One of the greatest challenges in mechanized tree care – how to set up a crane on grass – became a focal point at a recent OSHA hearing. I’d like to take this opportunity to share with you the cribbing procedures utilized by David Happ and Marquis Tree Service in addressing this challenge.
Personally, I’ve had to defend this process multiple times in court following unfortunate accidents. To date, the industry and government standards we rely on for guidance haven’t provided specific instructions for cribbing a crane on grass. However, it’s crucial to note that these same standards explicitly mandate the proper setup of a crane, whether using cribbing or not.
Testing the ground
Several factors need to be considered when setting up; undoubtedly, the first factor starts with the ground itself. When we set the crane up on a construction site, it is customary for the ground to first be prepared to accommodate it. Years ago, OSHA published a soil-bearing-capacity chart as an aid for this process. Knowing the surface type the crane will be set up on and what kind of load it will bear determines how much cribbing is to be used.
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Cranes in arboriculture, however, bring a new dimension to crane setup. The arborist crane operator, per ANSI Z133 safety standard, requires different skills than construction crane-operator skills to operate a crane safely. As there is no universal standard for the compression rate of lawns, the inability of a crane operator to calculate the compression of the outrigger to the ground bearing is replaced with testing the ground conditions with the weight of the crane itself.
Marquis Tree Service, a 24-year TCIA member company based in Billerica, Massachusetts, enforces strict policies, like many companies prioritizing safety. They require a minimum amount of cribbing on every crane and have a procedure for setting outriggers on cribbing, leveling the crane, loading its counterweights and assessing soil compaction.
During this process, the operator first raises the boom to its highest working angle. A ground worker then monitors the soil compaction while the upper structure is rotated 360 degrees. If excessive compaction is observed, the operator must call for “swamp mats” or “crane mats” to crib the entire crane.
Cribbing
I recently met with David Happ, Marquis Tree Service vice president, who showed me how the cribbing is done. According to him, these cribbing procedures were developed during “25 years of working with cranes in people’s yards.” Setting up on an unknown surface can be a challenge. Knowing that going in and being prepared for every scenario is just part of the job, according to Happ.
So, for this demonstration, he set up one of their Liebherr cranes with four different cribbing configurations, one for each outrigger, depending on the need for a larger or smaller footprint. Six-foot-long timbers are used as the standard, and then more cribbing is added as needed. (Photo 1)
Photo 2 shows the “smallest basic configuration for a relatively firm, known surface,” says Happ. “We’re not much concerned with any cosmetic damage here. In a typical configuration, we could see 80,000 pounds on an outrigger, about 40 pounds per square inch on that outrigger.”
As shown in Photo 3, adding 4- to 6-foot timbers on top of two 4-by-8 sheets of plywood reduces the overall impact on grass to approximately 25 pounds per square inch, “a configuration we’d typically use on grass or a potentially delicate driveway,” says Happ.
“We had these hardwood-timber mats made – 10 feet long by 7 feet wide – for when we have some unknown underground thing going on and we have some concerns. We can really spread it out. Maybe about 6 pounds per square inch. We can set up on almost any surface at this point,” explains Happ. (Photo 4)
Going to extremes
“In extreme situations, soggy or swampy situations, we’ve put three swamp mats under these pads and then built it up like this. This configuration gets you roughly 8 pounds per square inch. It’s a 4-foot by 16-foot hardwood swamp mat bolted together. We use that when an operator has a concern about a setup. We’ll have a log-truck driver bring out those swamp mats and we’ll build out some sort of cribbing underneath it, just to be sure.”
Then, to test the security of the outriggers with cribbing, Happ boomed the crane up to its highest angle, lifted the counterweights and then rotated the cab 360 degrees. If the operator and/or crew were to see any sinking or movement during this test, they would consider a different kind of cribbing or move the crane to another spot.
The computer screen in the cab displays how, when the counterweight swings over each outrigger, the pressure on it increases. As Happ says, “If we’re going to encounter any issue with ground stability, we’d much rather find it with no boom out, performing no work, than once we start actually picking trees.” (Inset)
By using cribbing that goes well beyond the construction-industry standard of three times the square footage of the outrigger pad (often going five times or more), thus spreading the weight over a greater surface area, the crane operators of Marquis Tree substantially reduce the pounds per square inch. The ground underneath will have less cause to depress and/or shift, lending the crane greater stability while doing the job.
Conclusion
As a customer’s lawn could have any amount of topsoil or unknown voids, OSHA’s soil-bearing-capacity chart just isn’t sufficient information to make cribbing decisions.
During our time together, I noted that the ground-bearing pressure on that lawn from one house to the next is going to be different. There’s no standard to tell you that this person’s lawn is going to bear so much weight. Having additional cribbing and then testing it with the counterweights provides so much more information for the crane operator to decide whether the crane will be stable as is, or if a readjustment is necessary before doing the job.
Additional cribbing also means less compression on the lawn, so less damage for the homeowner to deal with afterward. So not only will the crane operator create a safer job site with these cribbing techniques, but the company also will have a happier customer after the job is finished.
Jay Sturm and Cranes101 staff appreciate the generous time and efforts of David Happ and Marquis Tree Service in making this article happen.
Jay Sturm is founder of Cranes101, a 12-year TCIA corporate member company based in Bellingham, Mass.
TCI Magazine’s Educational Sponsored Content is a sponsored feature available only to TCIA corporate members. This article is sponsored by Cranes101.
