Timber Cribbing Use
Cribbing is an essential tool for rescue operations. In fact, cribbing is one of the most frequently used tools during rescue operations and 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". Cribbing provides 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 on soft surfaces such as mud, sand, snow, etc. This substantial base of support will assist in efficient weight transfer, and should be level or nearly so if at all possible. Make an attempt to keep all cribbing plumb and level to provide greater stability. Remember, stabilization is a dynamic process frequently needing an inspection to ascertain its effectiveness.
Three smart cribbing considerations are:
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, and
3. Crib or shore from a safe 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 three questions,
1. Are the needed materials readily available,
2. Are the tools needed readily available, and
3. Are the rescuers trained and possessed the expertise to perform the required operations?
Wooden cribbing pieces seem to be the most commonly used, with softwood a popular choice. However, hardwood and softwood 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. 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. Advantages of Softwood cribbing pieces include being lighter in weight compared to hardwood, and most importantly providing warnings of failure. These warnings include visible cracking or splitting of the wood, and sounds produced by such cracking. 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 while splits occur when wood cells tear apart, parallel to the grain of the wood. The properties of wood are allowing the noticeable signs of failure result from the two primary growing season‟s, spring and summer. Spring growth produces softer fibers while summer growth produces harder fibers. The softer fibers of spring growth 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 crushes a timber piece. Timber pieces with greater strength values in perpendicular compression (stated in psi) are better suited for wedges and bearing timbers (cribbing). Axial loading such as in shoring operations relies upon buckling failure. Greater strength in compression parallel to the grain is better suited for columns such as used in shoring.
In some instances, fifty pieces or more may be needed to stabilize an upright school bus. If your primary response vehicle does not‟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 needed timber 24/7/365? If not readily available, consider establishing a quantity to be stored at your agency. Pack this cribbing according to dimension or primary purpose into open mesh crates. These crates can be easily handled. 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 of the pieces. Cribbing pieces may be „toe-nailed‟ together to maintain integrity. A cordless or pneumatic nailer should be used to drive 16d framing nails into place. Optimally nails being used should be driven so that 2/3 of their length extends into the second piece of wood. Colored handles of rope or webbing may be attached near the ends of cribbing to separate the types and sizes. The ends of cribbing may be painted or labeled to identify various types and sizes as well.
Cribbing should frequently be inspected for physical and chemical damage or other deterioration. Cracks are obviously indicative of physical harm. Moisture is a bitter enemy of cribbing. Store it in a clean, dry and ventilated area with room for air movement among pieces if possible. If cribbing is found to be damaged it should be removed from service, and not used for training.
Varied lengths of cribbing may be used. However, an accepted value is that the height of a stack crib should't exceeds three times its width (footprint), provided all contact points are covered. For example, if the footprint of a stack crib
is 18” (calculated using 26” timber pieces, and allowing 8” of overlap measurement), the height should‟t exceed 54” (3:1). Therefore, rescuers may gain insight into cribbing length based upon this value, especially if the considerable height is anticipated. Although shorter lengths are most commonly used, longer cribbing pieces such as 4‟, 6‟, and 8‟ should be in a timber cribbing inventory.
As you see on the left picture, stack crib height shouldn't exceed three times its footprint
