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I am designing a high-rise structure for an undergraduate project. My design theory may be slightly unconventional but I decided to use a steel EBF core with moment resisting frames near the exterior of the structure. I've been using a Ta value (from ASCE-7 12.8-2) based solely on the special eccentrically braced frame which came out to Ta = 3.144 seconds. After modeling the structure in RAM I got my fundamental period to be 4.99s. My question is whether or not I am interpreting my Ta calculation correctly because of the fact that I am using a dual system under the rigid diaphragm parameter.

On a side note, this structure is in downtown L.A. (T_L = 8 seconds) what do you think about the 1st mode period at 5 seconds when comparing it to the Ta and T_L values on the spectrum? I have yet to take an EQ engineering course so unfortunately I have to teach myself whatever I can. Suggestions are welcome. Thanks for reading!