Abstract Summary
As the increasing requirement in nonlinear time history analysis of structures under bi-directional ground motions, the demand for the employment of RotD100 response spectrum-compatible bi-directional ground motions also increases. However, only spectrum-compatible bi-directional ground motions cannot guarantee a correct evaluation of the structural seismic response, but suitable seismic inputs. The orientation angle (OA) of bi-directional ground motions is a new concept and affects the nonlinear response of structures. And it will be preferable in the seismic design that if the concerned feature of bi-directional ground motions can be specified in a controllable manner. Thus, firstly, a new algorithm is proposed for generating RotD100 response spectrum-compatible bi-directional ground motions that OA remains constant regardless of natural period. This algorithm is based on the complex-discrete-Fourier-transform (CDFT), which takes the advantage of controlling the azimuth angle (AA) of ellipse at each natural periods in the frequency domain. The proposed algorithm is validated using a target RotD100 response spectrum and a suite of bi-directional accelerograms. It is shown that the performance of the proposed algorithm is satisfactory and its application is straightforward in the relative fields of earthquake engineering. In the second part, the strong motion characteristics of the RotD100 spectrum-compatible bi-directional ground motions using the proposed method are compared with a suite of bi-directional ground motions that were directly matched to be spectrum-compatible using a conventional method by modifying two horizontal components simultaneously. In the third part, the effect of the OA property of bi-directional ground motions on the nonlinear response of bridge piers involving a progressive damage process is investigated. As a representative example, idealized single column steel bridge piers with different geometric configurations were constructed using fiber model in OpenSees. The property of steel was modelled by a bi-linear hysteretic model with kinematic hardening and isotropic hardening and a modified rainflow cycle counter has been implemented to account for the effects of low cycle fatigue. Incremental Dynamic Analysis (IDA) was conducted with the two suites of bi-directional ground motions applied to the piers through 360° at an angle increment of 9°. The Intensity Measure (IM) was defined as the peak ground acceleration (PGA). The structural response differences under the two suites of bi-directional ground motions were discussed in hysteresis curves, IDA curves and the fragility curves, etc. It is shown that the difference in seismic demanding between the two suites bi-directional ground motions becomes significant as PGA increases. The seismic demands of bi-directional ground motions with OA remains constant is more seismic demanding when the piers are subjected to strong earthquakes (i.e., PGA is high). This sheds light on the importance of a reasonable suite of bi-directional ground motions in designing new bridges or retrofitting old bridges.
