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# seismic design manual - volume 1 - seaoc

• To: seaint(--nospam--at)seaint.org
• Subject: seismic design manual - volume 1 - seaoc
• Date: Thu, 11 Sep 2003 12:04:07 -0700 (PDT)

```Dave Adie wrote:

"don't have it in front of me so going from memory...

the single story example showing how to apply the out
of plane lateral seismic forces makes sense to me.
the two story example that immediately follows
confuses me.  they don't specifically show the shear
and moment diagrams (conveniently) but they do show
reactions.  these reactions look like two adjacent
simple span beams - right?  shouldn't this model be a
continuous beam with full height continuity?

tia"

Dave,

I've been studying this (excellent) reference for the
upcoming SE2.  I think you mean the example on page
113.  The example finds reactions of 572, 744, and 267
pounds per foot at the roof, upper floor, and ground
floor, respectively.  I calculate 577, 739, and 267
difference is roundoff.  This is calculated as two
simple span beams, yes.  If the (lateral) supports are
stiff and the wall panel deflects primarily in flexure
(both true here), you're right.

However, most engineers, when faced with a
tributary-area problem like this, would figure the
loads as was done here.  In the more common case of
you could run a continuous-beam analysis of the floor
slab, finding in the process that the first interior
beam will carry the most load.  However, if you really
got fired up and did a full 3D analysis of the slab
and stringers, including shear deformation of the
slab, you'd find that all the stringers carry nearly
identical loads.  In other words, the far simpler
tributary-area method is more accurate than a
continuous-beam analysis.  This, laziness, and
historical acceptability are the reasons the far less
complex simple-span distribution continues to be used.
AASHTO goes so far as to require it when distributing
wheel loads through bridge decks to obtain beam
reactions for substructure design.

But yes, you're right, in this case a continuous-beam
analysis would probably more accurately represent the
reactions.  And the resulting bracing will turn out
just the same, because the simpler analysis puts a
little more reaction at the roof and a little less at
the upper floor.

Long answer to a short question.

TKDA
St. Paul, Minnesota

On another topic:

Scott.  Buddy.  Maybe you should think of limiting
yourself to 8 or 10 posts a day.  Not to start a fight
or anything, but GEEZ.

OK.  Attitude off.

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