Dear @Muhammad.Irfan,
A high-level explanation of the calculation ElastoDyn uses to calculate tower-base loads is provided in my post dated Sep 5, 2014 in the following forum topic: Tower Base loads. For the full details behind ElastoDyn, find the theory documentation at: 4.2.7. ElastoDyn Users Guide and Theory Manual — OpenFAST v3.5.3 documentation.
Best regards,