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Seismic Design Manual, Volume 3

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Regarding the following message:

I recently was asked a question in regard to braced frame connection design
per CBC 2213.9.3.1.  The question was in regard to whether the "design
seismic force" in item 2 is E/1.4 per the load combinations in section
1612.3 for allowable stress design.  I had always thought this to be the
correct procedure if using section 2213; however, it was pointed out to me
that the SEAOC Seismic Design Manual, Volume 3 contains an example of a
braced frame design in which it appears that the 1.4 factor is omitted.  The
example is Design Example 1A, Special Concentric Braced Frame, step 6a.  Is
this an error, or is there some reason that the seismic design force for
brace connections should not be reduced by the 1.4 factor?

-Karen Casano

Response:
The SDM is inherently using a 1.7 factor to convert from nominal strength forces
into ASD forces.  It's jsut that all of the calculations are done at the
"nominal strength" level.  Step 6a calculates the nominal strength of bracing
connections that is required.  Later step 6d where the connection is designed
includes a multiplier of 1.7 throughout sections 6d and 6e to calculate the
nominal strength of components related to the connection.  On page 60, the
problem has an unfortunate mis-use of words where the problem states; "For E70
electrodes the allowable strength is:  Fw = 1.7(.3)(70 ksi) = 35.7 ksi.  Of
course, the ASD "allowable strength" is only 0.3 x 70 ksi = 21 ksi.  What the
problem is presenting is actually the "nominal strength" of the welds, not the
"allowable strength".

regards,
Martin