As you know better ,ASCE -7 SEC. 12.4.4 says:
“In Seismic Design Categories D, E, and F, horizontal cantilevers are designed for an upward force that results from an effective vertical acceleration of 1.2 times gravity. This design requirement is meant to provide some minimum strength in the upward direction and to account for possible dynamic amplification of vertical ground motions resulting from the vertical flexibility of the cantilever. The requirement is not applied to downward forces on cantilevers, for which the typical load combinations are used.”
It is supposed that vertical acceleration is 1.20 g , my question is :
if this acceleration is upward and downward so why does not ASCE have this combo.
UP >>>>> 1.20g -1.0g = 0.20g
DOWN >>>>> -1.20g -1.0g = -2.20g
The codes in general decide load cases based on worst case scenarios, hence the worst case here is to design the cantilever for a direction of loading opposite to the gravity and when combining it with the dead load it is not combined with the full dead load it is combined with a ratio of it (0.9 or 0.8) depending on which code as here this dead load is acting as a restoring force that needs to be minimized to get the worst load case in that direction.
please check the link
https://law.resource.org/pub/us/cfr/ibr/003/asce.7.2002.pdf
https://law.resource.org/pub/us/cfr/ibr/003/asce.7.2002.pd
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