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Excavation for the column footings cut through the existing basement floor below the former plaza. But because they are some distance away from the older basement walls and footings, there was no shoring of existing foundation required during construction.

The three columns also support east-west spanning girders that help carry the two intermediary floors. The north edges of these floors are suspended from the roof cantilever above and stop short of the curtain wall.

As a result, the three-story-high glass wall, whose relatively light weight bears on the existing foundation, looks like a continuous surface, with only minimal lateral bracing. It is made up of 7-foot- (2-meter-) tall glass panels. Near the top are horizontal mullions to cap the facade and to create interesting shadow patterns inside.

The lack of an opaque spandrel at the two floor levels creates a gap between the curtain wall and the floors and helps bring much-appreciated daylight down to the ground level of the north-facing enclosed space.

The Twisting Stair

The most striking design element of the atrium is the circular stair that turns 180 degrees between floors. It is supported on a central structural tube, and its tapered treads cantilever off of that. To preserve the sense of lightness, the code-mandated risers are of glass.

Hoepf explains that the idea for the stairs was inspired in two ways. "The spine and rib cage form was inspired by a fern," he recalls, "with its central stem and the tapered shape of its leaves as they branch off. I also thought of the huge cylindrical fuel storage tanks, with stairs that circle around the outside. When the sun hits them, it creates phenomenal, fluid shadow patterns."

The stair is further supported by the girders at each floor, and by a steel arm that projects from the south wall at each landing. "We could have structured the stairs without the landing support," Hoepf says, "but the structural tube would have become too big, and there would have been some unacceptably noticeable movement in the stair."

The Welcoming Wings

Another predominant feature of the atrium is the pair of angled wing walls. The trapezoidal spaces created between these walls and the existing buildings form emergency exit courts.

These non-bearing walls, which required no new foundation, serve several purposes. They mark a clear delineation between new construction and old. Their interior faces of textured limestone form a new surface for the atrium space.

Seen from the outside, they create a visual enclosure for the glass box to further distinguish it from the surrounding stone and masonry buildings. And they reach out in a gesture of welcome to the main entrance road to the campus.

Above each of these wing walls is a skylight that brings more illumination into the atrium. As daylight plays on the textured limestone, it adds visual interest to the space.

An additional skylight along the south edge of the new roof brings light over a new wall on the atrium's south side. This wood-paneled wall conceals the small steel columns that support the stair landings, and its occasional protrusions are designed to play with the light as it descends to the ground floor.

To further add to the visual and structural impression of lightness of the space, the free-standing elevator shafts — whose lightweight metal stud walls bear on the basement floor below — are pulled away from the south wall to create a deep reveal. The elevators were required to bring the three-building cluster into compliance with the Americans with Disabilities Act.

The result of the architect's design strategy is that the stairs and floors appear to be free-standing. Perhaps that is why, now that the building has been in service for two years, its occupants tend not to use the elevators. Those stairs are just too much fun.

B.J. Novitski is managing editor for ArchitectureWeek and author of Rendering Real and Imagined Buildings.

Architect of Record: Teng & Associates
General Contractor: Graycor Construction Company

The BP Amoco Research Center atrium is an infill circulation space. Photo: Steve Hall/ Hedrich Blessing Computer rendering with one of the wing walls to the left and the circular stair in the background. Image: Teng & Associates, Inc. The atrium was built over an existing plaza between three brick-and-limestone lab buildings. Photo: Teng & Associates, Inc. Plan of the atrium. Image: Teng & Associates, Inc. West-facing section through the atrium. Image: Teng & Associates, Inc. A preliminary sketch of the atrium Image: Thomas Hoepf The third floor of the glass wall has horizontal mullions to create shadow patterns within. Photo: Steve Hall/ Hedrich Blessing A skylight over each angled wing wall helps to bring daylight down to the ground floor. Photo: Steve Hall/ Hedrich Blessing   Click on thumbnail images to view full-size pictures.