Rotational Seismic Motion And Torsion Design

It has been validated that rotational ground motion is a factor in structural damage from earthquakes. However, it is not considered directly due to a lack of recorded rotational ground motion, knowledge of the rotational wave field and the code for structure design. This paper studies the method of simulate rotational ground motion, the characters of rotational ground motion and the method of torsion design of structures. The study is summarized as follows.(1)To verify the simulation methods of rotational earthquake ground motion, experiments were performed. Blast the rocks underground and get correlative translational ground motion. With dynamic wave method and two-points difference method, rotational ground motions were calculated. They are similar. It shows that the dynamic wave method to simulate rotational ground motion is effective.(2)The apparent velocity of the incident wave is one of the key parameters in the dynamic wave method to simulate rotational ground motion, but it is difficult to be estimated effectively when there is only a single record. This paper discusses a P-SV ratio method based on elastodynamic theory in a multi-layer isotropic elastic half space. The factors that impact the apparent velocity and equivalent incident angle are considered. The apparent velocity of the incident body wave is not obviously dependent on the site conditions and epicenter distance. The effect of the velocity structure to the surface wave dispersion is discussed based on the stiffness matrix theory. It shows that to assume the velocity to be equal to the S wave velocity of the top layer may greatly overestimate the low frequency rotational seismic motion.(3) To estimate the torsion input of structures caused by earthquake, a simplified method is presented. The torsional ground motion, the torsional foundation input and their differences are discussed. It shows that the ratio of foundation length to the apparent earthquake wavelength and the ratio of foundation length to width affect the difference. A formula of torsion earthquake input of structure is suggested for structure engineering purpose.(4) The rotational seismic response spectrum for structural seismic design is presented. The rotational seismic response spectrum can be established by modify the translational seismic response spectrum. The modify parameters that related to the apparent velocity of the incident wave, the ratio of the max derivative of translational motion and the max translational motion and the ratio of the normalized rotational seismic response spectrum and the normalized translational seismic response spectrum, are suggested.(5)Study on the intrinsic cause to make structures torsion is performed. The results show that the modal torsional component is a good criterion to justify the torsin resist ability of structures. The torsional response of structures under torsion seismic action is not sensitive to the symmetry and eccentricity. The design accidental eccentricity is discussed.