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TOPIC: Cribbing Crush
OBJECTIVE: Understand how a wood box crib performs at failure
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.
This University of Extrication column is Part 2 in a series on lifting and stabilizing heavy loads. In this column, we'll address wood box crib safety considerations, explain what a "balanced" crib is, and review box cribbing failure.
Rescue authorities recommend that when we build one layer on top of the other to form a box crib, we should position the wood blocks above slightly in from the edge of the layer below. This keeps the end of the wood below exposed and is done to ensure the best stability of the crib. We have been told that under load, the end grain of wood expands.
To better explain how this happens and what it looks like, we turn to a true expert on wood, Frank Maltese. (Yes, that is his real name.) He is the president and owner of Branch Corp., a Vermont-based company that manufactures wood cribbing kits called Crib Pac, Extrication Guards, which are individual sharps protection pads for use at rescue scenes, and the Extrication Blanket, a large sharps cover to protect the entire instrument panel and windshield area. This author can attest to the quality of these products and their effectiveness at incident scenes.
Frank conducted a series of destructive cribbing crush tests to show how different types of wood cribbing behave when loaded to destruction and is sharing the results with us. For all tests, he used a 100-ton-capacity hydraulic press. Various multi-layer double box cribs were constructed on the press using 4x4 wood blocks arranged with four points of contact between each layer. To construct a properly built box crib, each layer was specifically designed so that the ends of the wood below were exposed the correct distance. In addition, the height of each box crib was measured before, during and after each test to evaluate wood cribbing compression. Each box crib stack was then pressed to failure, and then analyzed and examined during the process.
When the load from the press was applied, the cracking sound of wood under load was noted. The pressure built up and then was held until all the sounding had stopped. From that point, the pressure was increased until the failure point of the box crib was evident.
In one test, a box crib consisting of two pieces each of dry maple, pine, spruce and wet maple was pressed. As the crushing load was applied, the softer woods compressed. What is important for us to note is that as the wood compressed at the contact points, the free ends of each crib expanded when the load became excessive. The more load applied, the more the ends flared out. This is exactly why a proper box crib has the ends of the cribbing exposed. As the wood changed its shape under heavy load, the "Lincoln Log" effect occurred. The layers bonded together in what is referred to as "saddling" into each other. Although the cribs were pressed to failure, as the saddling occurred, the cribbing became more stable; one less likely to move as it supports the lifted load.
In another test, an entire box crib constructed of 4x4 pressure-treated southern yellow pine cribbing was used. This is wood typical of what can be purchased at a local lumber supplier. Under load, the pine box crib compressed 3Â½ inches before failure at 20 tons. At 21 tons, the entire stack could no longer hold the load and would have collapsed if the load were increased.
Authorities tell rescuers to expect crushing to reduce the height of box crib between 10% and 20%. During this soft pine crib test, the 16-inch-tall box crib shrunk to 12Â½ inches; a loss in height of approximately 22%. The hardwood box crib crushed only 1Â½ inches; a compression of only 10%. So as a rule of thumb, it is correct when we are told to expect between 10% and 20% of compression; more for soft wood and less for hardwood cribbing.