The answer is just “NO.”
“Well I’ve heard it has to be 3x the outrigger float.”
“I’ve always used 4x times the float… better be safe than sorry.”
“Who cares what size the float is? I go with the crane capacity divided by 5.”
“There are no rules, I just stick whatever I have on hand underneath the pad,”
“I just use the pads that come with the crane, surely that is enough.”
STOP. Just Stop the Madness!
We hate to break it to you, but there are NO safe rules of thumb when it comes to cribbing. Nowhere in OSHA does it say that you will be ok if you use the rule of thumb for cribbing, instead in 1926.1402 OSHA states: that cranes must be assembled on ground that is firm, drained and graded sufficiently, in conjunction with supporting materials, such as blocking, cribbing, pads, or mats, to provide adequate support and levelness. For those not in construction, ASME B30.5-184.108.40.206 (i) states:
Blocking under outrigger floats, when required, shall meet the following requirements:
(1) sufficient strength to prevent crushing, bending, or shear failure
(2) such thickness, width, and length, as to completely support the float, transmit the load to the supporting surface, and prevent shifting, toppling, or excessive settlement under load
Finding “Adequate Support”
So how do you determine what “adequate support” is? You have to calculate it. No rules of thumb, no wing and a prayer, no best guess, just a calculation to determine the size of cribbing/blocking required.
A key thing to remember is that as a crane works and rotates over the various corners and quadrants, the load will shift and at any one time a greater load will be placed on one outrigger than on the others. It is for this reason that equally dividing the load around the outrigger pads is a mistake. If you consider that 100% of crane and maximum load may be exerted on any one outrigger pad at any time you can plan for proper outrigger set-up.
Sure it sounds simple, but we all know that having adequate information to complete the calculation on-site can be challenging depending on whom you talk to. The good news is, there is hope. It can be done. With some basic information on the crane weight, gross load weight and ground bearing pressure, then the size of outrigger cribbing for adequate support can be calculated.
Crane Tech teaches a 4 step process:
- Calculate the FORCE exerted on the ground (i.e. crane weight and attachments, plus load weight and rigging)
- Obtain the Ground Bearing Capacity (GBC) permitted for the soil you are set-up on. This is the PRESSURE the soil can withstand.
- Determine AREA by calculating FORCE ÷ PRESSURE
- Find the square root of the area to determine the blocking dimensions
When it comes down to it, a rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. While it is something that is easily learned and applied to make a preliminary determination, it is not something that Crane Tech believes should be used during crane set up. So when setting up cribbing and someone says, “just make it 3x times bigger than the outrigger pad”, just say NO! Take the time to calculate and plan – it can make the difference between a safe lift and a sunken one.
P.S. Always remember to follow your crane manufacturer’s instructions and check the crane level frequently to make sure your outrigger setup is adequately supporting your crane.
About Soils PSI
For further information on soil types refer to OSHA 1926, Subpart P, Appendix A. We also recommend that all users obtain soils engineering advice.