What holds that up? A feat of engineering

It hovers over South Tryon Street – the massive fourth level of the Bechtler Art Museum, jutting 30 feet beyond its most obvious support, a slim column 30 inches in diameter.

Looking up, you wonder: What's holding that up?

The technique that allows for such drama is the cantilever, where a beam is supported only at one end. It's magical because the cantilever seems to defy a basic principle of building: Weight must be supported. Instead, as structural engineer Robert Williams said, “There's something hovering in the air that ought not to be hovering in the air.”

Recent construction in uptown Charlotte uses the cantilever to an unprecedented extent. New condominium towers sprout balconies from their glass sides, for instance. But the most noticeable cantilevers are on two high-profile buildings blocks from each other: the Bechtler and the 20-story NASCAR Plaza adjacent to the future NASCAR Hall of Fame.

In an uptown where for decades power and prestige have been expressed architecturally through the vertical – the tall building – these two use a different way to stand out: the horizontal.

The most basic building scheme, and one of the oldest, is the post and lintel: two verticals supporting a horizontal. Think Greek temple, your house – or a doorway. With new materials and building techniques, the 20th century brought the cantilever into its own as architects and engineers looked to make a statement with structure.

“With a cantilever,” said Williams, who works for RTWA Engineers in Charlotte, “it's kind of that image of defying gravity, and that's the thing laypeople find fascinating.”

Mario Botta, the Swiss architect who designed the Bechtler, and the architects at Pei Cobb Freed & Partners in New York who did NASCAR Plaza, confronted different problems. But each found the cantilever a good solution.

Presence and blending in

The challenge for the Bechtler was size and context.

Part of the Wachovia cultural complex, the building will house the modern art collection of retired Charlotte businessman Andreas Bechtler. It will be only 30,000 square feet compared with 145,000 square feet at the new Mint Museum in Center City, which will stand across the street.

Moreover, the Bechtler will have towering neighbors – for instance, the 48-story Wachovia headquarters climbing skyward just to the south.

To give the smaller museum presence, the architects decided to flip it, according to Dave Wagner of Wagner Murray Architects, a Charlotte firm working with Botta. Rather than locate the main exhibition level on the ground floor, they put it on the cantilevered fourth floor.

It is supported on one side by lower floors and on the other by the slim column.

“Balancing a rock on a feather – that's the way we've talked about it,” Wagner said.

For the designers of NASCAR Plaza, the issue was blending with the adjacent NASCAR Hall of Fame.

The 20-story office building was added to the project after the architects completed the preliminary design for the Hall of Fame. The hall's dominant feature will be a swooping oval of stainless steel banding the top of the building, a reference to the shape of a race track.

The tower had to fit in but also in some way match the dynamic shape of the Hall of Fame.

“It's a difficult challenge to try and fit that (building) into the site,” said Bruce White, associate partner at Pei Cobb Freed. “The Hall of Fame is the big statement we're trying to make.”

Part of NASCAR Plaza is traditional, rectangular and made of buff-colored precast masonry. But the cantilevers projecting from all four sides are something else. They will be skinned in contrasting silvery glass and aluminum and give the building a more sculptural, dynamic shape.

Moreover, the 17 cantilevered floors are curved and become larger as the building rises, the cantilever going from 12 feet to 22 feet. So that portion of the building flares out, echoing the oval on the Hall of Fame but not overpowering it.

Stopping the sag

Use of the cantilever can cause problems, if the beam supporting the weight begins to bend. Think of a diving board. Engineers call it deflection, which can cause the floor and interior and exterior walls to sag. As Williams said, “It doesn't feel good to walk on a floor with deflection.”

In the '90s, engineers found America's best-known cantilevered building, Frank Lloyd Wright's Fallingwater, was sagging. Built over a waterfall south of Pittsburgh in the 1930s, the house has cantilevered balconies.

Wright didn't use enough steel to reinforce the concrete. And even though the contractor – unbeknownst to Wright – added reinforcement, the balconies would have fallen if they had not been repaired.

The designers of the Bechtler and NASCAR Plaza took pains to avoid Fallingwater's fate.

The Bechtler's fourth floor has diagonal bracing, which unites the ceiling and floor into one unit, making the whole level stiffer and stronger. The supporting column is made of high-strength concrete, an extra dense material that will support about 15,000 pounds per square inch. The typical concrete floor, said Wagner, supports 4,000 pounds per square inch.

When finished, the column will bulge and be covered with terra cotta tiles colored orange, as will the building. The present green color is waterproofing.

NASCAR Plaza has a system of posts at the edge of the cantilevered floors that can be seen now but will be invisible when the curtain wall is fully installed. If there is any slight bending, according to White, the poles will allow the floors to deflect together, preventing separation of the walls and floors.

Such smart engineering makes the cantilever workable. But more than nuts and bolts, the technique is about beauty.

Said White of NASCAR Plaza's slicing profile, “At Caldwell Street you can see the prow edge of the building, where we wanted to see the drama.”

The space beneath the Bechtler's cantilever, architect Botta said in an e-mail, will not only be dramatic but also a place where people can gather. “The covered plaza becomes a new urban space, an architectural gift to the city,” he said.