vertical acceleration - modal analysis
 

vertical acceleration - modal analysis
folks,
i have a question pertaining to the vertical accelerations per ubc 97.
in section 1631.2 - 5, it states that vertical accelerations can be defined by scaling the horizontal accelerations by 2/3.
in section 1630.1.1, a ev factor is used to include the vertical effects.
my question is thus:
i have performed a modal analysis and need to scale my base shears to 90% of the static base shear.
can this be done in any one of the options i list below?
option 1. compute e using equation 30-1 including the effects of vertical acceleration. scale the response spectrum results to 90% of the above computed value in both directions and not use section 1631.2-5
option 2. compute e using equation 30-1 but without the vertical effects. scale the response spectrum to the static values in x and y direction, and use a 2/3 factor on these scale factors for vertical excitation. etabs for example has a uz excitation component. i am plugging the 2/3 horizontal value into this input box for each direction separately.
please let me know if this approach is right.
thanks