Abstract: Dagu bridge, at Tanjin, is a tied arch bridge with asymmetric and inclined steel box arches and a steel box girder. Due to its structural complexity, the current Chinese design code is incapable of aseismic assessment. With reference to the new versions of foreign design specifications, appropriate design response spectra in three directions and ground acceleration time histories with three components are given. A 3D analytical model including geometric stiffness of the arch and the secant modulus of the hangers is established. The effective modal participation mass is used to determine the minimum number of modes for mode combination of response spectrum analysis and the reasonable time step for the step-by-step integration analysis. It is found that the CQC combination should be used in the response spectrum analysis. The time histories of three groups of simultaneous three-component ground accelerations are used as input for nonlinear response analysis by SAP 2000, ANSYS and an in-house program BER 2002. It is shown that the bridge possesses satisfactory aseismic behavior.