Outriggers: the soft spot and the crane that went over

A 30-ton boom truck with the load out puts most of its weight on one outrigger float: forty thousand pounds on a pad the size of a doormat. Most dirt starts to give at a quarter of that.

The points

• When an outrigger sinks, the boom tips out, the radius grows, the load gets heavier on the chart, and the crane goes over in about two seconds. Nobody outruns it, and the operator rides it down.
• The ground has to be firm, drained and graded so the crane sits level to the manufacturer's tolerance. If it is not, you fix it with mats, cribbing or a new spot, not with hope.
• Outriggers go fully extended, floats on, pads under the floats. Half-extended is only allowed if the chart has a column for it and the operator is reading that column.
• Look at what you are setting on: a curb, a catch basin, backfilled trench, a septic tank, a vault, the edge of a slab. The controlling contractor has to tell you about buried tanks, voids and utilities they know of, and you ask them before you set up.
• Level is not a feeling. Check the bubble before every lift and after every move, because one outrigger settling an inch during a heavy pick is how it starts.

Why it goes wrong

Pressure is the number that matters. A float 24 inches square is 4 square feet; 40,000 pounds on it is 10,000 pounds per square foot. Compacted gravel might carry that. Wet clay, fresh backfill, or a lawn over a leach field will not, and the chart assumes the ground holds. A 4 by 4 foot mat spreads the same load over 16 square feet and cuts the pressure to a quarter.

Out of level kills capacity fast. Load charts are built for a crane within about 1 percent of level. At 3 degrees out, many manufacturers put the loss of capacity near 50 percent, because the boom's side load and the shifted center of gravity are both working on the low side.

The soft spot hides. Backfill over a trench or a utility line settles for years and looks like the rest of the yard. Asphalt over a void will hold a truck driving across it and fail under a float sitting on it. Sidewalks and curbs are a few inches of concrete over whatever was there, and a float on a curb edge is a point load on the weakest part.

The rule gives you a lever. Under 29 CFR 1926.1402 the controlling entity has to make the ground adequate and has to tell you about known hazards under the set-up area: voids, tanks, utilities. If the operator or the assembly director says the ground is not adequate, the employer has to take it up with the controlling entity before the lift, not after.

The common mistake is the short-jacked side. The truck is up against a fence or a wall, so the operator extends the outriggers on the open side, leaves the other side in, then swings the load over the short side. The fully extended chart does not apply, and the crane tips toward the fence.

Watch the floats during the lift. A float that squeezes water out of the ground, a pad that cracks, a crib that starts to lean. Any of those and the load comes down where it is and the crane re-sets. The lift is not the priority; the crane staying on its wheels is.

Questions for the crew

1. What is under the outriggers today, and who told us that? Has anybody asked the GC about buried tanks or a septic system?
2. Are all four outriggers fully extended, and if not, is the operator reading the chart for the way they are set?
3. What is the heaviest pick and which outrigger takes that load? Is there a mat under it?

To close

Nobody signals the first pick today until the operator has walked all four outriggers, checked the bubble, and said the ground is good.