Need a book? Engineering books recommendations...
Fwd: Re: Is SMRSF permitted in Seismic Zone 2?[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: addseaoc(--nospam--at)euken.com
- Subject: Fwd: Re: Is SMRSF permitted in Seismic Zone 2?
- From: rlewis(--nospam--at)techteam.org (Richard Lewis)
- Date: 30 Jul 1997 14:20:51 GMT
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 1922.214.171.124, 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 19126.96.36.199 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 19188.8.131.52 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 19184.108.40.206. >>>> 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 19220.127.116.11. 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 --- Internet Message Header Follows --- Received: from server1.seaoc.org (bqe.com [18.104.22.168]) by host1.texramp.net (8.8.5/8.8.5) with SMTP id OAA09908 for <rlewis(--nospam--at)techteam.org>; Mon, 17 Feb 1997 14:41:12 -0600 (CST) Received: from emout26.mx.aol.com by server1.seaoc.org (NTList 3.02.10) id qa003916; Mon, 17 Feb 1997 12:34:21 -0800 Received: (from root@localhost) by emout26.mail.aol.com (8.7.6/8.7.3/AOL-2.0.0) id PAA04763 for seaoc(--nospam--at)seaoc.org; Mon, 17 Feb 1997 15:32:19 -0500 (EST) Date: Mon, 17 Feb 1997 15:32:19 -0500 (EST) From: BCASE1356(--nospam--at)aol.com Message-ID: <970211222634_-1575701282(--nospam--at)emout05.mail.aol.com> To: seaoc(--nospam--at)seaoc.org Subject: Re: Is SMRSF permitted in Seismic Zone 2? Reply-To: seaoc(--nospam--at)seaoc.org Error-To: seaoc-ad(--nospam--at)seaoc.org X-Loop: seaoc(--nospam--at)seaoc.org X-Info: [SEAOC] Owner: seaoc-ad(--nospam--at)seaoc.org X-POP3-Rcpt: seaoc-ad(--nospam--at)seaoc.org X-Sender: seaoc-ad(--nospam--at)seaoc.org Precedence: list X-ListMember: rlewis(--nospam--at)techteam.org [seaoc(--nospam--at)seaoc.org] __________________________________________________ Richard Lewis, P.E. Missionary TECH Team rlewis(--nospam--at)techteam.org The service mission like-minded Christian organizations may turn to for technical assistance and know-how.
- Prev by Subject: Fwd: Is SMRSF permitted in Seismic Zone 2?
- Next by Subject: Fwd: Re: Isolated pad footings - To tie or Not to tie?
- Previous by thread: Fwd: Is SMRSF permitted in Seismic Zone 2?
- Next by thread: Fwd(2): Firm Sued Over Fractures in Steel Frames in Quake