Outrigger on a Fresh Patch
A 50-ton rough-terrain crane is set up beside a newly poured concrete apron to lift a 9,000 lb pump skid at a 28 ft radius. The outrigger float on the curb side lands partly on the new concrete (cured 3 days) and partly on gravel. The chart shows 11,200 lb at 28 ft with outriggers fully extended. The concrete contractor says the mix reached 3,000 psi design but has no break results yet. The crane's load moment indicator shows 82% of capacity at the pick radius.
1. Is the setup acceptable as described?
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No. The outrigger is bearing on a partially cured, unverified slab and on mixed bearing materials, which can cause differential settlement and a tip. The chart value (11,200 lb) assumes a level, stable, fully extended setup; 82% of capacity leaves little margin for a float punching into green concrete. Require: cured-strength verification (break results or maturity testing), a load-spreading mat sized to the outrigger reaction, and a single uniform bearing surface. If not proven, relocate the crane off the new slab.
2. What would you calculate to size the mat, and what margin would you want?
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Estimate the maximum outrigger reaction, not just the load: reaction depends on load moment, boom position, and geometry, and can far exceed the 9,000 lb load on the loaded corner. Use the manufacturer's outrigger reaction chart or calculate from load moment and base dimensions, then size the mat so bearing pressure stays below the slab's proven capacity with a margin (commonly a factor of safety of 2 or more, or per the engineer's direction). Also check the slab's punching shear at the mat edge. Document the engineer's approval before lifting.