Seismic responses of high-flexibility structures under horizontal–rotational ground motions

Experimental and theoretical studies were conducted to investigate the seismic responses of high-flexibility structures to coupled horizontal–rotational ground motions. Rotational ground motions were simulated based on the translational ground motion measured using a large-scale seismic test (LSST) dense array. The amplitude of rotational ground motions decreases with increasing distance while increases with increasing earthquake magnitude and decreasing hypocenter depth. Taking a television tower as the prototype, shaking table tests were performed on the scaled model of such a high-flexibility structure under horizontal, rocking, and torsional ground motions, as well as coupled horizontal–rocking, horizontal–torsional, and horizontal–rocking–torsional ground motions. The dynamic equations of the high-flexibility structure under the coupled horizontal–rotational ground motions were deduced. In addition, the additional second-order effect formed due to the angular displacement of foundation was added in the form of the equivalent horizontal and lateral forces to the excitation term of the dynamic equations. Results show that the theoretical analysis results conform to the experimental results, thus verifying the correctness of the theoretical analysis. The torsional and rocking components of ground motions are non-negligible in the seismic response of the high-flexibility structure. Therein, the torsion angle under the torsional component is far larger than those under horizontal and rocking components. The additional second-order effect generated due to angular displacement under the rocking ground motion enhances the dynamic response of the high-flexibility structure, which leads to a significant asymmetric effect and thus increases horizontal displacement of the structure. The influences of the torsional and rocking components of ground motions should be considered in seismic design.