On Fifth Avenue, between 88th and 89th Streets in New York City, one of the most usual concrete buildings ever constructed- the Solomon R. Guggenheim Museum- is nearing completion. Featuring a spiral ramp making six turns at an approximate grade of 3 percent, stiffened by exterior webs at 30 degree intervals, and an inner court covered by a glass dome, this structure represents the great architect's conception of a natural facility for viewing modern art in a modern setting. The structural design features rigid slabs with spans up to 60 feet and cantilevers up to 25 feet, some with highly concentrated end loadings. The main ramp cantilevers 14 feet 6 inches to the inner court from a 13 inch deep rigid exterior beam of varying widths. Since the greater widths occur at the higher levels, the overall width of the ramp increases as it spirals upward. The inside edge of the spiral ramp which carries a 3 foot high plaster parapet, turns on a smaller and smaller radius as it rises upward. Similarly, the outside edge which carries the gunite curtain wall turns on a greater and greater radius at it rises upward. The relationship between these constantly changing radii and the gradually rising plane of the ramp is so arranged that at any point in the height of the structure a horizontal plane will always intersect a true circle. This is the result of a basic pattern of shapes. It is a conception of a large cone into which is fitted an inverted smaller cone whose apex is theoretically some 270 feet above datum zero. To lay out this complex structure would ordinarily be extremely difficult, but the work was simplified by an overall grid system of 8 foot units. All lines, radial points, and axes tied into this grid system in a precise pattern which gave the component parts of the structure an orderly natural scheme. All working points were tied by coordinates into the grid system, and for checking purposes the center point of the structure was physically carried up as the job progressed. The shop drawings and layout plans were accordingly superimposed upon this grid system.