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[SEAOC] Re: [SEAOC] Impact Loads

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Drew Morris wrote:
> 
> I am working on designing a series of columns that are designed to
> support glass walls that will contain marine mammals.  The
> specifications give the masses and velocities of the various animals,
> but not the methods of calculating the impact forces of these animals
> against the wall.  It was recently decided by the group of interested
> parties that using the kinetic energy of the animal and allowing it to
> deform a certain distance upon impact would be used to calculate the
> force.  Unfortunately, by using a energy method, the kinetic energy of a
> bird at 50 mph is the same as a sea lion at 5 mph, so the walls could be
> designed identically.  Is there a better method to be used?  One based
> on momentum or previous animal studies.  This is similar to what
> aeonautical engineers do when designing aircraft to survive bird impacts
> on windshield or engine fan blades.  Any help would be appreciated.

Drew,

You are working on an interesting problem.  The question you raised in energy 
balance method may be caused by the portion of the strain energy which was 
left out of the equation and was stored in the marine mammals during impact. 
 You are on the right track to look into momentum equation and animal 
deformation upon impact.

When we designed important building against aircraft impact before, the 
impulse momentum equation and fuselage deformation function based on test 
result were used in deriving the forcing function.  However, this may not be 
practical in your case, because most probabbly you do not have the mammal 
deformation function.  If you did, you would see larger force required to 
deform a sea lion than a bird.

It is not unusual for engineers to perform design based on incomplete 
information.  Under this case, conservatism will be applied to compensate for 
lack of knowledge in input data.  Thus, the energy balance method without the 
deformation energy stored in the marine mammals can still be applied.

Some additional suggestions are offered as follows:

1. The glass wall design may be out of your scope.  You may want to remind 
the owner to check its mechanical property for energy absorption during 
impact. 

2. Check the column design for at least two impact points, one at the center 
of glass wall with membrane tension and bending dominant, and the other next 
to the column with shear dominant.

Happy engineering,

Chang Chen, Ph.D., P.E.
Apollo Consulting, Inc.

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