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destructive testing of stacked cribbing used by Fire and Rescue Services.
Sounding: We consider sounding the cracking sound wood cribbing makes when a load is applied . In order to determine how this sound relates to the load carrying capasities of the stack, the pressure was increased until sounding accrued, the pressure was noted and the load held until all the sounding had stopped. Then the pressure was increased until it could no longer be held and this pressure was noted. Failure was determined when the pressure began to drop. At this point it was determined the the stack would collapse.
Compression: The
reduction of the high of the cribbing stack was considered compression.
As the pressing load was increased the cribbing stack high is reduced.
This is important because the more the hight changes the weight can be
shifted on to it. Also as the wood fiber is compressed the Lincoln Log
effect tacks place. That is the the cribbing is saddled in to one another
and is keeping it from shifting.
Mixing
stock of verticle hight did not change the loading capacitie,. An
increase in the compression height was noted due to the increased number
of interfaces in the stack.
A
plate of 3/4 in. 5 ply plywood was placed in a stack of Red Maple as a
gage to help determine how much of a load was being applied. The plywood
sounder and distorted at 40 tons then held all the way to 65 tons. We feel
that this can be a valuable tool to determine the safe loading capacities
when using hard wood cribbing. The plywood only compressed an 1/8 in. and
40 tons allowed for a small amount of saddling in the hard wood.
7" x 7" White oak.
Cribbing is a basic tool
used in construction as well as Rescue. Knowing how it performs and how
to use it is important.
Chart of compression results
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