[Beowulf] Re: removing tiles around heavy racks? (David Kewley)

David Kewley kewley at gps.caltech.edu
Thu Jan 5 00:04:37 EST 2006


On Wednesday 04 January 2006 15:44, David Mathog wrote:
> David Kewley wrote:
> > The first failure mode under such a load would probably be stringer
> > buckling, and that'd take a lot of force.  (Assuming your stringers are
> > something like mine: basically steel C-channels, about 3/4 inch square
> > cross-section, 2 feet long).  Yes, leaving the tiles in will greatly
> > reduce the risk of buckling, but for normal forces, buckling is pretty
> > unlikely anyway, I'd think.
>
> I tend to agree but only if the racks are firmly attached to the floor
> (and ideally, to another structural frame at the top).  There
> can be large accelerations both horizontally and vertically and under
> the worst case scenario (poor attachment of the equipment) that
> might cause racks to hop up and down (force wise) and
> pound the floor they are standing on.  If the rack is free to bounce
> around then the forces can be severe, for instance, if it manages to
> get up on one edge and then come down that way, or it walks over a tile
> edge.  Hmm, the feet on the raised floor are bolted onto the concrete
> below, right?  Else they can jump around too.

Things will hop only if the earthquake's downward acceleration is greater 
than 1G.  To my understanding, it takes a very large, damaging earthquake 
to produce that kind of vertical acceleration.

> > If your stringers are *not* trapped by or attached to walls, then
> > you'll be more subject to toppling the raised floor catastrophically,
> > although I'd imagine a properly installed floor would be highly
> > resistant to human-scale forces.
>
> I've never seen a free standing raised floor like that, they always
> go all the way to the walls.  I think primarily so that equipment
> isn't accidentally rolled off the edge, which would certainly be
> catastrophic for anybody unfortunate enough to be underneath it when
> it lands.

Our raised floor is not up against a wall at one end of our room, so yes, 
there's an edge there.  It's never been a problem to keep things from 
rolling off the end.  But there's no easy way to brace the raised floor, 
aside from intruding into the slab-level walkway there, which we've so far 
delined to do.

> > Right now, only the tops of our racks are braced, and in a sufficiently
> > large earthquake, the bottoms could pendulum out from under the braced
> > tops and the racks crash to the ground.  We have probably 40-50k pounds
> > (mass) of joined racks.  In a .5G earthquake, that means 20-25k pounds
> > (force) transmitted to the walls.
>
> It's a bit more complicated than that.  What you really have is
> something resembling a funky two story building, with the raised
> floor being its first floor and the racks being its second floor.
> Multistory buildings flex at each floor and they have resonant
> frequencies that cause them to fail in earthquakes that have ground
> motion frequencies that match.  The worst mode would be where that
> resonant frequency drives the bottoms of the racks across the narrow
> dimension in one direction and the tops in the other, which would
> maximize the lateral force on the floor.  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.

> > Yes, I'm a bit nervous about it.  Plus it'd be mighty embarassing for a
> > supercomputer owned (in part) by a world-famous seismology department
> > to fall over in an earthquake...
>
> They're braced at the top so they probably won't fall over.

Again, the tops won't fall over per se, but the bottoms could in principle 
pendulum out from under the tops, which would effectively tip the racks 
over.

> However 
> in a really severe earthquake the forces might break the floor enough
> so that it could collapse. That's a very big quake though, big enough
> that you have to start worrying about the welds which hold the building
> together.
>
> If Lucy Jones is giving post quake interviews in front of the
> rubble pile that was South Mudd embarassment will be the least of
> your worries!

That's exactly what we're banking on. :)

David
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