Air-Bag Operations – Part 3: Cribbing Skills

Any time an air-bag lift is performed, cribbing is a necessary part of the operation.  Cribbing is a temporary framework, usually constructed of timbers, that provides a solid base of support to material above.  This framework provides the means to...


The final variable we need for our calculations is the number of timbers per tier.  The more timbers, the more points of contact the system has.  For example a two per tier system has four points of contact (see photo 3).

Cribbing Capacity Formula:

PSI of Timber’s Perpendicular Load Capacity x Timber Dimension x Timber Dimension x Number of Points of Contact = Crib System’s Capacity in pounds (LBS)

Cribbing Capacity Math – Example 1:

4-inch x 4-inch being used for the system – Realizing true dimensions of 3.5-inch x 3.5-inch

Douglas Fir is the wood being used – 500 PSI perpendicular load capacity

Two timbers per tier construction – 4 points of contact

500 PSI x 3.5 inches x 3.5 inches x 4 Points of Contact = 24,500 pounds, rounded down to 24,000 pounds (12 tons)

Cribbing Capacity Math – Example 2:

Same variables as Example 1, but using three timbers per tier construction – 9 points of contact

500 PSI x 3.5 inches x 3.5 inches x 9 Points of Contact = 55,125 pounds, rounded down to 55,000 pounds (27.5 tons)

Commonly Accepted Capacities – Using 500 PSI Perpendicular Load Capacity:

Timber Size

Timbers per Tier – Points of Contact

Capacity in LBS

4” x 4”

2 per tier – 4 points of contact

24,000 LBS (12 tons)

4” x 4”

3 per tier – 9 points of contact

55,000 LBS (27.5 tons)

6” x 6”

2 per tier – 4 points of contact

60,000 LBS (30 tons)

6” x 6”

3 per tier – 9 points of contact

136,000 LBS (68 tons)

 

It is critical to understand that these capacities are based on weight of the load being distributed across all points of contact on the system.  The capacity of the system is reduced if the load is placed between the points of contact.

Conclusion

Cribbing is a vital part of any safe air-bag operation.  It is essential for members to understand the capacity of cribbing systems.  Without the proper crib system in place, the object being lifted could shift or fall and crush the victim or members.  Much more on the use of cribbing will be discussed in the next article.

JONATHAN HALL is currently a firefighter with the Saint Paul, MN, Fire Department. He previously served as a training and safety officer for the Township Fire Department in Eau Claire, WI. He is a certified fire instructor for the Chippewa Valley Technical College in Eau Claire. Jonathan frequently teaches firefighter survival and rapid intervention team concepts in both Minnesota and Wisconsin. He recently taught the "Truck Company Do's and Don'tsFirehouse Webcast.