Uprighting a loaded cement mixer or lifting the rear of a loaded van trailer are everyday tasks for heavy recovery operators. Place someone who is injured and trapped in a car under such a vehicle and a challenge begins. While heavy recovery operators may easily overcome this challenge, it may...
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Typically, there is sufficient force and tools needed to perform this task on a heavy wrecker. Moreover, it is important that fire-rescue responders realize the limitation of their equipment in this type of collision. They must understand the weight involved, construction methods, and materials of various trucks and trailers in order to appreciate these limitations. Without knowing the weight (resistance) imposed on the stabilization gear, it is possible to exceed the working load limit (WLL). Responders should be able to calculate the loads (resistances) in order to operate safely. Generally, vertical weight is calculated using the industry-accepted value of axle weight values: thin-profile steering tires of a Class 8 big rig (tractor-trailer) carry a front-axle weight rating of 12,000 pounds, larger front "balloon" tires are calculated using a front-axle weight of 25,000 pounds, and the drive axles of the tractor and trailer axles are calculated at 20,000 pounds each. If the axles are to be supported vertically, as in the case of an underride, these axle weight ratings are useful in determining the amount of stabilization needed.
When an object is lifted vertically, 100% of the weight is assumed by the lifting gear. Using a law of physics, "For every action there is an equal and opposite reaction" or simply "force pairs" responders can calculate the total amount of stabilization capacity needed. For example, consider a car that is involved in an underride with the front steering axle of a big rig. The big-rig axle weight rating is 12,000 pounds and the car is trapped underneath, which has lifted the front axle vertically. Thus, a total of 12,000 pounds of stabilization support is needed. Conversely, the lifting force required is also 12,000 pounds.
With an overturn of a big rig onto a smaller vehicle, the calculations are performed differently. For example, a fully loaded tractor-trailer (80,000 pounds) has overturned onto an auto. Responders will be stabilizing one side of the truck nearest the car, perhaps allowing the total uprighting prior to performing extrication. If the big rig were at zero degrees from horizontal (flat on the surface with no car involved), responders would only be lifting half of the overall weight which is 40,000 pounds. Further calculations are made using this figure as it becomes the maximum amount to be lifted.
As the horizontal angle of the big rig increases, the weight becomes less due to the application of the Class 2 lever principle and movement of the center of gravity (COG). At 30 degrees from horizontal, the lifting/stabilization would require 50% of the maximum of 40,000 pounds, equaling 20,000 pounds. These are recovery industry-accepted values that are useful during extrication operations. Obviously, they could be affected by certain cargos (liquids) and the way in which they are positioned after collision.
Once a weight (resistance) is known, stabilization gear can be applied. Typically, the best tools include timber cribbing and strut supports. Struts may be used effectively with big-rig incidents due to the vertical stabilization height needed and weights imposed upon them. The use of chain slings with struts is also effective. A minimum of Grade 80 (or greater) chain should be used during extrication operations. Preferably, half-inch-link-diameter chain is used to maximize the support provided by the strut. Generally, this requires the use of alloy steel strap shackles with strut attachments. A basket hitch can be employed using two struts and chain slings, which doubles the single-leg capacity of the chain sling if the "legs" of the sling are at 90 degrees to the load.
Caution should be exercised so as to not create a sharp bend in the chain. If a chain sling passes around a 90-degree bend in which the bend radius is less than two times the link diameter, the WLL of the chain sling is decreased by 50%. This can be avoided by "padding" the bend using cribbing.
STEP 3. Lower the Smaller Vehicle
Typically, this isn't possible, as the heavier vehicle has crushed the smaller one to some extent. Simply deflating the tires may provide a few inches of lowering. Generally, the suspension of the smaller vehicle will compress on heavy loading, creating an added problem during the lifting of the larger vehicle. As the big rig is lifted, the suspension will relax, thus letting the smaller vehicle rise. Actually, this will decrease the total amount of lift gained nearly five or six inches.
Suspension compression may be helpful, and can be accomplished using a ratcheting load binder strap (minimum of two-inch width). For example, when the front of a car has struck the rear of a tractor-trailer, the strap is connected to the front wheels of the car and the strap tightened. As the larger vehicle is lifted, the suspension will remain compressed. The larger vehicle must be lifted high enough to provide clearance for appropriate rescue measures.