By Cris Pasto, Spencer Fire Dept./Cepco Tool Co.
 
In the first two segments of this series we discussed equipment and techniques used in buttress stabilization of side-resting vehicles.  In this section we will take a look at the potential forces buttress equipment is exposed to in 3-point side-resting passenger vehicle stabilization.  The particular forces we will look at include the compressive column loads in the buttress stands as well as the tension loads experienced in base restraint strapping.  We will also walk through an example to illustrate how one might use this information to gain a better understanding of how one might apply the theoretical data to a real world situation.

Column & Strap Forces: Side-resting Stabilization

How much force does it take to stabilize a vehicle?  That’s a question with many answers.  If we are simply stabilizing a side-resting passenger vehicle in direct contact with flat ground, the answer is typically very little.  However, if we intend to suspend or lift the vehicle we potentially have a much different situation.  We should be using additional tools meant for the lifting application which may be special jacks or lift bags, and reserve the stabilization equipment for stabilization.  If we use the same equipment for both applications, we have no backup.  While lifting we should employ the “lift an inch, crib an inch” policy which is a simple task if we have a side wind type adjustment jack in our stabilization stand.  If the lift bag fails, the stabilization equipment should at least have the capacity to support the weight of the vehicle.  A chart (Table 1) is presented here with approximate passenger vehicle weight data for several classes of vehicles.

Table 1: Weights of Typical Passenger Vehicles

Using vehicle weight data in conjunction with a series of analytical relationships we can approximate compressive column loads in the stands and tensile forces in restraint strapping.  Variables which will impact these forces in addition to the vehicle weight include the angle the stand is set at, the number of stands, the position of the stands relative to the vehicle center of gravity, the number of base restraint straps, and the angle between the base restraint straps if more than one strap is applied.  To simplify this otherwise indeterminate problem, several approximations are made:

a. 100% of the vehicle weight is supported by the 3-point system

b. The stands are placed symmetrically about the vehicle center of gravity

c. The stands are all placed at the same angle (50, 60, or 70 degrees)

d. The ground is flat and frictionless (simulates a flat icy surface)

e. Straps are symmetric about the base

A diagram of a side-resting car (Fig. 1) shows the designation for the column loads and stand angles.  A second diagram (Fig. 2) illustrates how the stand loads act to create tension in the base restraint strapping.

 Fig. 1: Column Load Diagram

F1, F2, & F3 = Compression force in stands

a = Stand angle

W = Vehicle weight