Jacking and Skidding a Machine Base With Questionable Support
A 22,000 lb machine base must be moved 30 ft across a concrete floor with a 4-inch step down at a doorway. The crew plans to use four hydraulic jacks rated 10 tons each, steel rollers under 1-inch plate, and a come-along for the pull. The floor is 6-inch slab on grade, but a floor plan shows a 4-inch conduit trench crossing the path with a cover plate. The jacks will be placed on the plate over the trench. The crew plans to jack one end at a time, place rollers, and pull with the come-along while two workers guide the base by hand.
1. Evaluate the jacking arrangement and the support under the jacks.
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Four 10-ton jacks give 80,000 lb total capacity against 22,000 lb, which is ample in aggregate, but jacking one end at a time means only two jacks carry the load at that moment (40,000 lb capacity) and the load is eccentric. That is still adequate for capacity, but stability is the concern: jacking one end raises the CG and can tip the base if the other end is not restrained.
The critical issue is support. The jacks sit on a cover plate over a 4-inch conduit trench. A cover plate is not a structural bearing surface for a point load from a jack; it can deform, shift, or collapse into the trench, dropping the load. The slab is 6 inches on grade, which can carry distributed load but point loads from jacks concentrate stress and can crack the slab near the trench edge.
Controls: do not jack on the cover plate. Bridge the trench with a structural member sized for the jack load plus impact, or relocate jacking points away from the trench. Use cribbing or steel plates to spread jack load. Verify the slab's capacity or use load-spreading pads. Jack both ends in small increments with the load restrained, and never place any part of the body under a suspended load.
Judgement: the plan is not acceptable as written; the trench cover is the single-point failure.
2. Assess the roller and pull arrangement, including the step down and manual guidance.
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Rollers: steel rollers on 1-inch plate distribute load, but the plate must be continuous and restrained; rollers can skew and the load can walk off the plate. Roller diameter and the step down at the doorway are problems: a 4-inch drop will cause the load to tip or drop onto the rollers, producing shock loads and loss of control. The step must be ramped or the load lowered in a controlled manner, not rolled off an edge.
Pull: a come-along is a manual device with limited capacity and no controlled lowering; its rating must exceed the rolling resistance plus the slope/step resistance with margin. Pulling while two workers guide by hand puts people in the line-of-fire and near the pinch points between rollers and load. Use a tag line and keep hands off the load; establish a exclusion zone; use a snag-free pull line and a rated anchor.
Controls: build a ramp or use a controlled lowering arrangement for the step; use a winch or lever hoist with a rated capacity and a brake; calculate rolling resistance and add a safety factor; use a spotter and communicate with hand signals; chock the load whenever it is not being moved.
Judgement: the manual guidance and uncontrolled step-down are unacceptable. The move should be re-planned with a ramp, a rated pull device, and exclusion zones.