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Re: Site specific response spectra

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Chris Palmateer wrote:

>Due to the occupancy of the structure, a site specific analysis was
>performed by the geotechnical engineer...for short period structures,
>the accelerations are greater than the normalized response spectra
>given in Fig. 16-3 of the UBC...we are now designing the "structure"
>for this increased base shear...Do "elements" of the structure also
>need to be scaled up by the same factor?

I'll give you two answers:

1. Since the UBC allows you to "scale down" the results to the code required
base shear, as far as I can see there is no code requirement to "scale up"
element forces.  The sole purpose of the site specific response spectrum in
this case appears to be to determine a more accurate distribution of the
shear within the structure.

2. However, if the intent of obtaining a site specific response spectrum is
design for the potential of higher than code prescribed forces,
of code values for elements should be considered.  Since you state that you
are designing for increased base shear, it appears that it is your intent to
design for forces beyond code prescribed forces.  (This is not uncommon for
critical structures.)  In this case, I would consider the code element
as minimum forces, and I would calculate the acceleration at each level of
structure from a dynamic analysis using the site specific response spectrum.

Then I would base the acceleration of each element on these results.  (I
divide the calculated acceleration by the R-factor for design.)

Hope this helps.