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In a message dated 97-02-11 02:22:44 EST, you write:

<< Subj:	Is SMRSF permitted in Seismic Zone 2?
 Date:	97-02-11 02:22:44 EST
 From:	amirr(--nospam--at)paknet1.ptc.pk (ARW)
 Reply-to:	seaoc(--nospam--at)seaoc.org
 To:	seaoc(--nospam--at)seaoc.org ('seaoc(--nospam--at)seaoc.org')
 
 I am extremely grateful for the three enlightening responses to my posting.
I do not wish to repeat my arguments, but I still feel that the following
points, not touched by any of the three, merit your special attention:
 
 (a) How does one reconcile the wording of Section 1631.2.7 with that of
Section 1921.2.1.3, of the UBC? The former says that in Zone 2, the frame
SHALL AS A MINIMUM, be IMRSF while the latter says that it SHALL be IMRSF?
Would you not agree that it is ONLY by using an IMRSF, in Zone 2, that one
can reconcile both the said sections?
 >>
response:

I believe the intent of the code is to allow for the use of either an IMRF or
 a SMRF in seismic zone 2 .  There is a conflict with the wording.  I would
go along with section 1631.2.7.  If you provide the detailing for a SMRF, you
should be allowed to use this framing system.  The wording in section
1921.2.1.3 should be reworded to say that " 
....reinforced concrete frames resisting forces induced by earthquake motions
shall be as a minimum intermediate moment-resisting frames......."

>>>
 (b) How would someone using an SMRSF in Zone 2, enter the code for meeting
the requirements of providing special confinement details, for "frames
resisting forces induced by earthquakes"? Section 1921.2.1.3 of the code
unambiguously states that the frames in Zone 2 SHALL be IMRSF "proportioned
to satisfy ONLY Section 1921.8". On the other hand, the detailing
requirements -- that would naturally go with an SMRSF -- are specified in
Sections 1921.2 through 1921.7 of the code, which sections are applicable to
structures located in Zones 3 and 4 only, under the provisions of section
1921.2.1.4.
 >>>>
response:

If you are in zone 2 and want to use a SMRF, then you must comply with the
requirements for zone 3 and 4 since this is where the prescriptive
requirements for a SMRF are located.  By meeting the more stringent
requirements for a higher seismic zone such as zones 3 and 4, the building
official should realize that you are exceeding the minimum design
requirements of the building code.  If you decide to use an IMRF, then you
must meet the requirements for this system.  The difference between the SMRF
and IMRF is the Rw and ductility which is associated with the system.  When
using an IMRF you are designing for a higher base shear since Rw=8, but the
detailing of the members limits the allowable displacement of the moment
resisting frame system before damage occurs.  When using an SMRF, you are
designing for a lower base shear since Rw=12, but because of the detailing
requirements, you can handle larger displacements while still supporting
vertical loads.  I would expect the IMRF to possibly have larger framing
members than a SMRF because of the larger design forces.  With larger framing
members, the IMRF frames  would be stiffer and would be expected to have less
displacement, thereby helping to control P-delta effects.

>>>>>

 (c) How would someone using SMRSF in Zone 2, handle the "members not part of
the lateral load resisting system"? The provisions for such members are very
lenient for structures falling in Zone 2, as only Section 1921.7.2 is
applicable to such members, as stipulated in Section 1921.2.1.3.
 
 Very significantly, even ACI 318 does not have ANY special detailing
requirements for "members not part of the lateral load resisting system", for
structures falling in Zone 2. Naturally, this facility SHOULD not be
available if some one uses an SMRSF in Zone 2 and invites large inelastic
displacements, as a result. To my mind, these large displacements are very
likely to significantly endanger the structural safety of the gravity
members. Does this, too, not reinforce the belief that significant inelastic
action is not INTENDED in Zone 2, in the first place -- as would be the case
if we were to use an SMRSF -- by any of the above two codes?
 >>>>>
response:

If you use a SMRF, then all framing members, including those not part of the
lateral resisting system, must be capable of supporting the acting vertical
loads for the expected design displacements of the lateral resisting system.
 You must use the detailing requirements for zone 3 and 4 and check the
design forces as required by the UBC.  Your design force will still be less
than zones 3 and 4 since the design Z value will be for zone 2.  

When using a SMRF, you are anticipating that there will be large inelastic
displacements since you are using a Rw of 12, unless the actual capacity of
the framing system significantly exceeds the design forces ( example: the
capacity of the frames are twice the design loads).  By your very own
arguement, would you outlaw the use of the SMRF in zones 3 and 4 where we
expect large displacements.

Because a building is in zone 2 does not mean that you do not expect a
building to undergo large inelastic displacements if you have a large
earthquake.  We can assign seismic risk based upon reoccurrance intervals for
various magnitude earthquakes, or a certain expected ground acceleration for
each seismic zone for the same given return period, such as the 475 year
return period used for the UBC response spectra.  The likelyhood of an
earthquake of a given magnitude occurring in zone 2 is less likely than in
zone 4.   A magnitude 7.5 earthquake may only occur once every 2000 years in
zone 2, whereas in zone 4 a magnitude 7.5 earthquake might be expected to
occur once every 100 years.   When this 7.5 earthquake occurs in zone 2, the
concrete IMRF will undoubtably experience large inelastic  displacements
since you have designed using an Rw=8 instead of Rw=1 where the frame would
be expected to remain esscentially elastic. 

One other thing to remember is the construction cost of an IMRF vs. SMRF.  In
areas which have a low frequency of earthquake occurance,  the associate
seismic risk does not warrant the additional cost for an SMRF, based upon the
projected lifespan of the building. 

>>>>>
Perhaps if the word "permitted", used by me in my first posting, is replaced
with the word "contemplated", my case would become more plausible.
 
 Your help would be greatly appreciated.
 
 Sincerely,
 
 
 Rizwan Mirza, P.E.
 Lahore, Pakistan
 amirr(--nospam--at)paknet1.ptc.pk
  >>


I Hope this helps to answer some of you questions.  

Michael Cochran 
Brian L. Cochran Associates
Los Angeles, CA 


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