Outrigger Setup Over a Backfilled Utility Trench
A 90-ton hydraulic truck crane is set up on an asphalt street to lift a 9,600 lb rooftop unit. The boom is 105 ft long, and the pick radius is 50 ft. The load chart for the crane, with outriggers fully extended and beams at the manufacturer’s marked positions, shows 11,200 lb capacity at that radius. The rear street-side outrigger float sits on a 4 ft by 4 ft steel mat over a sewer trench that was backfilled two weeks ago. During preloading, the float sinks about 1 in. into the asphalt, and the cab bubble level shows 1.5° out of level. The site superintendent says the trench was “compacted with a plate tamper” and wants to proceed because the load is only 86% of chart capacity. The crew has 6 in. and 12 in. hardwood cribbing available but no engineered pad. Wind is calm.
1. Using the load chart and setup information, what is the immediate capacity status, and what does the 1.5° out-of-level reading do to that status?
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The nominal chart capacity is 11,200 lb at 50 ft radius, and the load is 9,600 lb, or about 86% of that value. That percentage is not the controlling issue. Load charts are based on a level crane, typically within the manufacturer’s stated tolerance, often about 1% or 0.5° to 1°. A 1.5° reading is outside that assumption and shifts weight onto the outriggers, changes the effective radius, and introduces side loading into the boom. The chart capacity can no longer be used as printed. The correct decision is to stop, correct the level condition, and verify the outrigger reactions and capacity with the manufacturer’s instructions for the actual configuration. Do not treat 86% as a safe margin while the crane is out of level.
2. Evaluate the ground support under the rear outrigger. What observations and calculations show whether the mat is adequate, and what would you require before lifting?
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The mat is 4 ft by 4 ft, or 16 ft². Even a modest outrigger reaction of 30,000 lb would produce about 1,875 lb/ft² under the mat, and the reaction could be higher depending on boom position and load. More important, the mat is bearing on recent backfill over a sewer trench. The 1 in. of settlement during preloading is evidence that the soil is yielding; a mat only spreads load, it does not create bearing capacity. Before lifting, require verification of compaction by a qualified person, an engineered pad or crane mat design, or relocation of the crane away from the trench. Recheck level after preloading and after the load is applied. If settlement continues, the lift cannot proceed.
3. What are the acceptable options to make the lift, and which would you choose?
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Acceptable options include relocating the crane to stable, verified ground; using an engineered pad designed for the actual outrigger reaction and soil bearing; reducing the load or increasing the radius only after confirming the revised chart capacity; or using a different lifting method. Hardwood cribbing alone is not a substitute for an engineered solution over a trench. I would choose relocation or an engineered pad, because both address the actual failure mode: loss of bearing under the outrigger. The crane should not be operated until the outrigger is level, stable, and supported by soil or a pad with adequate capacity.