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Re: Seismic Design

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Ah, not to be a downer, but I would say that you are talking about an
imaginary building is some fantasy land.

Here in the real world every building must be resist wind loads and as a
result have a lateral system.  I would argue that _NO_ building is
"governed" by wind/seismic vs. "governed" by gravity loads...expecially
when you are talking about a 20 story building.  Certain structural
members' sizes might be governed by one or the other, however.

Now, at a minimum, if you want to talk about these fantasy land building
just for the purposes of trying to "show" the difference between a gravity
system and a lateral system, then:

For a fantasy land building with no lateral system and no cladding
installed (interior or exterio), it could be comprised entirely of
"sticks" going purely horizontal (beams/girders) and purely vertical
(columns).  If the structural bays were the same size throughout, then all
the horizontal "sticks" in one direction would be one size (filler
beams) and the horizontal "sticks" in the other direction would be bigger in
cross-section (girders).  All the connections would be strickly at the
webs (i.e. double angle, single angle, shear tabs).  There would be no
walls and no braces (i.e. "sticks" at angles...unless the gravity framing
was odd).  All vertical "sticks" could be of the same size or the ones
around the perimeter could be slightly smaller.  This assumes a steel,
"frame" (as opposed to bearing wall) system.

Now, if you introduce a lateral system, it will either have walls added
(i.e. shear walls); diagonal "sticks" (i.e. braces); or some of the
horizontal & vertical "sticks" will get quite bigger in cross-section
(i.e. moment frames), most likely at the perimeter and the connections of
these "sticks" that get bigger will now have material that connection the
flanges together.

Again, this is purely for a steel, "frame" system.  It does not take much
work to "convert" it to a R/C, "frame" system, except that in R/C the
connections will look the same to the naked eye between a frame with a
lateral system and one without.

That is about as "dumbed down" an explanation that I can think of off the
top of my head.  And I frankly don't think it is that great of one...but
there you go.


Adrian, MI

On Sun, 28 Aug 2005 GSKWY(--nospam--at) wrote:

> Okay, stepping back a little, say you were trying to describe the difference
> between a building in San Francisco and one where you didn't need to design
> for either wind or seismic.
> I.e. you are comparing a building where lateral load controls to one where
> gravity load controls.
> If someone looked at the two buildings during construction, after they had
> been topped out, but before any exterior cladding, etc. had been put up,  what
> would they notice as being different about the two buildings.
> Assume it's a 20-story office building.
> Gail Kelley

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