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Re: WOOD MOMENT FRAMES, Rw = 3

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If the "K" you are referring to is to calculate the allowable axial stress
based on KL/r, a K value of 2 is not conservative since the flagpole is
cantilevered (Ref. AISC p. 5-135). AISC in fact recommends a K value of 2.1.

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 12:22 PM
Subject: RE: WOOD MOMENT FRAMES, Rw = 3


>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.
>>>
>>>
>>>
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