Cribbing and their weight limits

[mikedb1972]

Hi all, i have been trying to find some info, regarding the weight capacities and limits of wood cribbing. I am aware of the different lumber types and hardwood and softwood debates, and even if plastic is better or not.

This is not the question i have really, i have completed several vehicle extrication classes, including one from the REDS team out of raleigh NC. the heavy vehicle extrication class i atttended had a set of rules and reg's regarding when to deploy 6x6 or 8x8 cribbing.

I know that the standard 4x4 cribbing will hold approx 6000 lb per cross, standard lay of 2 each tier, will hold 24000 lb and with 3 tiers its 54k, but for some reason i cant remember the approx limit of the 6x6 and the 8x8. i was hoping someone could remind me of the limits for the larger sizes.

As said before, i understand theirs alot of factors that may affect this. i just wanted to know the general limits on the larger sizes.

thanks for any info you can provide,

Capt Mike Bingle
East Nash Fire/Rescue
Wilson NC

[BigRig]

The definitive answer lies in the crossgrain bearing capacity of the wood with force applied perpendicular to the grain. This will vary greatly depending upon the species of wood used.

It is necessary to know the species of wood being used in order to determine the capacity of the cribbing timbers. Armed with that information you can determine the answer you desire.

Many folks accept the FEMA published values for USAR work and I think that is acceptable given the species of wood matches the figures stated. Otherwise your capacity may vary.

You may wish to visit www.woodbin.com for a list of values. Feel free to email me directly at billyleach@gmail.com and I will send you a handout regarding cribbing.

Hope this helps.

[mikedb1972]

Thanks BigRig, email sent for the handout.

As far as the type of material use, we generally try and use oak. not sure if its white oak or what species of oak. but we have fair access to oak lumber, and it seems to hold up very well.

i have also read the info on the website you posted, and it has alot of info regarding the strength of various types of materials. one thing i am not sure on, is how is the ratings are used, they are using PSI strengths. are we supposed to use the contact area of the cross points to determine the holding capacity, or am i in left field on this.

Oak, White
Compress. Parallel to Grain, Max Crushing Strength 7,440
Compress. Perpen. to Grain, Fiber Stress at Prop. Limit 1,070
Shear Parallel to Grain, Max Shear Strength 2,000

using this formula with a 4x4 having 16 square inches of contact, using the 1070 psi (pounds per inch) would give me a rating of 17,120 lb. assuming the load is evenly distributed. this seems a little high considering if you have four contact points thats 68,480 on a tier of cribbing. as i said i may be in left field on this, and hopefully someone can help me figure out this college math test, LOL

maybe this is just a little above my head. thanks

[TimatRescue42]

BigRig is correct. Standard FEMA/Army Corps of Engineers load ratings are based on surface area and cross grain load capacity. Standard tables are based on the load capacity of Douglas Fir or Southern Yellow Pine, the most popular cribbing materials. They have a cross grain capacity of 500 PSI.

4X4 equation:

3.5inX3.5inX500psi=6125 pounds per contact zone (round down to 6000)
So: 2X2 box crib = 24,000 Lb. 3X3 box =54,000 Lb.

6X6 equation:

5.5inX5.5inX500=15,125 Lb/contact (round down to 15,000)
2x2 = 60,000 Lb. ; 3X3 = 135,000 Lb.

If you know the cross grain capacity of your wood, you can plug it into the equation.

This is all laid out in the FEMA Structural Collapse Technician Course, Module 2a, page 12. Everyone should print this page and put it with your cribbing. It also discusses the two most misunderstood and frequent mistakes rescuers make when building box cribbing: Failure to leave a sufficient overlap at the end of each piece of cribbing and building the boxes too high (3, 1.5 and 1 times the footprint). The fema manual is at:

www.fema.gov/emergency/usr/sctc.shtm

While you're there, check out the other modules. Great training!


Be safe,

Tim

[BigRig]

You are correct with the math using the figure of 1,070 psi. Each species differs regarding capacity, some greatly.

I suggest everyone understand the capacity of their cribbing timbers.

[cmcolfax]

Have not been on the forums in a couple of years but came across this old thread while doing some research.

The subject came up during some training this week.

Billy - I would love a copy of that handout to share with my guys.

Tim - I did print that FEMA resource and we discussed it this morning.

Thanks guys.

-cmc-

[BHFF22]

http://www.sover.net/~branch/Safety.html

[CANFF2706]

For more up to date FEMA US&R information on cribbing I would suggest going to the Disaster Engineer website Library page:

http://www.disasterengineer.org/Libr...7/Default.aspx

The information you are seeking can be found in the Shoring Operations Guide (SOG)

http://www.disasterengineer.org/Link...bid=57&mid=394

There is also a very good (& very technical) student manual on the Principles of Wood Design.

http://www.disasterengineer.org/Link...bid=57&mid=395

[fyrfytr240]

FOG book says it all.