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# RE: seismic design manual - volume 1 - seaoc

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: seismic design manual - volume 1 - seaoc
• From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
• Date: Thu, 11 Sep 2003 12:14:11 -0700

```Why is this analysis such a complicated problem? Maybe I oversimplify. I
dunno.

The load is applied to the studs which act as simple "beams", spanning
from sill plate to top plate. Trib load to the foundation, 2nd floor and
roof, respectively.

We've got plenty of things to worry about rather than try to make the
vertical distribution of loads act in accordance with a continuous beam
distribution. About the only issue I can see is debatable is the load on
a parapet wall, but I too just take the whole load and apply it on the
roof diaphragm.

Let's find something else to make complicated, shall we?

T. William (Bill) Allen, S.E. (CA #2607)
V/F (949) 248-8588
San Juan Capistrano, CA
http://members.cox.net/ballense/

:-----Original Message-----
:Sent: Thursday, September 11, 2003 12:04 PM
:To: seaint(--nospam--at)seaint.org
:Subject: seismic design manual - volume 1 - seaoc
:
:
:"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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