A sophisticated second-generation lift slab construction technique is bringing much of the efficiency of factory production to jobsites in Mexico. Reinforced concrete gearing walls and slabs are cast in a single stack at ground level in a patterned method invented by Mexico City structural engineer Pablo Cortina. Within two years of its introduction the system has delivered a million square feet of apartments in more than 200 buildings in Mexico City and Las Truchas, Mexico. Use of bearing walls, ingeniously hinged to unfold beneath the floor slabs, eliminates the expensive permanent lifting collars needed for conventional lift slab work. The expense of forming columns is also eliminated since the stack is lifted on reusable temporary metal columns set outside the structure proper. Only perimeter forms are required for the slabs and walls. Also, since the walls are cast horizontally, no form ties are needed and there are no tie holes to patch. Speed as well as economy characterizes the new method. Once the foundation slab is done, two 5 story buildings can be completely cast every 10 working days on large projects. Four-story apartments buildings have been routinely occupied less than 90 calendar days following ground-breaking. A 5 story building requires about 2 hours per floor for the cycle of lifting, wall-plumbing and rerigging.