Lili Xie

Quantitative Identification of Near‐Fault Pulse‐Like Ground Motions Based on Energy

Abstract Near‐fault pulse‐like ground motions have long been known to be capable of inducing significant seismic damages to the building structures. Reasonable classification of such ground motions has been a challenge to earthquake engineers. This study serves to propose an energy‐based approach that can be used to identify those ground motions with dominant pulses observed in the velocity time series; and time integral of the squared ground velocity is employed to represent the motion energy. For removing the influence of high‐frequency contents, the potential velocity pulse is first extracted with a pulse model. The starting and ending time points as well as period of the velocity pulse are subsequently determined by the peak‐point method. Records with peak ground velocities above 30  cm/s from a database containing more than 3600 recorded ground motions are selected and utilized to calibrate the final criterion. It is concluded that those ground motions whose dominant velocity pulses hold relative energy values of greater than 0.3 can be satisfactorily classified as pulse‐like. The proposed approach is further used to identify pulse‐like features in arbitrary orientations and pulse‐like ground motions possibly caused by forward‐directivity effects. Online Material: Tables identifying ground motions with near‐fault pulse‐like behavior.

Effect of seawater on incident plane P and SV waves at ocean bottom and engineering characteristics of offshore ground motion records off the coast of southern California, USA

The effect of seawater on vertical ground motions is studied via a theoretical method and then actual offshore ground motion records are analyzed using a statistical method. A theoretical analysis of the effect of seawater on incident plane P and SV waves at ocean bottom indicate that on one hand, the affected frequency range of vertical ground motions is prominent due to P wave resonance in the water layer if the impedance ratio between the seawater and the underlying medium is large, but it is greatly suppressed if the impedance ratio is small; on the other hand, for the ocean bottom interface model selected herein, vertical ground motions consisting of mostly P waves are more easily affected by seawater than those dominated by SV waves. The statistical analysis of engineering parameters of offshore ground motion records indicate that: (1) Under the influence of softer surface soil at the seafloor, both horizontal and vertical spectral accelerations of offshore motions are exaggerated at long period components, which leads to the peak spectral values moving to a longer period. (2) The spectral ratios (V/H) of offshore ground motions are much smaller than onshore ground motions near the P wave resonant frequencies in the water layer; and as the period becomes larger, the effect of seawater becomes smaller, which leads to a similar V/H at intermediate periods (near 2 s). These results are consistent with the conclusions of Boore and Smith (1999), but the V/H of offshore motion may be smaller than the onshore ground motions at longer periods (more than 5 s).

Dimensional analysis of earthquake-induced pounding between adjacent inelastic MDOF buildings

In this study the seismic pounding response of adjacent multi-degree-of-freedom (MDOF) buildings with bilinear inter-story resistance characteristics is investigated through dimensional analysis. The application of dimensional analysis leads to a condensed presentation of the response, and the remarkable self-similarity property for bilinear MDOF buildings with inelastic collision is uncovered. It is shown that when the response is expressed in the appropriate dimensionless form, response spectra for any intensity of the excitation collapse to a single master curve. The reduced Π set explicitly describes the interaction between the colliding structures. The effect of pounding on the MDOF building’s response is illustrated using three well-divided spectral regions (amplified, de-amplified and unaffected regions). Parametric studies are conducted to investigate the effects of the story stiffness of structures, the story stiffness ratio and mass ratio of adjacent buildings, the structural inelastic characteristics and the gap size values. Results show that (i) the influence of system stiffness ratio to the lighter and more flexible building is more significant in the first spectral region, where the maximum response of the building is amplified because of pounding; and (ii) the velocity and pounding force of the heavier and stiffer building is unexpectedly sensitive to the mass ratio of adjacent buildings.

Constant Ductility Energy Factors for the Near-Fault Pulse-Like Ground Motions

This study is focused on the constant ductility energy factors for bilinear system under the near-fault pulse-like ground motions. The variation of energy factors is studied in consideration of the earthquake magnitude, rupture distance, damping ratios, and post-yield stiffness ratios. The results indicate that the near-fault pulse-like ground motions would increase the energy dissipation of structures. The energy factors are significantly influenced by the earthquake magnitude. The damping ratios have more obvious influences on the energy factors than the post-yield stiffness ratios. A predictive model is proposed for the application of constant ductility energy factors for near-fault pulse-like ground motions.

The Inelastic Input Energy Spectra for Main Shock–Aftershock Sequences

This study investigates the input energy spectra for inelastic single-degree-of-freedom (SDOF) systems under main shock–aftershock sequences. The input energy spectra quantitatively reveal the effects of aftershocks on input energy, which verifies the necessity of incorporating aftershocks in energy-based seismic design. The investigation selects the sequences including one aftershock or two aftershocks respectively, according to the proposed criteria for selecting earthquake records. Then, the input energy for sequences is normalized by mass, m, and expressed in terms of the equivalent velocity, VE,seq. Next, the variation of VE,seq is studied in consideration of the hysteretic models, ductility values, periods of vibration, site conditions, relative intensities of aftershocks and number of aftershocks. The results indicate that the effects of aftershocks on input energy are significant in almost the whole period region. Finally, a simplified expression of input energy is proposed for incorporating aftershocks in energy-based seismic design.

Numerical Simulation of Masonry-Infilled RC Frames Using XFEM

AbstractEvaluation of the seismic performance of masonry-infilled reinforced concrete (RC) frames is challenging because a number of damage patterns can be induced by the interaction between the in...

A Probabilistic Methodology to Determine Elastic Acceleration Response Spectra for Pulse-Type Records through Multi-Resolution Analyses

This article focuses on the characterizations of pulse-type ground motions which are mainly caused by rupture directivity. Multi-resolution analysis is employed to decompose 53 typical pulse-type records which are selected from 12 large earthquakes into a series of ground motion components. A methodology for deriving design spectra, which could incorporate the special effects caused by pulse-type records is proposed based on the bi-normalized response spectra of ground motion components, and is named as Near-Fault Response Spectrum (NFRS). Analysis results indicate that NFRS might be a reliable candidate for the code-based design spectrum especially for structures constructed close to active fault.

A Simple and Quantitative Algorithm for Identifying Pulse‐Like Ground Motions Based on Zero Velocity Point Method

Pulse‐like ground motions, which are mainly concentrated in near‐fault regions, can induce severe damage on structures. This article presents a simple and effective method used for quantitatively identifying pulse‐like ground motions. First, zero velocity point method is proposed based on the properties of trigonometric functions. This method can reveal the time‐domain characteristics of ground‐motion velocity time history and can be used to detect the potential velocity pulse which is included in corresponding original ground motion. Second, pulse‐like ground motions are classified into two classes: single‐pulse‐like and multi‐pulse‐like ground motions. Further, single‐pulse‐like is divided into significant‐single‐pulse‐like and non‐significant‐single‐pulse‐like, while multi‐pulse‐like is divided into double‐pulse‐like, three‐pulse‐like, and four‐pulse‐like. Third, energy method is adopted to formulate the identification criterion for each type of pulse‐like ground motion. Finally, the energy parameters of the detected potential velocity pulse for a large number of ground motions are calculated, and the proposed identification criteria are applied to identify whether these ground motions are pulse‐like or not. A comparison of the methodology proposed in this study with previous identification methods is presented. Analysis results indicate that the methodology proposed in this study can accurately distinguish pulse‐like and non‐pulse‐like ground motions.

Consecutive combined response spectrum

Appropriate estimates of earthquake response spectrum are essential for design of new structures, or seismic safety evaluation of existing structures. This paper presents an alternative procedure to construct design spectrum from a combined normalized response spectrum (NRSC) which is obtained from pseudo-velocity spectrum with the ordinate scaled by different peak ground amplitudes (PGA, PGV, PGD) in different period regions. And a consecutive function f(T) used to normalize the ordinates is defined. Based on a comprehensive study of 220 strong ground motions recorded during recent eleven large worldwide earthquakes, the features of the NRSC are discussed and compared with the traditional normalized acceleration, velocity and displacement response spectra (NRSA, NRSV, NRSD). And the relationships between ground amplitudes are evaluated by using a weighted mean method instead of the arithmetic mean. Then the NRSC is used to define the design spectrum with given peak ground amplitudes. At last, the smooth spectrum is compared with those derived by the former approaches, and the accuracy of the proposed spectrum is tested through an analysis of the dispersion of ground motion response spectra.

Characteristics of strong ground motions in the 2014 Ms 6.5 Ludian earthquake, Yunnan, China

The 2014 Ms 6.5 (Mw6.1) Ludian earthquake occurred in the eastern Sichuan–Yunnan border region of western China. This earthquake caused much more severe engineering damage than the usual earthquakes with the same magnitude in China. The National Strong Motion Network obtained large set of ground motion recordings during the earthquake. To investigate the engineering interested characteristics of ground motion from Ludian earthquake and compare it with the Mw 7.9 Wenchuan and the Mw 6.6 Lushan earthquakes in western China, studies on the ground motion field, attenuation relationship, distance dependence of significant duration, and site amplification were carried out. Some conclusion is drawn. Specifically, the ground motion field reveals a directional feature, and the distribution characteristics of the two horizontal components are similar. The attenuation relationship for Ludian earthquake is basically consistent with the ground motion prediction equation (GMPE) for western China, except the slight smaller than the GMPE predicted at short periods. The distance dependences of ground motion duration are different in Sichuan and Yunnan regions due to the local physical dispersion and Q value. The site amplification factors are dominated by linear site response for lower reference ground motion, but the nonlinearity becomes notable for higher reference ground motion. This feature is basically consistent with the empirical model for western China. All the results indicate that the spatial distribution of ground motion, the attenuation characteristics, and the site amplification effect should be considered in characterization of near-field ground motion.