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Models:

 Barrier Collision Physics
or

Why you want a planter in your
Force Protection Program.

Large amounts of kinetic energy must be dissipated when a moving vehicle impacts a relatively immovable object like a barrier. The speed is important because it is multiplied by itself and the weight of the vehicle to calculate the kinetic energy that must be overcome. (KE = ½MV²)

This resistance of the barrier is the result of the strength to resist the impact multiplied by the duration of the impact. For example, a pile of hay can slow a speeding vehicle to a stop over time as does a massive concrete wall instantaneously. In this way, designs that create a progressive impact by virtue of the three dimensional shape are more effective for their mass than a conventional flat concrete wall. This also explains why a planter barrier with thinner walls and smaller foundation is more effective than a massive flat retaining wall with thicker walls, heavier rebar, and deeper foundations.

GFRC is the ideal material to take advantage of these facts of the physics of collision because of the ease of creating 3-D shapes that are also pre-finished and can be shipped to the job-site economically because of their light-weight construction.

Question:

Why would a structure less securely attached to the earth, with thinner walls, and smaller reinforcing steel bars be 1000 times stronger in resisting and stopping the impact of a 15,000 lb vehicle?

Answer:

A moving vehicle can be thought of as having kinetic energy (energy of motion). The faster it is going the more kinetic energy it has. Actually the kinetic energy of a moving vehicle is proportional to the square of the velocity, so a car going 50 mph has four times the kinetic energy of the same car going 25 mph.

The way a crash barrier works is to take the kinetic energy out of the moving vehicle. It can do this in several ways:

1. By being very rigid and strong, which keeps the vehicle from traveling further. As this kind of crash barrier is very rigid it cannot move very much, so it really doesn't absorb much energy. The vehicle has to absorb its own kinetic energy by deforming. If the vehicle itself is very strong and rigid, like a truck reinforced with steel, the impact forces can be extremely large.
   
2. By soaking up the kinetic energy, like a sponge. This means that the crash barrier doesn't need to be nearly as strong and rigid as it stops the vehicle by a plowing action. In many cases the vehicle will be almost unharmed, as the barrier itself will do all the work. It turns out that a correctly designed energy absorbing crash barrier must be relatively weak, as this permits it to deform readily. This keeps the reaction forces relatively low.

Under explosive conditions or crash conditions an energy-absorbing barrier is always better than a rigid barrier. This has been proven by many tests. Rigid barriers will fail under relative small impact velocities while energy absorbing barriers will be able to survive much higher velocities.

A soil filled planter is an excellent energy absorber. When a vehicle hits the planter the vehicle turns into an earth plow and loses its energy rapidly as it burrows through the barrier. The kinetic energy of the vehicle transforms into heating of the soil, due to friction, and into kinetic energy of the soil itself as it plumes up and out.

Another way of looking at this is to note how a barrier absorbs energy. The energy absorbed is roughly the reaction force multiplied by the distance that the barrier travels as it stops the vehicle, so the more distance the barrier travels through, the lower the force.

A rigid retaining wall, by its very nature, doesn't travel very far when it stops a vehicle. Maybe an inch or so at the most. So the force on the barrier is very large. A planter type barrier, on the other hand, stops the vehicle through a stroke of several feet. So the stopping forces are relatively low. This explains why a planter type barrier can have a much shallower foundation than a rigid barrier.

s/ David Lee