Ribbed cast in place columns will give a bold, distinctive look to an office tower under construction in Century City, California. The structure is the 15 story Northrop Building, whose columns are being shaped by an unusual process of concrete casting. Steel forms, designed to allow for close dimensional tolerances, are placed around steel reinforcing bars to form the front and sides of columns, which are cast in sections 13 feet 6 inches in height. Ten parallel tapered strips of extruded polyvinyl chloride, one and five-sixteenths inches wide by three-fourths of an inch high and coated with a form release agent, are attached to grooves cut into the inside face to the plywood. The strips extend the entire length of the form. These create the ribs when the concrete is cast. The forms are set in an upright position and securely fastened. Then the concrete is placed. Specifications call for lightweight concrete containing water-injected aggregate to minimize absorption and "flotation." Pozzolan is added to the ready mix to darken it so that the finished concrete will blend with the brick paving on the plaza.