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RE: WOOD MOMENT FRAMES, Rw = 3

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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.
>
>
>