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RE: UBC 1630, Ev Question

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This is one of those items where there is no consensus in the understanding
of why the provision exists.  There are really two camps on this issue.  I
believe that various individuals agreed to the Ev provision for one of two
reasons.  Since it passed they each think that their reason reflects the
real intent of the codes.

"Camp 1" is composed of engineers who are primarily concerned with concrete
buildings and who believe that the load factors in ACI 318 are "correct" and
the ASCE 7 load factors (which are also used by the AISC and the LRFD wood
folks) are "wrong".  Because they like a load factor of 1.4 on dead load
effects, they voted yes to the Ev factor because it gets the dead load
compression effect back to 1.4D in zone 4.  Of course, if the real intent is
just to change load factors for concrete design, Ev is the worst possible
way to do it because it also changes the factors for all other materials and
(as written in the code) also affects the uplift calculation.  The Blue Book
reflects the influence of this camp.

"Camp 2" is composed of engineers who believe that 1) load effects are not
material specific, 2) the ASCE 7 load factors are correct, and 3) although
traditionally ignored, some reflection of vertical excitation should be
included in our loading criteria.  ASCE 7 and the NEHRP Recommended
Provisions are crystal clear that Ev is an intended loading effect and not
just a game played for the sake of concrete.  Both of these latter documents
are also clear in indicating that Ev is both additive and subtractive.  As
Tom pointed out, the subtractive case can result in only 0.7D for some
cases.

The item in question has NOT been changed in the 2000 IBC.  This is because
no one from "Camp 1" proposed any changes.  If such a change had been
proposed, it would have been interesting to see how the politics worked out.

Now I'll give my personal opinions.  In many respects I belong to "Camp 2".
I don't believe that load effects are material specific.  I believe that the
statistics behind the ASCE 7 load factors are reasonable.  I believe that
changes that are intended to affect only one material should not be stuck in
the general provisions.  I KNOW that dead loads are almost always
overestimated by structural engineers (myself included).  We always use 20
psf for partitions; we throw around healthy loads for A/M/E; we round UP the
results.  As a result, I suspect that the "real" dead load in place in most
buildings is probably not far from 0.9D.  IF vertical effects are important
(and I'm not sure that there is significant evidence of such for
non-cantilever, non-prestressed elements), then they should be considered in
both directions.

> -----Original Message-----
> From: Mark E. Deardorff [mailto:MarkD(--nospam--at)DandDEng.com]
> Sent: Wednesday, February 02, 2000 11:51 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: UBC 1630, Ev Question
>
>
> According to the ICBO seminar I attended last fall it will be
> corrected in the 2000 IBC. I don't recall seeing it in the draft, however.
>
> > -----Original Message-----
> > From: Tom.Hunt(--nospam--at)fluor.com [mailto:Tom.Hunt(--nospam--at)fluor.com]
> > Sent: Wednesday, February 02, 2000 9:43 AM
> > To: SEAINT(--nospam--at)seaint.org
> > Subject: UBC 1630, Ev Question
> >
> >
> >
> >
> > UBC Section 1630.1.1 Equation 30-1 requires E = pEh + Ev
> > which is stated to be
> > used in the load combinations of Section 1612.  In Section
> > 1612.2.1, Basic Load
> > Combinations, "E" appears in two equations, 12-5 and 12-6.
> > HOWEVER, in reading
> > the SEAOC Blue Book, Ev was intended for equation 12-5 ONLY
> > (See Blue Book
> > commentary C105.1.1 and Equation 101-5).
> >
> > Was this a mistake or did ICBO/UBC conscientiously change
> > this??  For a regular
> > building, this has the effect of changing equation 12-6 to
> > 0.7D +/- 1.0E.  This
> > probably wont effect footing sizing since they are sized
> > using allowable load
> > design but this could throw concrete columns into tension and
> > could increase
> > anchor bolt diameter and embedment if you are using strength
> > design (ACI 349
> > approach).
> >
> >
> > Thomas Hunt, S.E.
> > Fluor Daniel