Box crib

A box crib or cribbing is a temporary wooden structure used to support heavy objects during construction, relocation, vehicle extrication and urban search and rescue. It is commonly used to secure overturned motor vehicles, and debris within collapsed buildings. Cribbing is often used in conjunction with other stabilization equipment, such as pneumatic or hydraulic shoring. Cribbing is also used in sub-surface mining as a roof support. Cribbing has largely been replaced by hydraulic shoring in modern mining applications.

Some forms of cribbing can also be used on movie sets and production sites for stabilizing dolly tracks, platforms, and various temporary structures when quick setup times are needed.

The stability of a crib is affected by a variety of factors: the material used (often a soft wood which gives audible warnings before failure), the number of contact points between the crib and the supported surface, the ratio of the footprint of the crib to its height, and the area of contact made between the crib and the ground and supported surface.

Cribbing is usually accomplished with blocks of wood, often 4"x4" or 6"x6" and 18"-24" long. Soft woods, like spruce and pine are often preferred because they crack slowly and make loud noises before completely failing, whereas stiffer woods may fail explosively and without warning.

Cribbing may also be made out of plastic, which unlike wood is not susceptible to rot or corrosion from fluids the cribbing may come in contact with (e.g. oil, gasoline, hydraulic fluid).

Cribbing equipment is normally of three varieties: rectangular blocks, wedges (also called shims), and "step chocks" (large wooden chocks constructed of wood of different lengths). Blocks are the bread and butter of cribbing and will be used in most cribbing evolutions. Shims are used to snug up contact between the crib and supported object or change the direction of the crib (tilt). Step chocks are often used as a quick solution for stabilizing vehicles on all fours or to quickly increase the height of a crib.

Cribbing structures are often categorized by shape. Different shapes of cribbing structures are chosen depending on the area available and the point being cribbed to.

A box crib is the simplest, most stable and most common method of cribbing. It is constructed by arranging sets (2 or more) of matched blocks in a regular log-cabin style to form a rising square or rectangular frame. The more blocks on each level, the greater the number of support points and therefore the greater the strength of the crib tower. In trench rescue training materials three basic types of box cribbing are the 4-point, 9-point and full crib. The four point type has two timbers on each level thus four points of contact. Three timbers on each layer makes nine points of contact. The full crib type has each layer filled with timbers. Each point of contact carries a maximum load depending on the size and species of the timbers.

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