By BILLY LEACH JR.
Cribbing is one of the most frequently used and essential tools during rescue operations and is considered to be among the strongest means of support. Gravity is inescapable; thus, cribbing is used to transfer the weight of a load into a “footprint” and provide a simple temporary support during rescue operations.
To correctly and effectively transfer weight from top to bottom, full and direct contact must be made with both the load and lower surface. Rescuers should begin with a solid base of support, especially in soft surfaces such as mud, sand, snow, and so on. This substantial base of support will assist in effective weight transfer and should be level or nearly so, if possible. Attempt to keep all cribbing plumb and level to provide greater stability. Remember, stabilization is a dynamic process that frequently needs inspection to ascertain its effectiveness.
Three smart cribbing considerations follow:
- 1. Avoid the area of danger—i.e., remain clear of the load’s footprint.
- 2. Mitigate the hazard if possible—i.e., upright a heavy vehicle off a smaller vehicle.
- 3. Crib or shore from a safe area into an unsafe area. Always place cribbing/shoring in a manner that provides both responder and patient egress.
Prior to cribbing/shoring, rescuers should ask themselves the following three questions:
- 1. Are the needed materials readily available?
- 2. Are the needed tools readily available?
- 3. Are the rescuers trained, and do they possess the expertise to perform the needed operations?
Hardwood and softwood seem to be the most commonly used materials for cribbing. Hardwood and softwood generally refer to the type of tree producing the wood and not the strength of the wood itself. Hardwood trees shed their leaves in the fall, while softwood trees retain their leaves/needles consistently. The softwoods most frequently used are Southern Yellow Pine and Douglas Fir, although other species are also used. Always attempt to obtain and use #1 Grade timber for cribbing/shoring.
Softwood is the preferred piece for cribbing for FEMA engineers; simply, hardwood doesn’t offer advantages provided by softwood. Advantages of softwood cribbing pieces include being lighter in weight compared to hardwood and, most importantly, it provides warnings of failure. These warnings include visible cracking or splitting of the wood and the sounds produced by such cracking. Generally, the signs of failure begin near the ends of the timber piece as “checks” and “splits.” Checks are separations in wood transecting the annular growth rings; splits occur when wood cells tear apart parallel to the grain of the wood.
The properties of wood that allow the noticeable signs of failure result from the two primary growing seasons—spring and summer. Spring growth produces softer fibers; summer growth produces harder fibers. Spring’s softer fibers produce the noise of cracking and the evident physical cracks during cross-grain loading. When building stack cribbing, the load is perpendicular to the wood grain, producing slow, noisy, and visible warnings of failure. This compression stress actually crushes a timber piece. Timber pieces with greater strength values in perpendicular compression [stated in pounds per square inch (psi)] are better suited for wedges and bearing timbers (cribbing). Axial loading such as in shoring operations relies on buckling failure. Greater strength in compression parallel to the grain is better suited for columns such as those used in shoring.
In some instances, 50 or more pieces may be needed to stabilize an upright school bus. If your primary response vehicle doesn’t carry this amount, is it easily obtainable? Preplanning for the need of cribbing is fundamental for heavy rescue. How can your agency obtain the timber needed 24/7/365? If it is not readily available, establish a quantity to be stored at your agency. Pack this cribbing according to dimension or primary purpose into easily handled open mesh crates. Consider storing a hand truck with the cribbing to transport a large quantity quickly using minimal personnel.
Wooden cribbing should be left unfinished and unpainted. Cribbing pieces rely on gravity and friction between bearing points for stability. Painted surfaces become slippery when wet and may hide damage or defects on the pieces. Cribbing pieces may be “toe-nailed” together to maintain integrity. Use a cordless or pneumatic nailer to drive 16d framing nails into place. Optimally, drive nails so that two-thirds of their length extends into the second piece of wood. Attach colored handles of rope or webbing near the ends of cribbing to separate the types and sizes. Paint or label the ends of cribbing to identify various types and sizes.
Inspect cribbing frequently for physical and chemical damage or other deterioration such as cracks and moisture (a bitter enemy of cribbing). Store it in a clean, dry, and ventilated area with room for air movement among the pieces, if possible. If cribbing is found to be damaged, remove it from service and do not use it for training.
You can use varied lengths of cribbing. However, an accepted value is that the height of a stack crib shouldn’t exceed 3× its width (footprint), provided all contact points are covered (photo 1). For example, if the footprint of a stack crib is 18 inches (calculated using 26-inch timber pieces and allowing eight inches of overlap measurement), the height shouldn’t exceed 54 inches (3:1 ratio). Therefore, rescuers may gain insight into cribbing length based on this value, especially if considerable height is anticipated. Although shorter lengths are more commonly used, longer cribbing pieces such as four, six, and eight feet should be in a timber cribbing inventory.
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| (1) Stack crib height shouldn’t exceed three times its ‘footprint’ if all contact points are covered. (Photos by author.) |
The 3 × 3 construction method (photo 3) uses three pieces per layer, each layer at right angles. Using the 2 × 2 construction method (photo 2) with four- × four-inch timbers, the weight bearing capacity of the stack crib is 24,000 pounds, or 6,000 pounds per column (12 tons total) if all four contact points are covered. The weight-bearing capacity would increase to 55,000 pounds, or 6,111.1 pounds per column (27½ tons total), if the 3 × 3 construction method was used and all nine contact points were covered. The 3 × 3 construction method increases the weight-bearing capacity. However, it uses only 50-percent more cribbing pieces. The weight-bearing capacity of a stack crib is calculated by the maximum perpendicular load to the grain (stated in psi) as accepted by structural engineers on the sum of all bearing points. Stack cribbing must be centered under the load if possible, maintaining the majority of the load in the center third of the stack crib.
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| (2) 2 × 2 construction method of building a stack crib uses two pieces of cribbing per layer, each layer at right angles. |
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| (3) 3 × 3 construction method uses three pieces per layer, each layer at right angles. |
Do not use the 2 × 2 construction method when using stack cribbing as a platform for air bag lifting systems unless the top tier of cribbing is completely solid and capable of supporting the force imposed by the air bag as it lifts the load. Ideally, connect the solid top tier of cribbing pieces by some means to prevent unwanted movement—i.e., “scabs.” High-pressure air bag lifting systems tend to inflate from the center outward and may dislodge a stack crib, resulting in catastrophic failure during a lifting operation.
Using six- × six-inch timbers and the 2 × 2 construction method, the weight-bearing capacity is 60,000 pounds, or 15,000 pounds per column (30 tons total). The weight-bearing capacity would increase to 136,000 pounds, 15,111 pounds per column (68 tons total) if the 3 × 3 construction method were used. These capacities are valid if the load covers all contact points.
The formula to calculate weight-sustaining capacity per column is total surface (in square inches) of cribbing piece × the compression strength perpendicular to the grain (stated as psi). The weight-bearing capacity values expressed here are based on the use of undamaged #1 Grade Southern Yellow Pine or Douglas Fir and are accepted by the Federal Emergency Management Agency for urban search and rescue response. It is vitally important for responders to determine specifically the strengths of their respective cribbing pieces using accepted engineering values. There is no strength loss for treated vs. untreated wood, provided the moisture content is less than 19 percent.
Cribbing pieces should be of #1 Grade, which provides greater strength and better cosmetic appearance. Manufacturers are now producing varied cribbing pieces using plastic. These pieces are formed into such tools as stepchocks, wedges, buttresses, “lock blocks,” and others. The surfaces of plastic cribbing are resistant to soiling and staining. The durability of these pieces is reported to be longer than wood, and the weight-bearing capacity is also greater.
The ends of cribbing pieces should overlap the preceding layer by the width of that particular piece (photo 4) for two primary reasons: (1) Should the cribbing pieces slip minimally, some degree of integrity will be maintained; and (2) failure will begin at the ends of the cribbing pieces, showing warning signs of deteriorating integrity. For example, when using four-inch timber, the ends of each layer should overlap a minimum of four inches.
Rescue situations may dictate that the cribbing pieces be placed in shapes other than a square (photos 5, 6). When a shape other than a square is used, the footprint will vary. Thus, the safe column height will vary. If the square shape of a cribbing stack is modified, the safe height of the stack is limited to 1× the footprint (1:1 ratio). For example, if the footprint of modified stack cribbing is 12 inches, the safe height of the column is limited to 12 inches. Stack cribbing should form columns that support the load. The pieces should be aligned vertically to form such a column and provide the required strength.
If all contact points of a stack crib aren’t covered, the safe and stable height of the stack will be affected (photo 7). If three of the contact points are covered when using 2 × 2 construction, the safe and stable height for the stack crib is 2× the footprint (2:1 ratio). If two of the contact points are covered, the safe and stable height of the stack crib is 1.5× the footprint (1.5:1 ratio). If only one contact point is covered, the safe and stable height for the stack crib is 1× the footprint (1:1 ratio). The weight-bearing capacity of the stack crib will vary also if all contact points aren’t covered.
Use wedges to fill voids between the load and cribbing pieces; they should be the same width and preferably the same length as the cribbing pieces (photo 8). If four-inch timber cribbing pieces are used, the wedge should be four inches in width. The length of a wedge shouldn’t exceed 6× its width—i.e., if four-inch timber cribbing pieces are used, a 24-inch wedge is the maximum size that should be used (6 × 4 = 24).
Proper placement of wedges serves to transmit the load into a column, with no more than two wedges stacked on one another. Stacking more than two wedges will likely produce instability by dislodging the middle wedge. Wedges can also be used to change the vertical direction of the stack crib, allowing rescuers to support a sloped load, which has two primary forces acting on it—gravity and friction. Gravity produces a vertical load force; friction produces a load that acts as a downslope. Friction is the resistance encountered when two solid surfaces slide or tend to slip. The degree of surface roughness has an influence on the coefficient (measurement) of friction. When a surface is soft and coarse, greater frictional resistance is produced. The coefficient of friction is expressed as an angle or its decimal equivalent—i.e., 15° = .27. Stack cribbing generally may be used to a height of less than three feet against a sloped surface with an angle less than 15°, or 30 percent.
Small protractors are useful in determining angles. When building a stack crib into a sloping surface, the height of the cribbing shouldn’t exceed 1.5× the footprint, or instability may result. Optimally, the stack crib should be constructed plumb and level with wedges used on the top tier or underneath the bottom tier to produce stability. Sloped surfaces may alter the direction of downward force on the stack crib, necessitating frequent monitoring of stability.
When placing cribbing pieces, NEVER put a part of your body between the load and the cribbing. Use a tool or another piece of cribbing to maneuver it into place. During cribbing operations, the use of personal protective equipment is necessary to ensure safety.
Cribbing is an essential rescue tool, often supporting tremendous weight while rescuers operate underneath. It is necessary that all rescuers understand the safe and proper use of this vital tool.
Table 3 lists representative wood species and their strengths in compression perpendicular to the grain. By no means are these the only wood species used for cribbing pieces. Investigate the wood used by your agency and determine its strength characteristics.
BILLY LEACH JR. is a captain for the North Carolina Emergency Management USAR Task Force 7 and Ash-Rand Rescue & EMS, Inc. and has been actively involved in the emergency services since 1976, combining career and volunteer experience. He is the developer and senior presenter for Big Rig Rescue™. He trains in vehicle rescue and has presented at the International Vehicle Extrication Learning Symposium, Search and Rescue Disaster Response Conference, FDIC, American Towman Exposition, NC Extrication College, Fire Department of New York’s Technical Rescue School, and many regional fire/rescue training seminars. He is a coauthor of Big Rig Rescue.
