Outrigger Over a Storm Drain at 98 Percent of Chart
A 100-ton hydraulic truck crane is set to place a 24,000 lb precast stair tower onto a second-floor deck. The load chart for the 62 ft boom at a 38 ft radius on full outriggers shows 26,500 lb. The chart's rated capacity is a gross figure, and the 900 lb hook block plus 1,150 lb of shackles, slings, and spreader beam all count against it. The rear outrigger reaction for this configuration is 82,000 lb. Each outrigger float is 24 in by 24 in. The site is compacted structural fill with an allowable bearing pressure of 2,500 psf. A 30 in diameter reinforced concrete storm drain crosses beneath the rear outrigger with only 36 in of cover; the utility owner's drawings show the pipe but give no load rating. A spotter is available, and the crane's load moment indicator was calibrated six months ago. The general contractor wants the pick today and says the pipe 'has been there for 30 years.'
1. Calculate the gross load and the percentage of chart capacity. Does this pick qualify as a critical lift under a typical 75% threshold, and what does that change about how you plan and run it?
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Gross load calculation
- Load: 24,000 lb
- Hook block: 900 lb
- Rigging (shackles, slings, spreader): 1,150 lb
- Gross load: 24,000 + 900 + 1,150 = 26,050 lb
Percent of chart: 26,050 / 26,500 = 0.983, or about 98.3%.
Critical lift: OSHA does not set a numeric threshold, but most company lift plans and ASME B30.5-based procedures classify a lift as critical at 75% of chart or above. At 98.3% this is clearly a critical lift. That triggers a written critical-lift plan, independent verification of the load weight (scale ticket, load cell, or engineered calculation), a documented pre-lift meeting, engineer review of ground conditions and rigging, and confirmation that the boom length, configuration, and outrigger setup match the chart.
The margin is only 450 lb. A tag line, mud, ice, or a mis-stated load weight exceeds it. The LMI is an aid, but the plan must be based on the calculated gross load, not on the display.
2. Size a mat for the rear outrigger and evaluate the storm drain. Show the calculation and state exactly what you would require before the pick.
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Mat sizing
Required bearing area = outrigger reaction / allowable soil pressure = 82,000 / 2,500 = 32.8 sq ft.
The 2 ft by 2 ft float alone is 4 sq ft, which is about 20,500 psf, far over the allowable. A 6 ft by 6 ft mat (36 sq ft) gives about 2,278 psf, only about 9% under the allowable and with no safety factor. Applying a 1.5 factor of safety: 82,000 x 1.5 / 2,500 = 49.2 sq ft, so a 7 ft by 7 ft mat (49 sq ft) is the minimum, and the mat must be structurally adequate in bending. A steel plate or engineered crane mat is preferable to timber of unknown grade.
Storm drain
The mat does not give the pipe capacity. Load spreads through the fill at roughly 45 degrees; with 36 in of cover, a 7 ft by 7 ft mat covers about 13 ft by 13 ft at the pipe crown, or 169 sq ft, so about 485 psf reaches the pipe. But the pipe's rating is unknown. 'It has been there 30 years' is not a capacity. Before the pick I would require the utility owner's written load rating or a structural engineer's approval, or I would bridge the drain with engineered plates, or relocate the outrigger so the load does not reach the pipe.
3. The GC pushes back, says the pipe is 'probably fine,' and argues the mat will spread the load. What is your response, and what is the minimum you would accept?
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A mat spreads load; it does not create structural capacity in a buried pipe. The 45 degree spread is a rule of thumb that assumes uniform, compacted fill, and it does not apply to a flexible or deteriorated pipe.
My response: the lift is on hold. The minimum I would accept is one of the following:
- A written capacity from the utility owner or a licensed engineer showing the pipe can carry the calculated pressure at the crown, or
- An engineered bridging system (steel plates or a crane mat designed to span the trench) with the design on site, or
- A revised lift plan that moves the outrigger away from the drain, even if that means a larger crane, a longer boom, or a different setup position.
I would document the hazard, the options presented, the GC's verbal assurance, and my stop-work decision, and notify the site safety officer and the crane owner. Verbal assurances and schedule pressure do not transfer the lift director's responsibility.