skylights made of Pyrex glass tubing. At the corners, where the walls usually meet the ceiling, the glass tubes continue up and over and connect to the skylights, creating a clerestory effect and admitting a soft light. The Great Workroom is the largest expanse of space in the building, and was intended for the secretaries of the Johnson Wax company, with a mezzanine for administrators. Construction In the Great Workroom, the columns expand from 9 inches (23 cm) in diameter at the bottom to "lily pads" 18 feet (5.5 m) in diameter at the top; skeptical building inspectors required that a test column be built and loaded with twelve tons of material. After the test column proved capable of supporting the specified load, Wright had the load progressively increased. Only at sixty tons load did any crack appear. The building was completed in 1939, considerably over-budget. It proved very difficult to properly seal the glass tubing of the clerestories and roof, and leaks occurred. The problem was not solved until the company replaced the top layers of tubes with skylights consisting of angled sheets of fiberglass and specially molded sheets of Plexiglas with painted dark lines to resemble the original joints in a trompe-l'œil when viewed from the ground. Wright designed not only the building but its furniture. His chair design originally had only three legs, supposedly to encourage better posture (because one would have to keep both feet on the ground at all times to sit in it). However, the chair proved unstable, tipping very easily. Purportedly, Wright redesigned the chairs after Herbert Johnson asked him to sit in one, and he fell out of it. Johnson Wax has continued to use Wright's furniture. Despite these problems, Johnson was pleased with the building and later commissioned the Research Tower and a house (known as Wingspread) from Wright. Research Tower The Research Tower was added in 1950 to the Administration Building, and provides a vertical counterpoint to its horizontality. It is one of only two existing high-rise buildings by Wright. Cantilevered from a giant stack, the tower's floor slabs spread out like tree branches, providing for vertical segmentation of departments. Elevator and stairway channels run up the reinforced concrete core, which Wright called a tap root. This single core was based on an idea that he had proposed in 1929 for the St. Mark's Tower, and which he used again