|SNAKING Casting a thousand curves in walls, cores, shafts, columns. (Photo courtesy of Conseco Doka/Hall Williams)|
Artifice for artifacts' sake is keeping the team shaping the curve-heavy National Museum of the American Indian running in circles not far from the foot of the nation's Capitol. It is taking many formsclimbing, reusable, adjustable, circular, stick-builtand other stretches of skill to create a natural-looking environment, inside and out, representing cultures and habitats of indigenous tribes.
"The effort we go through to make the stone [cladding] look natural is unbelievable," says William I. Kline, project manager for the joint venture architect, Polshek/SmithGroup, Washington, D.C., and Jones and Jones, Seattle. The same could be said for other elements of the five-story building.
The 23,318-sq-meter museum, currently rising on the last available site of the National Mall, is intended to resemble a "solid piece of rock carved over time by wind and water," says Kline.
There are six footprints, undulating perimeter walls, real boulders and constructed water features and a 45-m entrance overhang reminiscent of age-old cliff dwellings.
That's not the only cliffhanger in the project. With a change in architect, a lawsuit, design changes and delays, there has been no shortage of suspense during the $199-million project's 11-year history.
In January 1998, five years into design, the Smithsonian Institution dismissed the original architect, GBQC Architects, Philadelphia, in conjunction with Douglas Cardinal Architect Ltd., Ottawa, Ontario, for failing to submit architectural and engineering drawings as scheduled. GBQC then filed a lawsuit. Under a 1999 settlement, in which neither party admitted guilt, the Smithsonian altered the terms of dismissal from "default" to "convenience." That allows GBQC to compete for federal contracts. The museum also agreed to credit GBQC and Cardinal, a Blackfoot Indian descendant, for conceptual design. As part of a counterclaim, the museum received $453,593 from GBQC's professional liability insurer (ENR 11/8/99 p. 24).
The design team may be different but the charge, to create a 100-year building, has never changed. "The level of quality ...is beyond most of the standards we're referencing," says Kline. For example, concrete faces have an architectural finish, though they will be covered up by limestone cladding.
All of NMAI is an "exhibit," even the design and construction process. But the demands have a toll. Speaking of the many curves, Michael Hamm, senior superintendent for Bethesda, Md.-based Clark Concrete Contractors, says, "I get dizzy walking around in circles." He then calls the job "a pain in the neck."
The design calls for a buff building, some 33 m tall, clad in rusticated Minnesota limestone. The building is sited on 4.25 acres, with footpaths undulating around and through water features, boulders, plants and trees. Each level has a different, amoeba-like footprint, with serpentine walls and other vertical elements. Inside, a central 33-m-dia gathering space is topped by a nearly 40-ft-high dome.
|RUSTICATED AND ORGANIC Massing, landscaping, siting represent diversity of America Indian tribes, says Architect Jones (left). American Indian Jones is remaining memeber of original design team. (Model � 2001 Smithsonian Institution)|
Everything is highly symbolic and nonrepetitive. The amorphous shape reflects the diversity of the Indian people, says Johnpaul Jones, principal of Jones and Jones, the only member of the original design team still on the job. With tepees, igloos, long houses and even natural cliff dwellings, "it seemed best to do something organic," he says.
|(Photo by Nadine M. Post for ENR)|
The programmatic elements are not easily accommodated. Water features must not foul dry elements. Structured pathways and other slabs must be able to support 300 psffor boulders and other heavy exhibits. And maintaining a museum environment in a humid climate is tough. Vapor barriers and insulation separate heated sides of structural elements from those exposed to weather.
The concrete job is as multidimensional as its symbolism. There are more than 500 work points, each of which represent the center of a circle and can generate multiple radii. There are easily 1,000 different curves and little repetition, which means no one does the same things two days in a row, says Hamm, whose firm is a subsidiary of Clark Construction Group Inc., Bethesda, Md. Clark Construction is part of CLARK/TMR, the job's joint venture general contractor. TMR stands for Table Mountain Rancheria Enterprises Inc., a subsidiary of TMR, Friant, Calif., a federally recognized American Indian tribe.
With concrete work 60% complete, Clark has already gone through much of its learning curve. Rebar congestion led to a switch, during the foundation stage, to self-consolidating concrete. There was simply no room for vibrators. And generally, turnaround time for pours has been slashed from five days to two. Clark says the job is "essentially" meeting a schedule, revised to accommodate some design changes associated with eliminating a chiller plant.
|DIZZYING Circles shape museum site. (Model left � 2001 Smithsonian Institution, rendering right by Elizabeth Day)|
The menu of formwork types is long. For external slab edges, spandrel beam sides, some radial walls and "potato chip" columns that look almost rectilinear but are actually radial, wood-faced formwork panels are "stick-built" but at a remote yard. Building them in place would have been too time-consuming, says Clark, because of the many curves and sizes. For cylindrical columns, Clark is using steel forms.
For the vertical elements, including serpentine walls, circular cores and shafts, stick-built forms would have been "a nightmare" and a waste of material, says Clark. Instead, Clark is using off-the-shelf reusable systems modified to accommodate special conditions. The forms cost more but require much less labor. They are more precise and can be engineered to pour walls as tall as 6.2 m in a single lift.
The biggest issue for the main formwork supplier was "getting a handle on the geometry," says Michael J. Schermerhorn, marketing director for Conesco Doka Ltd., the Little Ferry, N.J., formwork supplier. "There was a lot of engineering time on this job," he adds, especially because the geometry was "not clear at the beginning."
For example, stair core A morphs in plan from an eccentric "J" at the first floor to a "b," a "D," a square "U" and back to a "D" at the top floor. To top it off, locations of box-outs change from floor to floor.
Once the team established the process of having geometry meetings every week to try to solve discrepancies before the forms were set, turnaround time improved, says Clark.
For perimeter walls, and stair, mechanical and elevator shafts, Conesco is using an adjustable system. Vertical side rails have a steel profile for clamping to adjacent panels. Turnbuckles connect panels horizontally, across horizontal steel walers. Panels are ganged in up to 22 m lengths.
Crews put a curved wall template on the form and crank the turnbuckles on the back side of the form until the form matches the template. Panels are stripped after a lift and, using a tower crane, are moved in gangs horizontally to the next position.
|FOOT OF CAPITAL Museum takes up last site of National Mall. (Photo by Carol M. Highsmith)|
On upper levels, climbing formwork, supported by the portion of the wall previously poured rather than by the ground, is used on the exterior face of the wall formwork wherever there is a long drop. It is most commonly used on building shafts, where there are no intermediate floor slabs.
During the first lift, crews cast in anchors to install the climbing formwork for the second lift. The nearly 3-m-deep climbing platform, as long as 6 m, is then suspended from the anchor points. It typically is a rectilinear platform that bears on parallel steel channels that anchor the platform to the wall and serve as rails to roll the outer form back for climbing.
The two-bracket configuration only works if the wall is straight for it is not possible to have parallel brackets anchored to a curve. The solution is to have four brackets per platform. The two outer ones, which anchor to the wall, are perpendicular to the face of the wall and not parallel to each other. The two inner brackets, on which the form rides back, are parallel to each other and not perpendicular to the wall.
At NMAI, climbing platforms are used on the dome's walls and outside of stair cores at the edge of the building. For the stair cores with a constant but tight radius, Conesco provided a combination of modular componentsplywood panels with vertical timber membersand custom steel walers laser-cut to conform to the curve. Panels are spliced together with custom plates, also because of the curve.
For elevator banks with larger radii, Conesco achieved the curve using straight steel walers and inserting "shaping wood" between walers and the vertical timber beams. That saved money over custom laser-cut walers, says Schermerhorn.
The concrete structure will have a steel truss roof system over the main entrance that cantilevers 45 m off the main stair core and a mechanical shaft, each to one side of the entry. The backspan of each truss will connect into the dome's concrete ring beam. "The uplift is counterbalanced by the gravity load from the dome framing on the ring beam," says Dominic M. Cullen, structural engineer with Severud Associates Consulting Engineers, New York City.
The fourth floor will be hung from the fifth-floor truss through a series of hangers. There are slip joints under the fourth floor to allow the trusses to deflect up to 37 millimeters, says the engineer. The window detail was also designed with a slip connection.
The museum, currently 30% complete, is to open in 2004. Those who can't wait can visit an exhibit, housed in a site trailer, tracing the design and construction process.