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Re: Stopping That Truck !!

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The problem is really nonlinear.  In a similar design, that I did years ago
we looked closely at the deformations and deflections and the energy used
to cause them rather than the approximate equivalent static force.   We
assumed a force that would yield the major components, applied that force
and looked at the deformations and the total energy that would be expended
in the deformations and deflections.  When we could show that the energy
expended was greater than the energy of the moving truck and that the
deformation and deflections were within the ductility limits of the
materials making up the barrier we concluded that the design was adequate.

The crash test organizations may be able to give you a better guess at how
much energy is expended in deformation of the truck.  We made some very
conservative assumptions in that regard.





"Caldwell, Stan" <scaldwell(--nospam--at)halff.com> on 01/23/2002 08:32:33 AM

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Subject:  Stopping That Truck !!

Greetings, y'all:

Every now and then, and interesting problem comes along that is challenging
in its simplicity.  It is even more interesting if that problem is entirely
outside the realm of all common building and bridge codes and guidelines.
Structural engineers simply are ill-prepared to tackle problems which
involve moving objects, momentum, kinetic energy, potential energy, force
due to deceleration, and acceptable structural failure.  We are struggling
with just such a problem right now:

A client in the defense industry wants to construct a security fence around
the perimeter of their industrial campus.  The fence is to be comprised of
bollards nominally spaced at 12 ft., connected with a continuous wire rope.
Each bollard will be comprised of a 12" X 12" concrete post over a 18" dia.
concrete pier.  The posts will extend about 3 ft. above grade, and the
piers
will extend 6 ft. to 8 ft. into black clay.  Reinforcement will be
continuous between the piers and posts, either with #8 rebars or with small
rolled shapes.  The 1" dia. wire rope will pass through the bollards about
30" above grade, and will sag about 2" between bollards.  The client has
specified that the fence must stop a 7500 lb. truck with a C.G. at 3 ft.
above grade, traveling at a speed of 20 mph or less.

Question:  What is the design force on the bollard?  on the wire rope?

Six normally intelligent structural engineers have calculated answers
ranging form 3 Kips to 980 Kips, with the majority thinking somewhere
between 50 and 100 Kips.  What do you think?

Before you jump to an answer, you should consider the following factors:

1]    The softest, and least known, spring is the soil ... and it's
stiffness changes seasonally.

2]    The next softest, and ill-defined, spring is the truck bumper and
chassis.

3]    The bollard can "fail" into a full plastic hinge (or beyond), just as
long as it stops the truck.

4]    The two most important variables are time and distance, which, of
course, are both indeterminant.

Who out there can think "outside the box" and solve this problem?

Stan R. Caldwell, P.E.
Dallas, Texas

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