Preliminary budget pricing for the Central Park Plaza in Omaha, Nebraska (two identical 17-story towers, each with a truncated right triangular shape) was based on a structural steel system, with fireproofed beams and girders, and a metal deck with concrete topping. When it became obvious that this system would not fit within the owner's budget, our firm, a forming and reinforcing steel subcontractor, was asked for suggestions. We proposed a concrete design system called the skip joist system. This produces an efficient concrete frame by spacing concrete joists 6 feet on center instead of the usual 3-foot-centers. In addition, we suggested that concrete beams running perpendicular to the joists be post-tensioned to keep them narrow and shallow.
Material calculations proved that this design was lean on concrete, with the total slab and beam concrete equal to slightly less than a 7-inch equivalent flat slab. All loads were handled by the efficient joist rib and post-tensioned beam system. Dead weight was low for this system and in fact was only slightly more than that of the structural steel system. In addition, the fire resistance inherent in the concrete joist system made applied fireproofing unnecessary, and erection time was shorter than for the structural steel system.
Hoisting equipment was the most critical item on this project, and the contractors used a variety of machines to meet the need. A 20-ton tower crane supplied the major reach. Stationed between the towers, its hook height of 244 feet and jib length of 196 feet provided access to the entire structure. Two mobile cranes and a material hoist provided additional capacity for unloading and material movement. Concrete was pumped with a truck-mounted concrete pump with placing boom. Placements were made on alternate days, pouring one half of one tower at a time.