass atrium, dubbed “the Veil,” that is suspended in front of an enclosed exhibit box and feature staircase. This unique structure provides a stage for the movement of people throughout the exhibit levels, contributing to the pedestrian experience along 10th Street. With its evocative form, powerful exposed structural steel sloped columns, and pleated glass veil, the museum serves as a catalyst to revitalize L’Enfant Plaza.

Washington is generally known as a “concrete town,” where cast-in-place concrete structures are prevalent. However, as design discussions began during the early phases of the project, it became clear that structural steel was the best choice for the building structure. Steel provided the greatest flexibility needed to achieve the desired aesthetics as well as engineering, constructability, and cost benefits, and the project is framed with 1,600 tons of structural steel.

An intricate series of monumental stairs and platforms constructed of AESS members of varying shapes and profiles exist within the volume of the glass veil, all of which had expressed connections to the built-up sloping columns. Structural steel provided the strength and stiffness needed to achieve the architectural vision for this space by keeping the structural members as small and aesthetically pleasing as possible. In order to carefully evaluate vibrational performance of the stairs and platforms to ensure occupant comfort, steady-state analysis (using SAP2000) was conducted for the entire system, which accounted for the interaction between these stair/platform structures and the building’s superstructure.

This unique combination of custom-built cantilevered columns, non-standard connection types (in the form of pins and large-diameter bolts for aesthetic reasons), and high-performance finishes (hot-dip galvanizing and intumescent paint) collectively made for a design unlike anything steel fabricator SteelFab had experienced before—and was a testament to structural steel’s ability to strike a balance between being strong and appearing delicate.

Weight was also a major factor that could be successfully addressed by a steel framing system. The museum superstructure lives above an existing four-story below-grade concrete structure built in the 1960s, and the addition of a new building above was not anticipated in that structure’s original design. In order to support the new museum superstructure loads, strengthening the existing concrete structure was necessary, so it was very important to minimize weight as much as possible. In addition, the typical floor system for the museum, consisting typically of W30 long-span composite beams and girders with lightweight concrete on metal deck, provided the open spaces required for the building’s programming while also minimizing the self-weight of the structure. Vibrations were carefully analyzed during the design process and, in many cases, controlled steel member sizing.

Structural steel was a natural framing choice in helping to resolve design challenges associated with the building geometry and layout. For example, the sloping geometry of building columns at the south and west faces causes an inherent lateral drift from gravity loads. Further compounding this issue, museum programming prevented placing traditional braced frames near the sloping south and west building faces, resulting in significant diaphragm torsion. To help address these issues, full story-depth “hat trusses” were placed above the highest museum level within the mechanical plant in line with primary building core braced frames. In addition, a three-story, sloping braced frame that is discontinuous to the ground was used at the south edge located just inboard of the façade to avoid intrusion on museum space, which required careful coordination between structural engineer SK&A, SteelFab, and the architects to implement successfully. The structural team also worked closely with other contractors to establish a construction sequence aimed at limiting lateral building drifts during construction due to the unbalanced sloping nature of the west and south building faces.

The events space, which is located above the museum spaces at Level 7, includes a 20-ft, 6-in. cantilever protruding above the roadway below to provide striking views of the U.S. Capitol. In order to alleviate vibrational concerns for potential rhythmic excitations, a combination of cantilevered beams, perimeter built-up steel box beams for torsional rigidity, and 3-in.-diameter stainless steel diagonal tension rods was used.

From a constructability perspective, steel construction had several advantages over cast-in-place concrete. The construction schedule for the project was aggressive from the start, and the use of structural steel allowed the building superstructure to be completed in a faster timeframe than would have been possible with cast-in-place concrete options. In addition, by implementing a design incorporating structural steel with un-shored composite metal deck construction, the need for expensive and intricate concrete formwork was eliminated. By not requiring formwork, the owner’s goal of maintaining occupancy of the garage, retail areas, and office space below during construction was preserved.

The museum’s unique design came with its share of challenges, starting with the building site itself, which was not much larger than the building footprint and was also nestled in the middle of a dense area, requiring extensive planning and coordination in order to make the project a success. The tight site could not accommodate much steel on the ground, and active roadways surrounding the site limited the ability to stage trucks for unloading. And in order to meet the project schedule, the mill order needed to be placed and the model started well ahead of demolition of the existing topping slab/roof structure that the new building sits on. Once as-built information of the existing structure was available, some fairly significant modifications needed to be made to the base plate patterns and column heights to accommodate the existing conditions.

Meeting the aggressive project schedule established by ownership meant that in some instances, certain steel members needed to be released for fabrication prior to completion of all design elements. For example, SK&A worked closely with SteelFab to permit the release of the primary floor and braced frame framing members before finalizing all façade steel concepts. This required thoughtful planning and discussion to avoid subsequent coordination issues.

In order to maximize headroom, numerous mechanical duct openings were required through steel beams and girders at all floors. This required careful coordination between the engineers and SteelFab to ensure openings were located and adequately strengthened with welded plates in the shop prior to the members arriving in the field.

The team was also tasked with ensuring that the finishes of sloping interior columns and exterior columns matched when the latter required the extra process of hot-dip galvanizing (this included coordinating with intumescent paint requirements and fabrication/erection tolerances that differed between the two).

Another complication became evident during non-destructive testing of the splices between the main structural framing and the sloping column cantilevers, which are W30×124 sections with ¾-in. web doubler plates on each side of the web. These cantilevers were then spliced to main building beams made with the same built-up section with a CJP splice. When ultrasonic testing was performed, the interface between the doublers and the web of the W30 beams created false-negative results. Structural engineer SK&A and the testing agency eventually developed a testing procedure that would satisfy the design requirements.

One of the most outstanding aspects of the museum is that the AESS is featured as a prominent portion of the design and not just used as a highlight or an accent. This steel, particularly the vibrant red exterior sloping columns, is the premier architectural feature seen not only by museum patrons and D.C. visitors and residents but even those passing by on nearby I-395.

These 11 perimeter columns, which support the building’s “upside-down pyramid” shape—and which became known to the project team as the “red slopers”—are 85 ft tall and built from 1-in.-thick grade 50 plate. The columns are located along the south and west faces, which slope at an angle of approximately 2.5 vertical to 1 horizontal and act as part of the building’s gravity load-carrying system. They taper and have reduced depths at the top where structural demand diminishes to reduce material cost and for aesthetics.

The cross sections of the columns look like back-to-back L’s while the profiles vary between 1 ft to 3 ft to follow the architectural design intent. The slopers connect to double HSS16×8×5∕8 vertical columns, referred to as “pin columns,” at the fourth floor and are attached to a pedestal of HSS16×5∕8 members by way of a 5.5-in.-diameter pin at the plaza level.

With the visual exposure and prominence of these AESS steel members and their connections, it was imperative for the team to get it right. To facilitate this process, SteelFab constructed a full-scale mock-up of the Level 4 connection early in the design phase to ensure structural and aesthetic compliance. This type of early and productive collaboration between Steel- Fab, SK&A, and the architects helped greatly with the project’s success.

In order to fully understand the structural behavior of these complex pin and sloping column connections, SK&A conducted a detailed finite element analysis using SAP2000 to ensure stresses were within acceptable ranges for all load combinations.

Due to the structural steel being such a significant part of the façade itself, significant coordination was necessary between the steel package and the other elements that make up the façade system. It was especially crucial at the sloping front of the building, where the curtainwall veil and monumental stair are both connected back to the sloping steel columns. Connection points for the curtain wall and stair had to be incorporated into the shop fabrication of these columns, meaning that the connection points had to be coordinated with each supplier’s internal tolerances and also allow for the project-specific steel erection tolerances. Both the stair hangers and curtain wall connections were attached to the columns by a 2-in. pin, so there was no room for error once the structural steel was fabricated and erected.

Another façade system that had to be coordinated was the aluminum panel rain screen system. In order to support this system, vertical W6 girts were placed around the entire building perimeter at 5 ft on center. Due to the tight project schedule, these panels could not wait for field dimensions to be taken between sloping columns prior to production, so the location of the steel columns had to fall within the prescribed AESS tolerances. The majority of this coordination took place by way of model sharing between individual subcontractors and a weekly (sometimes daily) 3D BIM (building information modeling) process in which SteelFab established allowable tolerances and individual system requirements.

There also were many different finish requirements for structural steel on the project, some of which necessitated different fabrication and erection details in order to accommodate various coating types and thicknesses. The following surface-prep and finish conditions had to be implemented for different elements:

To make sure the finish requirements for each piece of steel were correct, SteelFab traded color-coded models with the design team to visually check and ensure each piece came to the field with the correct finish, and the exposed steel was finished to a project-specific custom AESS level. (For details on the various AESS levels, see “Maximum Exposure” in the November 2017 issue, available at www.modernsteel.com.)

SteelFab’s involvement in the International Spy Museum project began approximately one year before it was awarded the contract for the structural steel package. During this time, conceptual and schematic design-level feedback regarding some of the feature elements was provided to Clark Construction and the rest of the design team. The willingness of the project team to engage a steel fabricator well ahead of the procurement stage helped steer certain design decisions in directions that maintained the architectural intent but allowed for more fabrication- and erection-friendly details.

Had this project been procured under a typical design-bid arrangement, it is not an exaggeration to say that three to four months would have been added to the structural steel schedule alone. A significant portion of the up-front work involved delving into the details of earlier discussions about coatings, connections, tolerances, and AESS expectations in general, and only with the full buy-in of all project team members was this kind of progress achieved in such a short amount of time.

Ironically, a museum dedicated to espionage sticks out like a sore thumb—a modern, steel and glass jewel box amongst brutalist buildings and concrete monuments. But in such a high-profile city, it doesn’t hurt to draw a bit of attention to yourself.

Chris Gregory (cgregory@steelfab-inc.com) is executive vice president of SteelFab, Inc.’s Washington, D.C., office, and Daniel Cocciardi (danielc@skaengineers.com) is an associate with SK&A Structural Engineers.

For more images of the International Spy Museum, see the Project Extras section at www.modernsteel.com.

Hickok Cole, Washington (Architect of Record) Rogers Stirk Harbour + Partners, London (Design Architect)