When loads are moved or lifted, gravity wants to return it to the ground by any means possible. Loads can shift in a split second, causing devastation to an operation. When lifting objects, it is imperative that cribbing is used to support the load. You’ll generally find cribbing cut from dimensional lumber of the sizes 4x4 and 6x6. Cribbing can vary in length; however 24 inches is a common size. You’ll also find 4x4 and 6x6 wedges and 2x4s as fillers. Adding to the cache of cribbing will be pieces of micro-lam and or 3/4-inch plywood.
Cribbing is an important aspect to your lifting operation. It needs to be done with care and on the money. The principal behind cribbing is not only to stabilize the load, but it also transfers the load force vertically into the ground. Crib stacks can have multiple levels. The rule of thumb to follow is the height should not exceed three times the width and should not exceed 30 degrees out of plumb. Outside of those two features, the third most important aspect you must adhere to is all the cribbing should line up with each other, giving you a solid contact point from top to bottom.
If you look at figures 14 and 15, Figure 14 depicts how not to build a crab stack while Figure 15 is a shining example. The contact points are where the strength is. On a 4x4, each contact point is rated for 6,000 pounds, giving a simple two-tier crib stack a load rating of 24,000 pounds. Using 6x6 lumber, those contact points are bumped up to 15,000 pounds with a load rating on a 2-tier stack at a whopping 60,000 pounds....yes that’s some serious weight. The next time you’re building a crib stack, don’t let someone tell you “it doesn’t have to be perfect” or “that will do,” those statements couldn’t be any further from the truth. Whether you’re building a crib stack for a 3,000-pound car or a 30,000-pound truck, you should build your stack one way every time...perfect!
Let’s now wrap this article up by touching on air bags. You’ll find three different types of airbags – low, medium, and high pressure. Low pressure can operate using around 7 psi, medium 15 psi and high pressure weighs in at 118 psi. The difference in size is...we’ll let’s just say noticeable (see Figures 16, 17, 18). The lifting capabilities between the different bags also changes. When an air bag inflates, it looks something like a pillow. Because of that there is a two-bag maximum stack rule. Anything over two just becomes too much of a balancing act.
Other notable points are to always put the larger bag on the bottom and ensure the air connections are facing out towards you. When lifting on a crib stack you want a solid top and, depending on the ground you’re lifting on, perhaps a solid bottom. An example of a ground type that would require a solid base is grass and or dirt. Some departments choose to always use a solid base and I think that’s great forward thinking. The reason for the solid top is that if you inflated an air bag resting on two 4x4s, the inflation of the bag would want to spread them apart. By using a solid top (Figure 19) this problem is avoided. The solid top transfers the force created by the inflation of the airbag vertically down through the crib stack and into the ground. Airbags are a very powerful means of lifting loads and it is imperative that you keep the load that’s being lifted stable the entire duration of the operation. The stabilization method used is “lift and inch, crib an inch” meaning as the load is being lifted you continuously follow the load with your crib stack. Wedges are used to continue stabilization until a full piece of lumber can fit.
Lifting operations can be simple or complex and a size-up of the load prior to lifting is a must. You need to determine how moving one load will affect other loads nearby or directly in contact. Like any other rescue skill, a little training goes a long way.
Until next time, keep those loads stable and your forces vertical.
MICHAEL R. DONAHUE is a 14-year veteran of the fire service assigned to Rescue Company 1 in Elizabeth, NJ. Mike is the owner and founder of Progressive Rescue, a company dedicated to further firefighter's in all aspects of the job. He holds the title of rescue specialist with New Jersey's Urban Search and Rescue Team (NJ-TF1) and he is actively teaching at Middlesex Fire academy and the Middlesex County College as their Fire Science Program Coordinator. Mike has been on two Firehouse.com podcasts: The Buzz on Technical Rescue: Rope Rescue Operations and The Buzz on Technical Rescue: Special Operations Roundtable. He has taught as a HOT instructor at Firehouse Expo and is the Specialized Rescue Forum moderator for Firehouse.com. You can reach Michael by e-mail at email@example.com.