Signal-Rite has designed, built and had approved a rigging system hung from a construction crane for moving concrete forms out from underneath concrete slabs.
The concrete slab poured on top of the form covers about a third of an 80ft (24m)-long form, including all of one end. The slab obstructs access to the natural lifting points on one end.
The Signal-Rite rigging system consists of a 52in (1.3m) spreader bar and four wire rope slings. Two 7/8in (22 mm) wire rope slings run from the edges of the spreader bar to each side of the form just beside the slab. Beneath the chain hoist, balanced with 150 lb (46kg) of counterweight, two lines attach to the form about 20ft (6m) from the first two points. Two tether lines keep the hoist from rotating.
The hooks are spread as widely as possible on the form to resist tipping. But the obstruction of the concrete slab above means that the crane hook is not located above the centre of gravity of the form. If a crane tried to lift the form with four slings of the same length, the form would swing, and then hang at an angle.
This is where the chain hoist comes in. First front, and then rear, pick points are hooked up as the form is rolled out of the building. As the crane hook lifts up, the chain hoist hook is lowered by remote control, to keep the form horizontal. During the process, the hoist can take up to two-thirds of the form’s weight.
Signal-Rite has built three sizes of spreader, and specced a different sized Harrington chain hoist for each; a 5 US ton hoist with three falls of chain, 10 US ton hoist with four falls of chain and 15 US ton hoist with six falls of chain.
The company teamed up with Harrington supplier and parent firm, Kito of Japan, to develop a modified kit hoist whose power cable and remote pendant cable both come out the same side. Signal-Rite chose Harrington because its hoists are more forgiving of side pulling than other brands, said president Jeff York. The hoist chain angle ranges from vertical (90 degrees) to 65 degrees.
Signal-Rite has sold more than 20 systems over the past 18 months and it has been ‘wet stamped’ (approved) by professional engineer Avery Miller of Oakland, California. Customers include Atlas Construction Supply, Penta Group, Marnell Corrao, Patton Systems, Nibbe Brothers, Webcor Builders and Ceco Concrete.
