The problem: to roof an overall area measuring 103 by 480 feet at a height of 75 feet above floor level, and to do this without interfering with the operations of a 450 foot cement kiln already installed within the structure. The 40 foot wide arches called for would span 80 feet between supports with 12 foot cantilevered extensions at each end. The solution: an ingenious and yet strikingly simple adaptation of portable forming. Steel bow string trusses covered with plywood were used to form the arches, and portability was obtained by means of two movable steel towers erected on the side of the building available for work space. On the other side of the building, bents were clamped to the building columns to support the full load during and immediately following casting operations. The arches were cast in pairs and after each casting operation the trusses were lowered to clear the edge beams and then moved forward by shifting the supporting towers and bents.