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• To: seaint(--nospam--at)seaint.org
• Date: Fri, 10 Oct 2003 09:17:53 -0400

```> From: "Joseph Grill" <j.grill(--nospam--at)comcast.net>

> I've gotten that far with this and with other problems of a similar nature.
> I was just hoping that from that last guard rail type problem someone has
> joined the list with a great "snap your fingers" easy way to go about it.
> Guess I'm back to armwave the movement to get an answer.

Typical approach to this type of scenario is to make it bigger and
stiffer, which is completely contrary to the dynamics. Making the
structure stiffer leads to higher "impact" forces. The goal of this
approach is that at some point the assembly becomes "massive" enough
that the stiffness of the assembly becomes insignificant w.r.t. to the
rest of the system for a reasonable constructed cost

1/net stiffness = (1/log stiffness) + (1/structure stiffness) + (1/other
stuff stiffness)

However, sometimes it is impractical or unwise to achieve that "massive"
stiffness (e.g. failure is catastrophic rather than plastic deformation
or there is simply not enough space). A mighty great concrete thrust
block is useful if you have the space. Usually, you trade maintenance
cost for constructed cost, one way or another.

Determine what deflections the assembly can tolerate and make sure that
you respect that. Calculate the impact force. That's your minimum
without other mitigating design. If it's in a manageable range, do a
design iteration and see how sensitive the force is to your structural
conditions - the force may rise beyond reasonably manageable.

Check the price of a shock absorber to reduce the impact force - even a
bunch of old car coil springs from the dump would be useful if the
maintenance crew is sharp. A manufactured system gives a greater degree
of design control and they might do the calculations for you!

Deceleration energy is inversely related to the stopping distance. It is
only linear if the system response is linear - some spring systems have
a stiffness that increases with compression so you get a big force
despite a longer total deceleration distance.

So, don't spare the steel but engineer wisely.

--
Paul Ransom, P. Eng.
Civil/Structural/Project/International

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