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RE: WOOD MOMENT FRAMES, Rw = 3[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: RE: WOOD MOMENT FRAMES, Rw = 3
- From: "Dennis S. Wish" <wish(--nospam--at)cwia.com>
- Date: Tue, 14 Apr 1998 12:08:53 -0700
I see your point and agree with you. One suggestion, the City of Santa Monica requires that a flagpole used outside (eccentric) to the structure for resistance of lateral loads only, be designed with a K factor of 2.0. This would push the load to twice the height of the column - essentially increasing the stiffness requirement of the column. Wouldn't this be the easiest, most conservative approach regardless of the column being in the plane of the wall or eccentric to it? Dennis Dennis S. Wish PE La Quinta, California wish(--nospam--at)cwia.com ICQ# 6110557 http://wwp.mirabilis.com/6110557 "The death of democracy is not likely to be an assassination from ambush. It will be a slow extinction from apathy, indifference, and undernourishment." Robert Hutchins -----Original Message----- From: Bill Allen, S.E. [mailto:billallen(--nospam--at)earthlink.net] Sent: Tuesday, April 14, 1998 10:48 AM To: seaoc(--nospam--at)seaoc.org Subject: Re: WOOD MOMENT FRAMES, Rw = 3 You have totally misunderstood my point. Assume a structure like a townhouse where you have "flagpoles" or "inverted pendulums" resisting seismic loads at the garage entry. The remainder of the LFRS consists of plywood shear walls. Based on the 1997 UBC, the entire lateral force resisting system resisting loads perpendicular to the garage opening will have to be designed based on an Rw=3. This includes the shear walls at the far end of the townhouse (furthest away from the garage opening). Based on the SEAOSC seminar, many (most? all?) felt that this was conservative and not accurate, but, unfortunately, the 1997 UBC had already gone to press. Some building officials (I don't know what jurisdiction(s)) said they would amend their plan check procedure if SEAOC would present a position paper. To my knowledge, SEAOC has not provided this position paper therefore this code provision is still in effect. I suggested that a possible "work around" would be to design the structure based on Rw=6 (or 8 if all plywood shearwalls) and then apply 3Rw/8 to the design of the "inverted pendulums" which would bring the design of the "flagpoles" to near an Rw=3 without designing the whole structure for Rw=3. Regards, Bill Allen -----Original Message----- From: Dennis S. Wish <wish(--nospam--at)cwia.com> To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org> Date: Tuesday, April 14, 1998 10:33 AM Subject: RE: WOOD MOMENT FRAMES, Rw = 3 >Whoa, stop me here and explain something to me. I am under the understanding >(at least from the '94 code) that the system is designed for a Rw stipulated >in table 16-N-Structural Systems. In working stress design, only the member >strength due to the internal reactions at the connections (beam to column, >column to fndt) are increased by 3Rw/8 - specifically as stated in 2211.5.1 >of the '94 code "In addition, in Seismic Zones 3 and 4, columns in frames >shall have the STRENGTH to RESIST the axial loads resulting from the load >combinations in Items 1 and 2 following...." >Following to the exception "1. Need not exceed either the maxium force that >can be transferred to the column, by elements of the structure, or the limit >as determined by the overturning uplift which the foundation is capable of >resisting.." >This means to me that the system is designed by Rw factors as stipulated in >Chapter 16 while the strength of the columns and foundation alone (rather >than the entire system) are increased 3Rw/8 to resist overturning. >Furthermore, to accomplish this, the connections need to be designed for the >increased lateral load needed to resist the demand due to overturning. >This is a far cry from suggesting that the entire mixed system be designed >for a lateral load of 3Rw/8 (if this is what you are suggesting). >The reality of this would be that unless wind governs, the cost of >construction in seismic zones 3 or 4 would become unaffordable to most >families unless they reduce their living to a 900 square foot box. This may >be simplistic, but there are a lot of financial ramifications for being >overly conservative. >As far as the comments about "personal interpretation of the code". Our >codes are built upon emperical rationalization not facts - for this reason >they are tweaked out in each code cycle - we hope for the better. However, >the codes are created by a group of individuals who, I believe, do the best >they can to create an ecconomical interpretation of design, but are no more >sure of what the effects will have than the rest of us following their >suggestions. >Be careful about accepting the letter of the code as gospel when there most >certainly are interpretations to be made and conditions where the letter of >the code can no apply - isn't this where engineering judgment comes into >play? >Dennis S. Wish PE >La Quinta, California >wish(--nospam--at)cwia.com > >ICQ# 6110557 >http://wwp.mirabilis.com/6110557 > >"The death of democracy is not likely to be an assassination from ambush. It >will be a slow extinction from apathy, indifference, and undernourishment." >Robert Hutchins > >-----Original Message----- >From: Bill Allen, S.E. [mailto:billallen(--nospam--at)earthlink.net] >Sent: Tuesday, April 14, 1998 8:16 AM >To: seaoc(--nospam--at)seaoc.org >Subject: Re: WOOD MOMENT FRAMES, Rw = 3 > > >Another way would be to add "inverted pendulums" to the 3Rw/8 list. This >way, the design forces would be 2.25 times the rest of the building (if Rw=6 >is used) or an "effective" Rw=2.67 for the flagpoles. This approach would >require less re-wording of the Code, I believe. BTW, I would design the >grade beams, etc. using the higher force. You don't want the columns to be >O.K. and the foundation to fail. > >Be careful about your "personal" interpretation of the Code. You may run >into a building official with a less than accommodating attitude. > >Regards, >Bill Allen > >-----Original Message----- >From: Parkerres <Parkerres(--nospam--at)aol.com> >To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org> >Date: Tuesday, April 14, 1998 8:07 AM >Subject: Re: WOOD MOMENT FRAMES, Rw = 3 > > >>Bill - >> >>Thanks for the repsonse. I, too, feel that the Rw=3 for the rest of the >>building, or at least that direction, is excessive. I will adopt the >City's >>shrug method and design accordingly. Personally, I think it would be >clearer >>if the Code left Rw=6 for cantilvered columns, and then added a note in the >>text saying that cantlivered column systems should be designed for twice >the >>calculated load. (This is similar to masonry shear walls, which are >designed >>for 1.5 times the calculated load). This would eliminate the Rw=3 problem >for >>the building, yet still give the desired design results for the >cantilevered >>columns. The Code could then explicitly deal with the question of whether >or >>not the doubled load also applies to the footing design and to the drift >>design. >> >>Bruce Resnick, SE >>Parker Resnick Str. Eng. >> >> >> > > > > > > >
- Re: WOOD MOMENT FRAMES, Rw = 3
- From: Bill Allen, S.E.
- Re: WOOD MOMENT FRAMES, Rw = 3
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