Rule of Thumb for Outrigger Support

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!

rulesayNO_medWe 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 B30.5-3.2.1.5 (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:

  1. Calculate the FORCE exerted on the ground (i.e. crane weight and attachments, plus load weight and rigging)
  2. Obtain the Ground Bearing Capacity (GBC) permitted for the soil you are set-up on. This is the PRESSURE the soil can withstand.
  3. Determine AREA by calculating FORCE ÷ PRESSURE
  4. Find the square root of the area to determine the blocking dimensions

For example:Calcoutriggerpads

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.

17 Responses to “Rule of Thumb for Outrigger Support”

  1. jim trail says:

    how would you calculate pad thickness? I know based off the soil types but was curious if that’s included in the formula?

    thanks

    • Crane Tech says:

      Jim- Thank you for your question. Calculating pad thickness is dependent on a variety of factors such as the weight of the crane, the weight of the load to be lifted, the ground bearing capacity, the square footage required, the cribbing materials to name a few. If you are concerned about a particular product being thick or dense enough to support a load, then we recommend that you consult with an professional engineer and/or a manufacturer of cribbing/pads/matting. When it comes down to it, if the cribbing/pads/matting collapse or breaks, then it wasn’t of sufficient thickness and/or strength.

  2. Bobby says:

    could you please provide the crane outrigger load calculation for 70ton crane.
    Crane boom length is 26.6m, Crane radius is 12m, lifting load is 6Ton

    • Crane Tech says:

      Thank you Bobby for your question. The article gives the calculation that we use to determine the force that could be exerted upon an outrigger and to thereby, determine what should be sufficient blocking for your crane. If you are unsure of how to do this and need further assistance, this is something that we teach as part of our Mobile Crane Operator course.

  3. Doug Greathouse says:

    What is the best way to determine allowable bearing pressure, or as you call it “Ground Bearing Capacity.”

    • Crane Tech says:

      Thank you for your question Doug. The ground bearing capacity must be determined by a soils engineer and provided to you. This is the only 100% accurate method.

  4. Anupam Chandra says:

    Hello,
    Can you please tell how to design the outrigger when lateral loads are coming on the outrigger?
    For example e.g., because of the acceleration and deceleration of the payload, the outrigger cylinder will encounter some lateral loads but since hydraulic cylinders are not designed for lateral loads, how are outriggers designed for that?

  5. Abdelazeem says:

    Hello
    How are you?
    i like to know how can i calculate the loads on any crane outrigger? during load raising

    thanks

    • Crane Tech says:

      Please see the first step of how to calculate the FORCE that may be applied to any crane outrigger. This result is the load that might be transferred to any outrigger as the boom moves through its work path.

  6. Jhelson C. Banzon says:

    Sir good morning, I want know where they gate 40PSI?that is formula?

    • Crane Tech says:

      You have to have the soils PSI provided to you by a soils engineer or qualified person. In the U.S. For further information on soil types refer to OSHA 1926, Subpart P, Appendix A.

  7. Ameer says:

    Hi

    I would like to know how I can calculate the outrigger opining length based on the load weight since we are working on the main road and we require to minimize the opining of outrigger as much as we can …
    Awaiting your reply please …

    Thanks & Best regards

    Ameer

  8. Yasir says:

    Good Day!!

    The force, that we calculated above, is shared between four ourtiggers. I think if we consider all the force on one outrigger, we are exaggerating the load. Don’t you think the load on any outrigger can’t be more than 75% of total Force? and accordingly, the area required should be (FORCE x 0.75)/GBC

    Kindly elaborate if it’s not correct way.

    Many thanks

    • Crane Tech says:

      Yasir – Thank you for your comment. We have seen studies that show as a load moves through its lift and if it passes over a single outrigger, the force upon it can be close to 100% of the load. It is for that reason that our article states “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.”

      If you choose to think that is exaggerating the load, then that is your call, but we’d rather be safe than sorry.

  9. Chas Dart says:

    Hi,
    Thanks for a really informative article, I knew it already but you set it out in a very clear and concise way.
    Thank you.
    Chas

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