PSI - Issue 44

A.Castellani / Structural Integrity Procedia 00 (2022) 000–000

20 © 2022 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license ( https://creativecommons.org/licenses/by-nc-nd/4.0 ) Peer-review under responsibility of the scientific committee of the XIX ANIDIS Conference, Seismic Engineering in Italy Keywords: Structural design of buildings; Rocking ground spectra; Seismic hazard; Site effects; Rotational seismology; Plane wave approximation; Seismic wave propagation. 1. Introduction Takeo (1998), indicates that rotational ground motions may be many times larger than expected from classical elasticity theory based on a propagation model. Igel et Al (2007), in the introduction to the BSSA special issue on rotational seismology and engineering applications, include some background information, and, in particular, a summary of the events that led to the special issue. The review of 51 papers, and 11 short notes, is reported. Direct observations are published by Smerzini and Paolucci (2008). They evaluate earthquake induced transient ground strain, basing on records obtained on the near-field by two DAIs, at Parkway Valley New Zealand, and UPSAR California. They point out the limitations of the simplified evaluations used in engineering practice, which uniquely consider the wave passage effects on the ground strain. About the peak acceleration dispersion, a similar result as that above mentioned for the DAIs, is hint by observation of some structural damage, following an earthquake. The quake of 2009, Mw 6.3 L’Aquila event, caused extensive damage in L’Aquila town and along the Aterno river valley, in two small towns, Onna and Monticchio, Di Giulio et Al, (2010). Although building vulnerability and near-source effects are strongly responsible for the high level of destruction, site effects are invoked to explain the damage heterogeneities. The local ground frequency resonance is varying from 2 to 3 Hz and it was ascribed to the alluvial sediments with a thickness of about 40 meters that overlay a stiffer Pleistocene substrate. Research on the local variability may thus collect complementary information from the structural damage, and from that remarked in the surface of the DAIs. Other measurements are reported by Spudich et Al (1995), and in particular by Liu et Al (2009). The letters present data concerning 52 earthquake records collected at a single station in Taiwan, Fig. 1. Data are in function of the ratio PRV/PGA. PRV is the peak rotation velocity, and PGA is the maximum acceleration along the two horizontal components. In practice, it is a measure of rotation of the soil motion, after a normalization with respect to intensity. The ratio is shown in function of the distance from epicentre. The same ratio, computed according to DAIs, is in good agreement with the average value of these data, as it is shown in fig. 2. Statistical estimate from the set of data of fig. 1 identifies a modicum dependence of the ratio on the distance from epicentre. Meanwhile, no one record is referring to a distance equal or less to the focal depth, and only 18 to a distance less than 1.5 the focal depth. Nevertheless, this amount of data, which is the richest so far available, provide a confirmation of the recurrent conjecture that in the near field rotations might be higher, for the same peak horizontal acceleration, Kawakami and Sharma (1999). The papers mention much higher amplitudes for the two horizontal components of the acceleration than that for the vertical component, everywhere mention is explicit. This circumstance is generally characteristic of medium to far regions. A direct observation of ground rotations caused by the M = 6.0, 2004 Parkfield earthquake and aftershocks, has been published by Spudich and Fletcher, (2008). The paper is mainly devoted to the mathematical process to ascertain reliable values for rotations. Results are not in a format suitable for a direct comparison to those of Fig.1, however the order of magnitude of rotation rate is respected. The DAI provide only surface motion. Accordingly, rotations measured through this evaluation do not take into account the derivative of quantities with respect to the vertical axis. The evaluations reported in the following, should be referred to as rotations in a two dimensions’ representation . (See for instance, Flugge Handbook, chapter 32, where effectively such evaluations are mentioned as two-dimensions’ representation .) However, the order of magnitude of rotation rate is respected. See Fig.1 and 2. Alberto Castellani et al. / Procedia Structural Integrity 44 (2023) 19–26