Longitudinal joists are spaced at 2 feet on center to allow for standard ¾-inch plywood sheeting to be placed as a sub-flooring skin for carpet or tile. Floor joist cross-stems are spaced at 8 feet or less to brace the longitudinal stems against buckling and to provide floor diaphragm action.
Each precast concrete longitudinal stem is 3 inches wide by 15 ½ inches deep. Two to five longitudinal stems may be connected together with cross-ribs to form a panel. A panel consisting of two longitudinal stems is 2 feet 3 inches wide. With prestressing, the panel can span up to 40 feet without any intermediate supports below. This advantage provides the homeowner with more layout alternatives in planning basement recreation rooms, lower level bedrooms, or workout areas.
Precast concrete cross-ribs permit panel erection without bracing or bridging. Panels can be made of pretensioned, post-tensioned, or conventionally reinforced concrete members. In the middle zone of the panel stems, 8-inch diameter openings provide adequate space for mechanical and electrical hardware within the floor depth.
The efficacy of providing relatively large openings in precast concrete flexural members was based on extensive theoretical and experimental research at the University of Nebraska, results that were later adopted in the PCI Design Handbook. This precast concrete floor may be covered with plywood, oriented strand board (OSB) or any other plate material capable of spanning the 2-foot gap between ribs. The underside of the floor panel may be finished with gypsum wallboard or other ceiling finishing systems.Roof beam concept
It was determined early that a total precast concrete roof system would not be economical because the roof's appearance needed to match the architectural style of other houses in the development complex.
Design restrictions led to the idea of using precast roof beams. In addition to bracing the walls during construction, the beams serve as platforms to support the upper trusses at much shorter spans than the distance between the exterior walls. The concrete roof beams are anchored to the exterior walls. The wood trusses are anchored to these beams, creating a system that can resist the wind uplift of tornados.
Spaced at 15 feet on center and spanning 35 feet 4 inches, the precast concrete roof beams allow the economical use of conventional wood roof trusses in spanning between the beams. The roof design provides a flat floor in the attic area for a loft or studio space. The upper story design of the NU Concrete House made about 600 square feet of extra loft living space available.
During production, one floor panel per day was typically produced. The panels were made as large as could be handled safely at the plant and on the construction site.
The largest panel was 45x11 feetx10 inches and weighed about 17.5 tons. The first sandwich wall panel, which was the smallest precast component fabricated for the house, was formed as a triangular shape to serve as the dormer gable en.
Construction began by pouring a cast-in-place (CIP) footing. Short CIP round piers were also placed at the garage and walkout areas where local building codes require footing construction to extend below the frost line to avoid frost heave (42 inches). Precast panels were placed either directly on the footings or the pier tops.