Authors: Gopala Krishna Rodda; Dhiman Basu
Addresses: Department of Civil Engineering, Indian Institute of Technology Gandhinagar, Gandhinagar-382355, Gujarat, India ' Department of Civil Engineering, Indian Institute of Technology Gandhinagar, Gandhinagar-382355, Gujarat, India
Abstract: Rotational ground motions are not being measured by the accelerographs deployed in the free-field, but rather extracted from the recorded three-component translation data, usually at one single station (single station procedure, SSP). These SSPs differ from one another in terms of approximating the underlying wave propagation scenario in the field. Increasing order of complexity does not always guarantee better product and may lead to unrealistic results because of the assumptions of compatibility. This paper aims to develop a simplified procedure for estimating rotational motion that may compete with far more rigorous approaches. The proposed procedure makes use of the existence of appropriate translational components (ATCs) and time derivatives that lead to the rotational motion with due scaling through apparent wave velocity. First, a spectral window is proposed and a set of invariant characteristics are identified (with respect to the limited studies carried out in this paper). Next, an empirical rotational window is derived that enables computation of the apparent wave velocity. Three seismic events recorded in a dense array comprising 15 surface-stations are considered for assessing the proposed simplified method.
Keywords: torsional ground motion; rocking ground motion; spectral window; cut-off frequency; single station procedure; SSP; wave propagation; rotational ground motion; rotational window; rotational motion; appropriate translational components; ATCs; time derivatives; wave velocity; earthquakes; seismic design; seismic arrays.
International Journal of Earthquake and Impact Engineering, 2016 Vol.1 No.3, pp.253 - 288
Received: 09 Jun 2016
Accepted: 17 Jul 2016
Published online: 19 Jan 2017 *