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RE: FW:

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This is my take on the overstrength factor.

The strength level design for members is really based upon underlying
assumptions about maximum deformations that will occur in ductile members,
with forces capped at yield capacity, and real force levels that are less
than what would occur in a fully elastic structure due to the different
behavior when you dynamically load structures into the nonlinear range. This
design philosophy will work if you can accept permanent deformations and the
structure has enough ductility to survive the number of cycles that are
going to occur in an earthquake.

I think the overstrength factor is intended to take into account that actual
forces at critical locations can end up higher than the assumed yield
strength of the member if things go on long enough for strain hardening
effects to occur, or if material strengths are actually higher than assumed,
which can often happen. I think at connections, in particular, things may be
less ductile and it is prudent to design them so they stay elastic and
remain stronger than the connected members no matter what.

-----Original Message-----
From: Dave Adams [mailto:davea(--nospam--at)laneengineers.com]
Sent: Monday, October 14, 2002 10:49 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: FW:


Rick:

I am currently NOT reducing the seismic loads by 1.4 (using full "E")
when using the equations for omega-0 since that's the way the equation
is referenced and that is ICBO's instruction for the use of the
overstrength equations.  However, it seems that (... and I may be way
off base) for ASD, by not dividing E by 1.4 we are doing things
conservatively, since I believe a "rough" equivalent of R (1997 UBC) =
Rw/1.4 (1994 UBC), which would include for the old 3Rw/8 overstrength
factor.

We're adding overstrength to a strength-level force, but designing with
service level methods, which seems to be too conservative.  The omega-0
factor is the same for ASD & LRFD and the 1.7 allowable stress increase
for "strength" applications has not changed from code to code.  The way
the code is written and instructed, the 1.4 is not supposed to be taken
out for overstrength, but it sounds like an error to leave it in.  Do
you have any thoughts or background on the overstrength sections of the
code?

Also, Division V is, in fact, for allowable stress design.


Best regards,
Dave K. Adams, S.E.
Lane Engineers, Inc.
Tulare, CA



-----Original Message-----
From: Rick.Drake(--nospam--at)Fluor.com [mailto:Rick.Drake(--nospam--at)Fluor.com]
Sent: Monday, October 14, 2002 8:20 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: FW:



UBC Chapter 22, Division V  are not ASD design provisions.  They are
strength design provisions.  Neither E nor P-sub-E should be divided by
1.4.

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA

*********



 

                    "Vasu, Ravi"

                    <Ravi.Vasu(--nospam--at)jaco      To:     "'seaint(--nospam--at)seaint.org'"
<seaint(--nospam--at)seaint.org>                         
                    bs.com>

                    10/14/02 07:49       cc:

                    AM

                    Please respond       Subject:     FW:

                    to seaint

 
..... 
 





> I found out the following discussion/recommendation by seismology
> committee of SEOAC.
> It says that in ASD design, that in load combination that has omega,
the
> design
> seismic force has to be divided by 1.4.
>
> http://www.seaoc.org/Pages/committees/seismology.html
>
> Any comments?
>
>
> I am still waiting for comments on question # 2, below.
>
>
> Ravi
>
> -----Original Message-----
> From: Vasu, Ravi
> Sent: Friday, October 11, 2002 8:58 AM
> To: seaint(--nospam--at)seaint.org
> Subject: ASD load combinations in UBC '97, vol. II
>
>
>
------------------------------------------------------------------------
--
> Your following message has been delivered to the list
>   seaint(--nospam--at)seaint.org at 07:19:49 on 11 Oct 2002.
>
------------------------------------------------------------------------
--
>
>
> I have two questions(UBC 1997, Vol. 2) here.
> Thanks in advance.
>
> 1  a. Refer to UBC '97, section 2213.5(Allowable stress
> design(ASD)), pg. 2-255(column requirements)
> The load combinations have 'omega' to check column compression &
> tension.  AISC, '97 'seismic provisions for structural buildings',
> pg. 135(section c4.1) says that for all Allowable stress design,
> E(from UBC) should be divided by 1.4.
> So, in the above UBC section 2213.5, I think, from section
> 1630.2,(to calculate Pe) should be divided by 1.4(since that section
> in in ASD).  In fact, in LRFD(section 2211.6.4), for the same check
> on columns, we do not need to divide E by 1.4(from AISC on '
> seismic...).
>
> b. Similarly, for bracing connection design
> and moment connection(UBC section 2213.8.3.1 & 2213.6 under "ASD"-
> pg. 2-258 & 2-256), in the combinations with 'omega' times the
> 'design seismic force', the E (from section 1630.2 procedure) should
> be divided by 1.4 or not?.  Pl. verify.
>
> 2.  In UBC section 2213.8.3.1(pg. 2-258), the forces for bracing
> connection has 'The maximum force that can be tranferred to the
> brace by the system'(3 rd item).  Does it mean, the load
> combinations to use to find the force is from section 1612.3(pg.
> 2-4) ONLY?  or any other load combinations required?    If the load
> combinations are only from section 1630.2, then forces from items 1
> & 2 in section 2213.8.3.1 may not GOVERN at all.?
>
> I appreciate your time & response very much.
>
> Thanks
>
> Ravi
> Telephone:832-351-6856
>
>

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