Building consists of a core tower say 130 feet by 155 feet - 5 stories. At the Main Level (1st floor), the building footprint is larger. Say 30 feet southward and 30 feet eastward from the core. From the main level, we have lower levels called ground, basement, mechanical 2 and mech 1 levels. Mech 1 level being the lowest level. Building sits on a slope - slope goes downwards in the southeasterly direction.
For simplicity, let's say the lateral system is concrete shear walls full height. Diaphragm is composite metal deck.
At the main level, under seismic loading, there will be a shear transfer through the diaphragm extension.
1994 UBC requires the seismic forces be increased by 3Rw/8 if supporting a discontinuous lateral system sych as columns supporting shear wall above or for transfer diaphragm that transfer seismic shear from lateral elements that are offset planwise.
My questions is: Does the diaphragm have to be designed for 3Rw/8 for the main level for my building? Due to the large rigidity of the perimeter walls, significant amount of shear will be transfered thru the 30ft diaphragm extension and forces in the perimeter of the core will "unload". My concrete shear walls at the cores are continuous all the way to the Mech 1 level thus there is no discontinuity but perimeter walls being so rigid, it will behave like a plan irregularity #4.
Anyone who can still remember the 1994 UBC may have some input. My gut feeling is to use 3Rw/8, regardless. Thank you all in advance.