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Re: 1997 AISC seismic - IBC seismic design category D - OBF brace connections - column strength

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Sometimes a clue can be found in an unlikely place, Chapter 18 of the 1997
UBC.

Section 1809.2 of the 1997 UBC says that in Seismic Zones 3 and 4, "The
foundation shall be capable of transmitting the design base shear and
overturning forces prescribed in Section 1630 from the structure into the
supporting soil...."  I interpret this to mean design the foundation
without omega-sub-zero or 1.5 chevron brace factor.

Section 1809.3 of the 1997 UBC says that in Seismic Zones 3 and 4, "The
connection of superstructure elements to the foundation shall be adequate
to transmit to the foundation the forces for which the elements were
required to be designed."  I interpret that to mean design the brace end
connection for the same forces as the brace (omega-sub-zero, 1.5 chevron
brace factor, whatever).

I think that we area long way from applying strength concepts like
omega-sub-zero to foundation design.  The geotechnical engineers are still
accustomed to giving us soil data in allowable stress format.  Another
issue to be resolved is energy dissipation through foundation rocking, and
its effect on inelastic structural response.  This could take the effect of
modifying omega-sub-zero and/or R, depending on foundation characteristics.

Rick Drake, SE
Fluor Daniel, Aliso Viejo, CA

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                    Jake Watson                                                                                    
                    <jwatson@inconn      To:     seaint(--nospam--at)seaint.org                                                 
                    ect.com>                                                                                       
                    05/02/01 06:51       cc:                                                                       
                    PM                                                                                             
                    Please respond       Subject:     Re: 1997 AISC seismic - IBC seismic design category D - OBF  
                    to seaint              brace connections - column strength                                     
                                                                                                                   
                                                                                                                 . 
                                                                                                                   




As long as we are in this debate, I would like to re-open an old can of
worms.  Where exactly does the Omega level force end for braced frame
design (1997 UBC)?  I would like to ask this from two perspectives,
first from the legal point of view and secondly from a philosophical
point of view.  Does the footing need to be designed for Omega? Do the
anchor bolts need to be designed to Omega?  Does the base plate need to
be designed to Omega?...  There are several hints by various people and
papers, but I haven't found a definitive answer yet.
--
Jake Watson, E.I.T.
Salt Lake City, UT

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