1. #1
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    Default It's Supposed to be Cribbing...Not a Jenga Game!

    As a rescue instructor, I always have the participants stabilize a vehicle that is resting on its' roof. More times than not, what they come up with is a secure vehicle held up by some well-placed struts. That's what I should see.

    There is usually one team however that tries to stabilize a roof-resting vehicle with just cribbing. So what that usually results in is a failure to understand the safety height limitations of a box crib.

    FEMA Urban Search & Rescue teams will build box cribbing up to a height equal to THREE times the length of the individual cribbing blocks. That's their protocol. But, they might nail it all together. I believe working with a roof-resting car is different than working with a damaged structure or building.

    So, over the years of seeing the successes and failures of box cribbing used to support a vehicle, I have come up with the recommendation that in vehicle rescue, the maximum height of a box crib should NOT exceed TWO times the length of the cribbing being used. In other words, with 18 inch long cribbing, the box crib should not be more than 36 inches tall. Actually, the lower the better.

    Besides my height recommendation, make sure that when you build each layer of the crib, the blocks sit in from the end of the block below it a distance about equal to their width. If your cribbing is a 4"x4"x18" block, then each layer sits in about 3 or 4 inches from the end of the blocks below it. That locks the cribbing together when under a load.

    Also, all wood blocks used in a box crib should have the grain of the wood running horizontally; not vertically. Vertical grain can fail a wood cribbing block by splitting or splintering when under a serious load.

    First is a photo of a box crib, using wood blocks, that clearly shows that its' total height is equal to the length of the blocks being used.

    The second image shows a tall box crib that is about as high as I would like to see one built at an incident scene. Any higher than two times the length of the cribbing and it begins to get a lot unstable!

    What guidelines or protocols does your department use when establishing the construction of box cribbing at a vehicle rescue incident? Let's share ideas...
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    Ron Moore, Forum Moderator
    www.universityofextrication.com

  2. #2
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    Default

    Hey Ron,

    Missed running into you in San Diego. You need to get out more…

    I am going to disagree with you a bit on this subject. Cribbing is an area where I find a great deal of confusion in the students I teach. They have often been taught by the “old guys” who have “always done it this way”. Unfortunately, the “old way” is often wrong. In our business wrong equals dangerous.

    To combat these misconceptions I always use established standards when I teach. To do otherwise is to not only open ourselves up to negligence but to, frankly, get ourselves or our patients hurt or killed. To my knowledge, the only established standard on Box Cribbing is the FEMA standard. This cribbing standard was established (I believe) by a combination of FEMA, the Army Corps of Engineers and the Department of Mines. In my opinion it is the ONLY standard we should be using.

    It is available (on page 12) in PDF format at:
    http://www.fema.gov/pdf/emergency/usr/module2a.pdf

    Two main points on this sheet point to the two biggest mistakes I see done by students (and you called out in your note above);

    1) You must leave an overlap of 4” at each joint or contact point
    2) Maximum height is 3 times the WIDTH of the structure, NOT the length of the cribbing

    The misconception between cribbing length and what FEMA calls the “width” is the most common point of confusion. What is width? If you look in the section in the lower right of the FEMA sheet you will see that it is the outer distance between the contact points, or the footprint. It is NOT the length of the cribbing! This makes sense because the part of the cribbing hanging out in space is not contributing to the stability of the structure. Everyone can understand that a structure with a small base is easier to tip over than a structure with a wider base.

    With 18” cribbing you need to subtract 8” (2 times the 4” overlap) to get a width of 10”. 3 X 10” equals a maximum allowed height with 4 points of contact of 30”. You should never see a box crib built with 18” cribbing taller than 30”. Ever. Per FEMA your 36” recommendation needs to come down a bit.

    However… In extrication we almost always build box cribbing stacks which get applied with two points of contact (two wedges between the crib stack and the car). Per FEMA, the maximum allowed height with two points of contact is 1.5 times the width. With 18” cribbing and a 10” width, this is 15”. You heard that right… 15”!

    My recommendation is to go to 24” cribbing. Now your width is 16”, your max height with 4 points of contact is 48”, and your 2 point max height is 24”.

    I suggest you laminate the FEMA cribbing sheet and keep it in your cribbing compartment. That way when you are told to build a crib box that puts you and your patients in an unsafe situation you can have a reference to use to try to stop it. To build it anyway even though you know it is wrong is called “criminal negligence” in a court of law. Contrary to popular belief we firefighters are not immune from negligence.

    -Tim
    Last edited by TimatRescue42; 03-04-2011 at 11:26 AM.
    www.rescue42.com

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    Thanks so much, Tim, for taking the time to explain the proper cribbing height criteria. I really appreciate getting this much detailed information in this Forum. You really are an expert on stabilization and I completely respect you and your contributions to this topic.

    Here's an example image of what I see the most in areas where struts have not been accepted as the solution to vehicle stabilization, whether it is at a scene or in training.

    One thing that is worse than having an unstable stack beneath an unstable vehicle is having rescuers think that they have stabilized the load.
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    Ron Moore, Forum Moderator
    www.universityofextrication.com

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    Default

    Also use longer cribbing so you can go higher.

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