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RE: seismic design manual - volume 1 - seaoc

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Bill,

Since when have the forces from UBC equation 32-2 been distributed vertically in a triangular fashion based on hx/hr? That would be the case if hx were variable up the height of the panel. As last I checked, it's the fixed elevation at point of attachment and it doesn't vary, just like hr is a fixed point that doesn't vary. What you're really doing is taking the average of two rectangular distributions.

Also, if a guy wants to sharpen his pencil, why do we have to badger him for it. He's not working for you, so what care do you have?


From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: RE: seismic design manual - volume 1 - seaoc
Date: Fri, 12 Sep 2003 07:15:03 -0700

I'll respond to my part.

Your post did not mention that the wall was concrete nor did I take the
time to thumb through the SDM (you didn't mention which example it was,
which would have made it easier), but I have now. I believe you're
referring to Example 36 on page 113.

I STILL believe you are taking a simple problem and making it
complicated. No sarcasm intended.

For a simple span beam, M = WL^2/8.

For a continuous beam, the negative moment (at the 2nd floor support), M
= WL^2/8 (if the load is the same on both spans).

If the wall is reinforced at the middle of the section, considering the
spans as simple spans, the analysis is conservative and you will have
saved some (IMO useless) analysis time for more important things.

I find it interesting that you are not concerned about the reactions
considering continuous spans, because the load at the 2nd floor is 25%
higher when considering continuity.

The most important point I want to make (and it is a serious one, not a
sarcastic one) is that it might be a mistake to get one's nose too close
to the paper and try to make the problem more complicated than it should
be. Just look at the loads you are applying to the wall. How accurate do
you believe they are? If the forces (themselves a good guess at best, an
arbitrary factor x DL at worst) are distributed verically in a
triangular fashion (based on the function of h_sub_x over h_sub_r) and
we are only taking the average between the points of support (again, for
simplicity), how accurate can the analysis be? I sincerely believe you
are trying to make a silk purse out of a sow's ear.

Yes, I agree that one should think about continuity in the design, but
when the analysis is based on simple spans which produces conservative
results, I don't see taking the analysis any further.

Regards,

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

:-----Original Message-----
:From: da [mailto:dadie(--nospam--at)sbcglobal.net]
:Sent: Thursday, September 11, 2003 9:52 PM
:To: seaint(--nospam--at)seaint.org
:Subject: Re: seismic design manual - volume 1 - seaoc
:
:here's my thoughts to the 3 (mostly) serious responses...
:
:************* Jake Watson, P.E. **************
:
:What is more conservative?  If you place the reinforcing in the center
or
:the wall, you will end up with more steel by assuming two simple spans.
:Secondly, you would need to do a cracked analysis and take into account
the
:stiffness of each floor if you want a true continuous two span wall.
The
:double simple span is not necessarily correct, just more conservative
and
:significantly easier to calc.
:
:************* Jake Watson, P.E. **************
:
:**da**  jake, i agree - thank you **da**
:
:
:
:************* Mike Hemstad, P.E. *****************
:
:However, most engineers, when faced with a
:tributary-area problem like this, would figure the
:loads as was done here.
:
:**da** that, in itself, doesn't make it right, does it?  **da**
:
:In other words, the far simpler
:tributary-area method is more accurate than a
:continuous-beam analysis.
:
:**da** is it? what about the negative moment at the floor level?  this
is a
:wall design, not a slab on grade, elevated slab, roof reaction design.
:**da**
:
:This, laziness, and
:historical acceptability are the reasons the far less
:complex simple-span distribution continues to be used.
:
:**da** not by me... **da**
:
:But yes, you're right, in this case a continuous-beam
:analysis would probably more accurately represent the
:reactions.
:
:**da**  like i said above, i don't care about the reactions at this
point,
:i'm just trying to find the real flexural stresses in the wall...
mike,
:thank you for your responses.... **da**
:
:
:
:**T. William (Bill) Allen, S.E. ***
:
:> 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.
:
:**da** i would agree wholeheartedly if the second floor interrupted the
:wall
:and there were 2 physical wall elements, one above the floor and one
below.
:but we are talking about a monolithic wall with full height continuity
:**da**
:>
:> 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.
:
:**da**  you aren't serious are you? **da**
:
:> Let's find something else to make complicated, shall we?
:
:**da** i think i'm supposed to take this as sarcasm, right?  you can
call
:me
:silly but i have a real soft spot for mechanics and free body diagrams.
:continuous beam analysis isn't THAT hard, is it?  **da**
:
:
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