Intense geometry: the National Museum of the American Indian: imitating the natural elements of wind, water, and light presents design challenges for the contractor on a very high-profile project

Concrete Construction, August, 2002 by S.C. McCraven

Shaped as if by the forces of wind and water, a curvilinear concrete edifice clad in limestone is rising in a place of honor on the National Mall in Washington, D.C. Scheduled to open in 2004, the Smithsonian Institution's National Museum of the American Indian (NMAI) will pay tribute to and preserve the culture and traditions of Native peoples of the Western Hemisphere. "The Way of the People," an extensive document inspired by input from countless individuals in Native communities, guided the plan and design of this structure, producing an astonishing building that recalls natural forces and elements. Prismatic rays of light will enter the museum's central space to illuminate areas within--marking significant moments of the solar calendar. Fluid forms that mimic nature, however, challenge the builder, Clark/TMR, A Joint Venture of The Clark Construction Group of Bethesda, Md., and Table Mountain Rancheria Enterprises of Friant, Calif.

Challenges for Clark/TMR

No straight lines "There wasn't a straight line anywhere--not one square corner," Tony Embry, Clark general superintendent, remembers thinking the first time he looked at the blueprints. This 260,000-square-foot project is "totally in the round," says Embry, "with every wall presenting a different radial curve." Clark/TMR knew that the formwork for the curved and switchback vertical elements would be an important key to successful construction.

D.C. security issues One might think that security would be a problem due to the proximity of the U.S. Capitol to this 4.25-acre construction site. "Surprisingly, we don't have site-access problems due to security measures, except that we cannot allow anyone up on the crane in line of sight when dignitaries visit nearby buildings," explains Brad Koch, project engineer. So far, vacating the 200-foot stationary cranes has not delayed project schedules.

Tight elevation tolerances "I was particularly nervous about the effect of seven different fixed form panels used in the shear walls. We had only 2 inch of elevation tolerance for the resulting spandrel beam at grade," relates Ray Register, senior superintendent. The spandrel beam is the curved slab edge providing angle support for the limestone facade. "Steel trusses for two huge slabs are suspended from shear wall camber points in this design," explains Register, "making for restrictive quality-control measures and tolerances."

Compound cantilevers and camber The NMAI design involves three cantilevered floors suspended off vertical mechanical and stair shafts. Cantilevered elements can deflect unpredictably under loading--especially if concrete strength varies significantly. How much camber will there be in the end? There are no easy structural calculations for reassurance. Because wedge inserts for the relieving angles for the stone facade are being cast into the concrete work as the building rises up out of the ground, engineers are concerned about staying within the 1/2-inch final elevation tolerance.

Blueskin membrane A continuous blue rubber barrier skin covers all the external structural elements of the museum. This insulating system will help maintain the specified interior temperatures and humidity conditions necessary to preserve delicate museum artifacts. "It looks a little like a `Blue Smurf' building wrap and includes 2 inches of rigid insulation and 1 inch of air space behind the limestone facade," says Ed Hussung, superintendent. The blueskin membrane system must also follow the reverse curves and variable geometry of the NMAI structural elements.

Surveying and QC With three floor slabs suspended from two vertical concrete shafts, the importance of accurate and continuous surveying becomes apparent. Clark/TMR provides two separate crews of field engineers. The first crew provides layout and quality control for all concrete operations; the second crew monitors the work of the first crew, while providing tolerance control and guidance to the remaining subcontractors. A registered surveying firm double-checks important calculations and building baselines to ensure compliance with the rigid construction tolerances.

Foundation and subgrade More than 1050 driven steel piles reach down to the Potomac strata beneath the structure. The original soil on the site was removed down to a 3-foot undercut. A 24-inch base material of #2 or #3 stone and 12 inches of ground aggregate capstone were used to provide a quality subgrade. One advantage of the stone base is that on rainy days work crews don't have to trudge through mud.

Phased building occupancy. By spring of 2004, just 880 days into this 3-year construction project, the Smithsonian Institution will start a phased occupancy of the basement level in order to begin exhibit preparation and support work. Clark/TMR will still be working full-tilt on other portions of the building. Three basic components will have to be functional to provide for museum staff safety and security: HVAC, fire alarms and sprinklers, and safe access (including operating elevators).