ABSTRACT
For ordinary bridges, a dynamic modal response spectrum analysis is usually performed under the design earthquake. e modal responses are combined using the complete quadratic combination (CQC) method. e resulting orthogonal responses are then combined using the “30 Percent Rule.” Two cases are considered when combining orthogonal seismic forces. Case 1 is the sum of forces due to transverse loading plus 30% of forces due to longitudinal loading. Case 2 is the sum of forces due to longitudinal loading plus 30% of forces due to transverse loading.
