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In Part 1 of this series, we discussed the rated capacities of wood box cribs. It was stated that a double, 4x4 wood box crib will theoretically support 24,000 pounds; 12 tons of load. As you can see from these tests conducted by Frank Maltese, there is a significant safety factor built into this rule of thumb. A box crib built of 4x4 spruce, a soft wood, supported 21 tons of weight during Frank's testing as did a box crib of 3 5/8-inch yellow pine cribbing. The hardwood maple 4x4 box crib, however, supported 60 tons or more before failure - five times more weight than the rule of thumb estimation for cribbing capacity.
A final safety note for consideration when working with box cribbing: To be able to support its maximum capacity, the box crib will have to be equally loaded. The load should be applied evenly across the contact layer and uniformly distributed down and through the entire box crib to ground. In Part 1, we identified a best practice of making the bottom layer a solid layer of cribbing when working on a soft surface. Another challenge is that if an offset load is applied, not only will the capacity of the box crib be significantly reduced, the possibility of catastrophic failure of the cribbing is a reality. Safety personnel must monitor all cribbing when supporting a lifted or unstable load. Check for compression. Check for saddling. And check for balance. The loading must be uniform or the evolution should be stopped and the box crib readjusted.
TASK: Given the need to support a lifted or otherwise unstable load, the rescue team shall explain cribbing safety considerations and understand failure points of a wood box crib.
RON MOORE, a FirehouseÂ® contributing editor, is a battalion chief and the training officer for the McKinney, TX, Fire Department. He also authors a monthly online article in the Firehouse.com "MembersZone" and serves as the Forum Moderator for the extrication section of the Firehouse.com website. Moore can be contacted directly at Rmoore@firehouse.com.