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A cornerstone for peace

November 2012 » Features » GOVERNMENT/PUBLIC FACILITY
New United States Institute of Peace building meets its values and mission through inventive structural solutions.
Cristobal Correa, P.E.
Schematic drawing showing the process of the geometric rationalization for the roof structures.

There aren't many buildings that require an act of Congress to be built, but the United States Institute of Peace Act passed by the U.S. Congress in 1984 officially established an independent peace institute in Washington, D.C. At first, the United States Institute of Peace (USIP) didn't have a home; it had no building to call its own. Finally, in 2011, 25 years after the institute was created, an official headquarters became a reality. Situated on the National Mall, USIP has become a cornerstone of the American effort to bring peace to areas ravaged by conflict.

The New York office of international multidisciplinary engineering firm Buro Happold worked with Boston-based Safdie Architects on the project, providing structural, MEP, and sustainability engineering expertise. The engineers and architects collaborated closely on the effort to design a building that supported USIP's values and mission.

Due to the USIP's location on the historic National Mall, the architects endeavored to incorporate classic design features found throughout the capital, such as domed roofs and majestic façades, and integrate them into a more contemporary design aesthetic so that the building would stand out even among its impressive peers.

Construction of the roof supported by scaffolding as it nears completion.
Photo by Bill Fitzpatrick.

The five-story, 150,000-square-foot LEED Gold-certified building is composed of three independent structures connected by two atria. The atria act as public spaces, with expansive glass façades and roof systems that fill these areas with light. In addition to these atrium areas, the complex includes an auditorium, a conference room, offices, and two levels of underground parking.

A second façade
Two of the most exceptional elements of the structure are its glass roof and exterior enclosure. These are the most iconic features of the building. The curved roof shapes link the building to the neoclassical architecture that is characteristic of the National Mall.

"The overarching roof structures covering the two atria were among the most important features of the project," says Paul Gross, principal-in-charge of the project at Safdie Architects. "In addition to allowing natural light to endow the space within the structure, the glass roof subtly transforms the prototypical Washingtonian dome into an abstract symbol of peace. This complex roof would not have been realized without the collaboration between Safdie Architects, Buro Happold, and the German manufacturer Seele. This partnership enabled the design team to develop a complex structure with a gracefulness that embodies the spirit of the Institute's mission."

One of the structural engineers' main challenges was figuring out how to make the roof forms – which emulate symbols of peace – structurally sound and easily buildable. This involved understanding glass fabrication technology as well as how the glass panels might be laid out, given the unusual architectural forms. Based on their previous experience working on projects with irregular roof forms, such as the Kogod Courtyard at the National Portrait Gallery in Washington, D.C., the engineers quickly analyzed the design challenges and devised a solution by using 4-foot-by-4-foot glass panels for the roof. These panels maintain constant depth but have variable thicknesses, depending on structural loading.

The panels are double-glazed units with fritted laminated glass on the exterior face. Steel sections measuring 4 inches by 8 inches hold each pane of glass in place. The glass units are attached directly to the structural steel grid by means of a turning, locking mechanism; there is no additional mullion system. A fabric layer that is offset from the inner face serves to hide the steel grid structure, soften the natural light, and create an acoustical buffer for the interior.

To create a geometrically complex glazed enclosure within a fixed budget, the design team decided to work with a design assist procurement. Early in the process, the design team prepared very advanced descriptions and specifications for the façades and roof, which included performance criteria as well as an analysis and sizing of elements. After providing this information to a selection of specialty contractors who bid on the project, the team selected Seele.

"As part of the design assist process, we got involved very early," says Heiko Mertel, senior project manager at Seele. "This meant that we could explore different structural and detailing options to find the right solutions for the project. Our team was also able to leverage our own BIM models with Buro Happold's to resolve details and to coordinate our shop drawings. Collaborating with Buro Happold resulted in an incredible custom roof that fully satisfied the architect's aspirations."

The design process included constructing mockups of portions of the roof in order to understand the visual impact of fritted glass and the use of flat panes (initially, the team had considered curved glass panels but the cost was too high). These mockups were also instrumental in understanding the quality of the natural light in the interior spaces.

The roof components were fabricated at Seele's facility in Europe and then transported to Washington for installation. The installation of the roof required hoisting, assembling, and welding together many steel sections, which were then clad with the glass and inner fabric panels.

Aerial view of the United States Institute of Peace under construction.
Photo by Bill Fitzpatrick.

The visual aesthetic of the roof was especially important as USIP lies along the flight path to Reagan International Airport. During the day, the roof surfaces appear white, while at night they are illuminated with artificial light that complements the nearby Lincoln Memorial. The roof thus acts as an additional façade.

Exterior view of the south atrium façade showing the façade truss.
Photo by Timothy Hursley.

Structural interdependence
Structurally, the base building is composed of three individual which the glass roof spanning between them. To minimize the loads going through the roof structures and to make them as slender as possible, the roofs do not structurally link the buildings. This allows them to move independently under wind and seismic loads. The roof span floats over the concrete buildings with horizontal restraint provided only at key locations on one side of the span. Because the roof shapes are not regular, the support restraints are horizontally angled in different directions. The engineers had to determine the right stiffness of support to minimize the parasitic structural loading in the roof.

The façade's arching shape imparts a geometrical flexibility so the engineers had to conceive and model their design analytically. They then detailed the façades with spring supports that allow them to be partially restrained by the buildings without generating too much additional axial load in the façade transoms. All of these support details involved extensive modeling and work with the fabricator in order to accommodate the different geometries.

For the larger south atrium, the façade also provides support for the roof shell, as well as support for the dramatic cantilever that is a signature piece fronting the building. The cantilever creates lateral loads resisted by a light horizontal truss that has been incorporated into the façade.

Supporting the base buildings
The base building is a concrete structure incorporating concrete columns, beams, and areas of flat slab construction. The exterior of the building is clad in precast concrete panels with extensive roof and façade areas in steel and glass. To provide unobstructed views of the atria for the building occupants, the engineers embedded 12-by-14-inch columns in the wall construction.

Part of the building program involved an agreement to incorporate parking space for adjacent buildings that already existed on the site. This agreement created the need for a below-grade parking footprint that was larger than the above ground building. The garage also needed

to incorporate access ramps as well as structural supports, which were placed strategically to navigate on-site obstructions, including a 19th century brick sewer line and high pressure, high temperature steam lines. Due to the shape of the above-grade building and the diverse program, the engineers used transfer structures to adjust most column locations. This allowed them to achieve the architectural design intent and also accommodate the parking.

Interior view of the south atrium.
Photo by Timothy Hursley.

The majority of the foundations were designed as spread footings bearing on rock that was close to the basement elevation. However, due to the active brick sewer, which runs diagonally across the site and bisects the building, the engineers designed a deep foundation system adjacent to the sewer. This transferred building loads to rock outside the sewer's zone of influence. Underpinning was also required on one side of the building to avoid damaging the stability of adjacent historical buildings.

Because of the irregular shape of the floorplate, the engineers performed finite element analysis of the floors to minimize floor depth and control deflection. In addition, Buro Happold fully integrated ventilation and piping systems as they worked through and around the floorplates. The electrical systems for the roof terrace were also completely embedded within the architectural concrete in this area in order to retain the visual aesthetic while still meeting strength and durability requirements.

A brighter outlook
The engineers at Buro Happold created a sophisticated structural design for this iconic building. The clean look of the structure conceals complex engineering beneath its glass roof and façade. Each element of the design – from the foundation to the roof – works together to create this striking new addition to the nation's capital. The new headquarters of United States Institute for Peace proudly stands on the historic National Mall as a symbol of peace for generations to come.

Cristobal Correa, P.E., is an associate principal, and Stephen Curtis, MICE, is an associate with Buro Happold in New York City.

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