Re: VAB suffers some damage -- threat to contents
From: Lou Scheffer (lou_at_cadence.com)
Date: 09/07/04
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Date: 7 Sep 2004 10:38:31 -0700
John Doe <jdoe@doe.org> wrote in message news:<413CF470.9B0ADDAA@doe.org>...
> "Greg D. Moore (Strider)" wrote:
> > The rating most likely refers to structural damage, not facade. In other
> > words, its like losing shingles on your house, but the house doesn't
> > collapse.
>
> A rating should refer to a building's ability to protect its
> contents/occupants from outside elements. [...]
>
> The length of the event should have no bearing on structural damage. Either
> your building can sustain such winds indefinitely, or it can't and will
> progressively weaken and fall apart. And it it is weakened by this storm but
> doesn't fall apart, what happens during the next storm ?
This is not true for metal frame structures. For most metals,
specifically including structural steel, there is lower limit where
the deformation is elastic, meaning the structure will return to the
original shape with no damage at all when the force is removed (like a
spring). Then there is a higher limit, in the plastic regime, where
the structure suffers permanent deformation, but does not break. The
structure may then require serious maintenance afterwords, for
although it protected the contents, another similar event (or even if
the first event was longer) might cause it to fail.
Of course it would be nice to stay within the elastic limits for all
conceivable events, but this is usually prohibitively expensive for
large structures. So buildings are usually designed to take any
number of smaller events without damage, with a higher limit for 'once
only' events, after which major maintenance will be required. So for
most buildings, there should really be two types of ratings. I'd
suspect the often quoted 125 mph for the VAB is the survive once
number.
This distinction can be seen all the time after earthquakes. Some
buildings survive undamaged (elastic region), some are damaged but
survive (plastic deformation), and some collapse. Modern building are
designed to survive a moderate earthquake unscathed, and a really
major one without collapse. Only facilities such as emergency centers
are designed to survive very rare events without damage - it's
(statistically speaking) cheaper to accept that occasional rare events
will cause major damage, and repair it when it occurs.
Lou Scheffer
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