My
Dad works in the guardrail business, so I’ve spent the majority of my childhood
being lectured about the importance of guardrail and how it “saves lives on a
daily basis.” From him, I’ve learned about the various purposes guardrail
serves, but the one purpose that always grabbed my attention was the fact that metal guardrail can lessen the force of
impact of car crashes, allowing drivers to sustain fewer injuries and reduce
the amount of casualties.
A
guardrail’s end terminal functions to absorb the force of impact of cars that
crash into it by collapsing. The head of the end terminal will push back onto
the guardrail, forcing the metal sheet perpendicular to it to feed through a
small opening, turn, and ribbon out the side to absorb the kinetic energy.
According to the Federal Highway Administration, a
standard end terminal should be able to withstand four different collisions to
pass inspection, but the one I will focus on will be a 0 degrees head-on impact
with a 2000 kg pick-up truck going at a speed of 100 km/hr (27.8 m/s), or 62
mph.
To
put things into perspective, the force of impact of crashing this pick-up truck
into a solid brick wall would be as follows:
W
= KE
Fd
= ½ mv2
F
= (mv^2) / (2d)
F
= [(2000 kg)(27.8 m/s)^2] / [(2)(0.75 m)]
F
= 1.03 x 106 N
(52.6
g’s)
The only “give” in this scenario is the small amount
of distance the front of the pick-up truck collapses (0.75 m). Chances of the
driver surviving this type of crash are slim at best from the sheer amount of
force felt in such a short amount of time.
When
the truck crashes into the end terminal, the force on the driver decreases
significantly because the transfer of kinetic energy (and subsequent force)
occurs over a longer period of time. This calculation assumes that 11 meters of
guardrail ribbons out with 0.5 meters of the front of the truck collapsing.
W
= KE
Fd
= ½ mv2
F
= (mv^2) / (2d)
F
= [(2000 kg)(27.8 m/s)^2] / [(2)(11.5 m)]
F
= 6.72 x 104 N
(3.43
g’s)
Guardrail
helps reduce serious injury and death during car crashes by absorbing the force
of impact of the vehicle during a head-on collision. Guardrail’s ability to
ribbon out allows the transfer of kinetic energy and the absorption of the
force to occur over a longer period of time, significantly reducing the force
of the collision felt on the driver.
Sources:
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