[Beowulf] Re: removing tiles around heavy racks?

David Mathog mathog at mendel.bio.caltech.edu
Fri Jan 6 12:54:30 EST 2006


> David Kewley wrote:
> > David Mathog wrote:

> > I'm not explaining that well,
> > but like this:
> >
> > racks    \\\\\\
> > floor    ++++++ --> a lot of force
> > risers   //////
> 
> In our case, our racks are not yet fastened to the raised floor nor to
the 
> slab.  In fact, we decided to leave them on their wheels.  So they'll not 
> be bound strongly at the plusses in your diagram, and the lateral force 
> won't be too great.  But yeah, in the common case where the racks are 
> attached to the raised floor, you'll have the issue you name.

My gut feeling is that leaving the racks up on their wheels is more
dangerous than dropping the feet and coupling them to the raised floor.

Inertia is going to provide enough force in even a moderate earthquake
to get the wheels moving relative to the floor and that's going to put
a huge load on the top brace since the resulting pendulum motion,
if perpendicular to the top brace, is going to twist that brace.
If the racks were standing on their feet, or better yet, bolted
to the floor, the whole thing would tend to move as a unit so the
stresses between parts should be less. (Assuming the raised floor is
strong enough to handle the lateral loads.)  

With the wheels down there's a possible failure mode
where all racks roll out from under the top brace, which
twists under the 25k lb load, and then the combined
mass of the racks pulls down the top brace.  That is, the entire row
of racks might act like one massive pendulum attached to the top
brace.  Model it as a mass of N rack weights on a lever arm half
the distance from the top brace to the floor.  Assume .5g lateral
acceleration in the worst direction.  That's about 4 ft * .5 * 25000 lb
worth of torque, at least in the initial phase of the motion just
as the racks start to move relative to the top brace.  Seems like a
heck of a lot of torque for the brace to withstand.  

As for hopping, you're right that if the units were sitting flat on
the concrete floor it would take >1g acceleration to make them hop.
However they are sitting on a more complex structure and that can result
in accelerations that exceed 1g even if ground motions don't, if the
resonances in that system match frequencies in the ground motions. 

Regards,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech
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