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<br /> <br /> <br /> <br /> <br /> <br />3rd & Broadway, Santa Ana, CA <br />Volume 2 - Structural Calculations June 17, 2024 | Project # S21043.00 <br /> pg. 27 <br />4.2.3 DETERMINATION OF THE MODAL PERIODS OF STRUCTURE <br />The analytical model is subjected to a response spectrum which has been generated according to the <br />building code. The analysis proceeds in two stages. The first is a P -Delta analysis which determines the <br />effective stiffness of the structure when loaded. The second stage is a dynamic lateral analysis Ritz <br />Vectors to determine the modal periods which are then used in combination with the response spectra to <br />determine the base shear. This base shear is then scaled to 100% of the static earthquake forces in <br />order to find the final DBE-level response spectrum base shear and member forces. A minimum of <br />approximately 90 percent mass participation is included in each principal orthogonal direction of the <br />structural response. <br />A summary of the primary modes for the DBE analysis are included in the tables below and in the <br />following figures. <br /> <br />