Vehicle Stabilization Flashcards
What is a wheel chock?
A stabilization device that prevents a vehicle from rolling.
What can be used to supplement wheel chocks?
If the vehicle’s mechanical system is still operable, then you can leverage it to supplement the wheel chocks. Shift an automatic transmission into park, or put manual transmission into first gear, engage the parking or emergency break, and turn off the ignition.
What tools are used to prevent vertical versus horizontal movement of a vehicle during rescue operations?
Wheel chocks are used to prevent horizontal movement (i.e. rolling forward or backward) while cribbing is used to prevent vertical movement (i.e. sinking downward in places as extrication operations are performed, such as a dashboard lift which could simultaneously push the car downwards).
What is “shoring”?
The temporary support of vehicles during emergency incidents. Shoring:
* Provides stability
* Removes the suspension qualities of the vehicle
* Brings an object into a solid state
* Minimizes hazards associated with the movement of objects
* Protects rescuers, victims, and property
What is a benefit of using wood cribbing, as opposed to plastic, for emergency shoring?
It provides the rescuers with warning signs when it becomes overloaded so you can mitigate the situation, be it installing additional shoring material or evacuating the area. These warning signs are:
* Visual - gradual crushing under the load
* Audible - groaning
How much can wood cribbing crush before failing?
Up to 20% of its height
What type of wood is most commonly used for emergency shoring / cribbing?
Soft woods such as Douglas Fir and Southern Pine.
What are some advantages of Douglas Fir compared to a hardwood like Oak?
- Relatively lightweight
- Inexpensive
- Good for unknown loads
- Gives warning of overload
- Most common type of lumber in the US
- Disposable
Why is cribbing loaded perpendicularly to the grain?
Although the bearing stress is higher parallel to the grain (650 PSI versus 500 PSI), failure in this configuration is undesirable. Longer wooden posts overloaded along the longitudinal grain can buckle and fail suddenly. Conversely, when the wood is overloaded perpendicularly to the grain, crushing is observed visually and audibly.
What is a box crib?
A box crib is a taller shore made from smaller pieces of cribbing. It resembles Lincoln Logs because the pieces of cribbing are stacked perpendicular to one another with 4 inches of overlap at each end.
What is the maximum height of a box crib?
To ensure stability, the maximum height to width ratio of a box crib is 3:1. However, it’s recommended that the maximum height of box cribs using 4x4 lumber is 4 feet and cribs using 6x6 lumber is 6 feet.
What is the width and footprint of a box crib?
The width is the distance between the outer edges of two points of contact on a piece of cribbing in the box crib. It is not the length of the piece of cribbing!
The footprint of the box crib is the area in the center that’s surrounded by all 4 points of contact. The larger the footprint, the greater the stability of the box crib!
How do you calculate the load bearing capacity of a box crib?
- Count the number of points of contact - For example, a 2x2 box crib has 4 points of contact and a 3x3 box crib has 9
- Calculate the surface area for the points of contact - This is the actual width, as opposed to the nominal width, times the length for the piece of cribbing. (Note that the actual width is a half inch shorter than the nominal width due to drying and planing.) For example, the surface area of a piece of 4x4 cribbing is 12.25 inches (3.5 x 3.5) and the surface area of a piece of 6x6 cribbing is 30.25 inches (5.5 x 5.5).
- Calculate the load carrying capacity for the points of contact - This is the surface area multiplied by the allowable bearing stress of the wood, which is 500 PSI when loaded perpendicularly to the grain. Therefore, the load carrying capacity for a point of contact in a box crib made with 4x4 lumber is 12.25 x 500 = 6,125 lbs which we round to 6,000.
- To obtain the overall crib capacity, add the capacity of the individual points of contact.
What is the overall load carrying capacity of a 2x2 box crib made with 4x4 lumber?
In this box crib, there are 4 points of contact, each of which has a surface area of 12.25 inches (3.5 x 3.5). The loading bearing capacity of each point of contact is 12.25 x 500 PSI or 6,000 lbs (since we round). Adding these gives us 24,000 lbs.
What is the overall load carrying capacity of a 2x2 box crib made with 6x6 lumber?
In this case the load bearing capacity of each of the four points of contact is 15,000 pounds so the overall capacity is 60,000.
