A hollow concrete cylindrical core standing 165 feet tall is just 23 feet 4 inches in outside diameter and 10 inches in wall thickness. Built at the Dayton International Airport, the new aircraft control tower was erected at a rate of one 15-foot lift every four to five working days using a single-shift crew of only nine carpenters and iron workers. Fast erection was possible because the general contractor, Danis Building Construction Co., Miamisburg, Ohio, selected a new, more versatile, and safer retractable-carriage type jump-forming system for forming the core's radically curved wall.

The forming system Space-Lift is a fully engineered all-steel forming system that allows the jump-forming technique to be employed for either straight or curved walls. Made by Symons, Elk Grove Village, Ill., the retractable carriages of nine 8-foot-wide Space-Lift units, each fitted with a 16-foot-high Flex-Form panel gang, were sufficient to encircle the core's 72-foot circumference. The core is a perfect cylinder of consistent wall thickness from top to bottom.

The tower core's inside wall of 10 feet 10 inches radius was created using a conventional Symons hub-and-spoke soldier-form platform supporting nine gangs of 16-foot-high Flex-Form panels bent to their tightest recommended curvature. A hole was cut in the center of the hub so a laser beam could be shot up the tower centerline for vertical alignments during construction.

Doors built into the gang on one interior unit could be opened to shorten the formwork's radius slightly, allowing the panels to pull away from the wall about an inch for stripping and raising all nine units at once.

Flying not desired"For the wall, we wanted a jump-forming system whose formwork would not need to be disconnected and flown to the ground for cleaning, placing 32 outer wall weld-plate embeds for each lift, and other work," reports Mike McTall, the Danis project manager. "Such a system would make for a faster cycle, less crane time, assure greater precision of the radius as the units were jumped, and to promote the safety of our crew, especially on the many windy days we expected, which would make flying formwork next to impossible. I'd estimate that the build cycle would have taken six to seven days compared to the four to five days we actually required had a nonretractable jump system been chosen. It may have required a larger crew as well."

The project manager said the formwork system was rigid enough and designed to withstand the pressure of 15-foot lifts of 5000 psi concrete without ties. The outer and inner units physically interlock to effectively create tension and compression rings. Still, lifts had to be poured slowly and evenly around the circumference, no faster than 4 feet per hour using 32, 1-yard bucketfuls. Maturity loggers were embedded in the concrete to determine, along with cylinder breaking, when stripping and jumping operations could begin.

"Not needing ties hastened work by removing potential interferences with the precisely mounted outer wall embeds, plus an additional 32 inner wall embeds, and by eliminating much concrete patching," McTall adds.

The outside embeds will support a decorative/structural, steel-and-glass lattice curtain wall that will wrap around the concrete core and resemble the shape and cross-hatch pattern of an ice cream cone. Interior embeds will support steel stairs, steel platforms every 15 feet, and an elevator off to one side. The concrete core eventually will be topped with a 75-foot-high, enclosed steel structure having levels for electrical and mechanical equipment rooms, an elevator lobby, a break room, and the air controller cab on top.

Engineered for curved walls

The new jump-forming system employs unique curved brackets for mounting gangs to the frame's straight steel components and its formwork-supporting carriage that rolls back from the wall along parallel tracks. The brackets have adjustable mountings that provide angular offsets with the frame elements to accommodate wall curvature. Similarly, the system's special jump shoes for curved walls are mounted perpendicular to the wall but are secured to the frames' leg supports with pivot pins and a locking mechanism. There is no need to shim the shoes with wedges to orient them square to the frame. Upper and lower jump shoes, bolted to inserts in the previously completed concrete lift, kept the forming system firmly anchored to the wall.

Nine double-frame units support 4x8-foot panels stacked four high. The carriage on each unit rolls back 32 inches, giving ample room for cleaning, oiling, installing embeds, tying off rebar mats, setting jump shoes for the next lift, and patching concrete from the preceding lift. The carriages are moved using a standard ratchet wrench activating a rugged rack-and-pinion drive mechanism.