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OBJECTIVE: Understand the components and ratings of a wood box crib and best practices when constructing a box crib at a vehicle rescue incident.
TASK: Given the need to support a lifted or otherwise unstable load, the rescue team shall be able to use wood cribbing to construct and position a box crib capable of accomplishing the task.
This University of Extrication column is part of a series on lifting and stabilizing heavy loads. In this series, we'll cover lifting tools in detail, review best practices for lifting and stabilizing techniques, and discuss several real-world scenarios. We start out our series talking about cribbing so rescue personnel can refresh themselves on the basics of wood cribbing as it is used for vehicle rescue applications. In Part 1, we look at the make-up of a wood crib and explain state-of-the-art principles that responders should follow to assure that the box crib we build at a rescue scene is safe, secure, and adequate to do the task at hand.
As a rescuer and an instructor for many years, this author has experienced a fair share of box crib "incidents." Unexpected shifting of a lifted load, "exploding" cribs, even a catastrophic failure of a wood crib member are all times we remember and would rather forget. None of these events are things that one wants to have happen at a rescue scene but sometimes they just do and we must learn from these experiences.
The box crib that we are talking about in this series on lifting heavy loads is a specially designed criss-cross assembly of wood arranged in a square or cube fashion. It can be a "double" or a "triple" box crib. A double has two pieces of wood forming each layer and a triple box crib uses three.
Rescue authorities recommend that when we build one layer on top of the other to form the box crib, that we position the layer above slightly short of (or 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. Under load, the end grain of the wood below slightly expands which allows the load from above to settle into the cribbing. Ideally, the exposed end should be equal to the thickness of the wood itself; four inches for a 4x4 dimension piece of wood for example. This is not always practical with 18-inch-long wood so at least two inches should be exposed for these shorter length pieces.
Another new and very important point for vehicle rescue personnel to consider is the best practice of constructing the bottom layer of a box crib that will support a heavy load as a solid layer when locating the cribbing on gravel, dirt, or even asphalt. A solid layer can also be placed at the top of the crib to support the load of a lifting tool; a must-do when lifting with rescue airbags.
To maximize our safety and efficiency when lifting or supporting a heavy load, we need to understand the safe load rating of a wood box crib. As we speak of a heavy load in this series, for our purposes, we'll consider any load of approximately 10,000 pounds of weight or more as a heavy load. A bus, construction machinery or a large truck involved in a crash or rescue incident would be examples of loads that meet this criteria.
When using wood to build a box crib support system for a heavy load, it would be beneficial to know the approximate rating or capacity of the wood crib members we are entrusting our lives to as well. Just how much weight can your box crib safely hold?
The standard load rating of a double box crib constructed with true-dimension 4x4 wood is estimated at 6,000 pounds per point of contact assuming there is proper overlap at the corners of the crib. This capacity is calculated with what is called the "perpendicular to grain load" rating as the sum of the bearing surfaces. The double box crib you built has four points of overlap; points where one layer of the crib sits on top of the layer below it. With each contact point rated at 6,000 pounds, the total capacity becomes 24,000 pounds (12 tons).