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RE: Story Drift: 1994 UBC vs. 1997 UBC

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Thanks, Ben. Yes, this does clear things up (somewhat).

First of all, I had omitted two terms: "Omega sub o" and the 1.4 Load
Factor. In this case Omega * Load Factor = 2 * 1.4 = 2.8 which is close to
the "three times greater" I was referring to.

So, to recap, if one were to check/design a flag pole column (or moment
frame for that mater) based on stiffness (which usually governs in the
structures I work with) the following items need to be considered:
Multiply the "design" (i.e., working stress for now) loads by the load
factor of 1.4 and "Omega sub o" to arrive at a "correct" "Delta sub s". Then
use Eq. 30-17 to determine "Delta sub m" and ensure Dm<=0.0025*h.

Beyond this clarification, I hope that it is obvious that the code language
is not very clear (at least to me). Between "strength levels" vs. "working
stress levels", "using the 1.33 stress increase or not", "elastic vs.
inelastic", The code bounces around so much, I would be shocked if a
significant number of structures are designed with a correct interpretation
of this mess.

Editorial aside, let's now look at an OMF, shall we?

For an OMF:
"Omega sub o"=2.8
R=4.5
Using the same logic:
Delta97=0.7*4.5*1.4*2.8*D94
Delta97=12.35*D94
0.025/12.35 =0.002*h

Is THIS correct? If so, this is 2.5 times MORE restrictive than the 1994
UBC. (inverted pendulum 2.8 less, OMF 2.5 more, what is it?)

I guess I'm also still a little confused about the inclusion of the 1.4
factor for these purposes. From my understanding, there is basically a
difference of 1.4 between the old Rw and the new R, so is this already
accounted for in the "Delta sub m" calculation or not when comparing to 1994
UBC?

I'm not trying to be contrarian or critical, but I'm just trying to apply a
new code with new terms and new methodology while still have a ghost of a
chance of meeting a design budget (an issue obviously not considered by code
authors).

Further, I seriously doubt if I'm the only one having difficulties here.
Technical changes aside, I have only known one language (English) all my
life but I can only speculate how difficult this might be for someone whose
English is a second language.

Regards,

Bill Allen, S.E.
ALLEN DESIGNS
Laguna Niguel, CA


> -----Original Message-----
> From: Byainc(--nospam--at)aol.com [mailto:Byainc(--nospam--at)aol.com]
> Sent: Thursday, August 12, 1999 7:45 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Story Drift: 1994 UBC vs. 1997 UBC
>
>
> bill
>
> I think there is something wrong with the way you are trying
> to work your way
> backward to compare the two methods.
>
> let me try to take a crack at it the following way. it may
> clarify the case a
> little bit:
>
> assume the "calculated" displacement under 94 UBC was D94 and
> for 97 UBC is
> D97. To get the equivalent drift under 97 UBC the following
> adjustment would
> have to be made
>
> Delta97 = .7 (Dm)R
>
> where
> Dm = (omega)(D97)= 2(D97)
> but D97=(1.4)D94   due the strength level forces
> =>    delta97= (0.7)(2)(1.4)(D94)(2.2)= 4.3(D94)
>
> now if you divide the 97 drift limit by 4.3:
>
> 0.025/4.3 = 0.0058 ~ 0.005 So actually not much has changed.
>
> I hope this helps
>
> Ben Yousefi, SE
> san Jose, CA