Conventional lift slab construction is a method of casting all floors of a building at ground level, then jacking them into final position on the building's columns. In 1973 a patented modification was developed in which reinforced concrete bearing walls can be horizontally cast interleaved in the same stack with the slabs and then automatically unfolded into position as the stack is lifted. Growing interest in the method together with its documented success in withstanding seismic shocks have prompted this update on the forming and lifting techniques.
Bearing wall buildings up to 6 stories high have been built using the newer system, although 4- and 5-story structures are by far more common. Since the ingeniously hinged supporting walls unfold beneath the floor slabs, there is no need for the expensive permanent steel columns or collars required for conventional lift slab work. Formwork costs, which could range from 35 to 60 percent of the cost of a conventional building frame, are kept to a bare minimum here. Buildings have no permanent columns, and so no column forms are needed. Only edge forms are required for slabs and walls, and since the walls are also cast horizontally, there are no form ties and no tie holes to patch. The steel columns used for erection are removed and reused.
As with regular lift slab work, the first step is to cast the foundation and ground floor slab. The base slab is coated with bond-breaking material; reinforcement, edge forms and wall-separating inserts are set in place, and the lifting and hinge hardware installed. Then the first-story walls are cast directly on the ground floor slab. Perimeter forms are set back from the slab edge a distance equal to the wall thickness, so that as the walls later pivot outward into position during lifting, a flush wall-to-slab fit exists after erection.