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).
Heavily loaded cribbing can crush so that it will lose from 10% to 20% of its height. This is a good thing as far as providing warning of overload, but may present problems regarding stability and the need to keep the cribbing properly placed.
A triple box crib of 4x4s has nine points of contact between each layer. Its capacity is therefore 9/4 as much as a double box crib. A triple 4x4 box crib has a calculated weight carrying capacity of 60,000 pounds, again assuming proper corner overlaps.
Cribbing Height Limitations
Stability of your box crib is dependent on its height-to-width ratio. The width of a box crib isn't the length of the wood making up the crib; it's actually the distance at the narrowest point of the crib. This is typically the outer points where one layer and the layer above it overlap.
Federal Urban Search & Rescue authorities advise their personnel that the height of a box crib should not exceed three times its width when used at a structural collapse incident; a 1:3 ratio of width to height. To make things a bit simpler for vehicle rescue applications, a good rule of thumb is that a box crib constructed of 2x4 and 4x4 wood, built in a square or cube shape at a vehicle rescue incident with all four corners in solid contact with the ground, should not be higher than two times (twice) the length of the wood being used; a 2:1 ratio of length to height.
When you consider using typical 18-inch-long cribbing purchased at a retail lumber store and cut to length, it is actually 3Â½ inches in its widest dimension. With the proper 3Â½-inch overlap at each end, this common box crib actually has a usable width of only 11 inches. Under the US&R 3:1 width-to-height ratio, it should only therefore be used to build a box crib to a height of approximately 33 inches. Following our rule of thumb for cribbing height at vehicle rescue incidents, two times the length of the 18-inch cribbing is 36 inches. This calculated limit of 36 inches is close to the rule of thumb height limit and is easier to figure at an incident. Higher than this, for rescue purposes, and the box crib becomes potentially unstable. If a rescue team used 24-inch-long cribbing at a vehicle rescue incident, that box crib could reach a height of four feet (48 inches) and still be relatively stable if properly constructed. If only two corners of the box crib are in good solid contact with the lifted load and ground, then the safe working height should be reduced accordingly.
A box crib in a triangle shape or one built in a parallelogram form is not as stable as the desired cube-shaped box crib. These unusually shaped box cribs should be limited to a height to cribbing length ratio of 1:1. In other words, the cribbing should be no taller than the length of the cribbing being used. With 18-inch-long cribbing, the crib should be no higher than 18 inches for example.
The two main objectives of cribbing placement are to maintain the integrity of all unstable or lifted loads and to properly transmit or redirect the weight (force) of the lifted or unstable loads to solid ground.
Cribbing must be placed at structurally strong points on the load being supported and must be located where they will not interfere with the removal of any trapped victims or anticipated rescue work.
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 deploy a box crib capable of accomplishing the task.
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.