Predominant Period Of Ground Motion Research Articles

The response law of far-field seismic ground motion of the Wenchuan earthquake and its damaging mechanism in the Loess Plateau

A series of housing collapses and other serious damage was caused by the 2008 Wenchuan MS 8.0 earthquake in the seismic intensity Ⅵ areas of the Loess Plateau, which is hundreds of kilometers away from the epicenter, and which showed a remarkable seismic intensity anomaly. The seismic disasters are closely related to the seismic response characteristics of the site, therefore, the systematic study of the far-field seismic response law of the Wenchuan earthquake in the Loess Plateau is of great significance to prevent the far-field disaster of great earthquake. In this paper, the seismic acceleration records of several bedrock stations and loess stations from the seismogenic fault of the Wenchuan earthquake to the Loess Plateau were collected, and the attenuation law of ground motion along the propagation path and the characteristics of seismic response on the loess site are studied, and the mechanism of amplification effect of ground motion is analyzed based on the dynamic feature parameters of the loess site obtained through the HVSR method. Taking a typical loess site of thick deposit as the prototype, a series of shaking table tests of dynamic response of loess site models with different thicknesses were carried out. Amplification effect, spectral characteristics of acceleration in model sites were analyzed under the action of a far-field seismic wave of the Wenchuan earthquake. The results show that seismic attenuation on the propagation path along the NE strike of the seismogenic fault to the Loess Plateau is slower than that in other directions, and the predominant period range of ground motion on bedrock site of the Loess Plateau presents broadband characteristics. Because the natural periods of loess sites with thick deposits are within the predominant period range of bedrock input wave, loess sites appear significant amplification effect of ground motion, the horizontal acceleration of ground motion exceeds 0.1 ​g, the seismic intensity reaches 7°. The thicker the loess deposit is, the more significant the change of spectral characteristics of ground motion on loess sites, and the narrower the predominant period range of ground motion becomes, and the closer it is to the natural period of loess sites. Therefore, for some old houses on thick loess sites, the poor seismic performance and strong seismic response eventually led to their collapses and damages because their natural periods are very close to the predominant period of ground motion of the Wenchuan earthquake on thick loess sites; For these damaged high-rise buildings, the resonance effect might be the main reason for their damages because their natural periods are included in the predominant period range of ground motion of the Wenchuan earthquake on thick loess sites.These research results would provide a basis for seismic disasters prediction and evaluation and seismic design of construction engineering in the Loess Plateau.

Seismic performance of nonlinear soil-structure systems located on soft soil considering foundation uplifting and soil yielding

In most of researches on soil-structure systems, it is assumed that the foundation is bonded to the ground where no foundation uplift is allowed. Uplifting makes changes in force–displacement behavior of the soil-structure systems, which in turn alters structural demands. In this research, a set of non-dimensional parameters is considered which controls the behavior of uplifting systems. The effects of foundation uplift on response of soil-structure system are investigated parametrically through time history analysis for a wide range of systems subjected to harmonic excitation and also ground motions recorded on soft soil with predominant period. It is seen that the response of systems with more dominant soil-structure interaction effects will influenced more by foundation uplift. It is shown that the period of soil-structure systems to the predominant period of ground motion has an important rule on the response of uplifting systems. While this period ratio is greater than one, the total displacement of uplifting soil structure systems almost equals to elastic soil-structure system when uplifting is prevented but the drift of the structure as a part of soil-structure system while uplifting is permitted decreases in comparison with corresponding systems which foundation uplift is prevented. If the period ratio becomes smaller than one, systems which are allowed to uplift experience larger displacements in comparisons with corresponding case which foundation uplift is prevented. It is shown that neglecting the influence of foundation uplifting in seismic performance assessment procedure of stiff and squat structures located on soft soil is not conservative, because in this situation structures can experience larger drift due to foundation uplift in comparison with the case that uplifting is prevented while subjected to excitation with long predominant period. Also for nonlinear soil-structure systems it is shown that foundation uplifting and soil yielding can reduce the ductility of the structure as a part of soil structure system. It is observed that traditional structures with longer period of vibration which are designed based on conventional spectra will be influenced less by foundation uplifting and soil yielding.

Inelastic displacement ratio for high damping SDOF systems built on soft soil sites

The main purpose of this study is to estimate the inelastic displacement ratio, C1, of highly damped systems built on soft soil sites. For this purpose, results of linear and nonlinear time history analysis of 79 single-degree-of-freedom (SDOF) systems with damping ratios of 0.05, 0.1, 0.2, 0.3 and post-yield stiffness ratios of -0.01, 0, 0.05 and 0.1 under a large number of ground motion records obtained at soft soil deposits, were used. Oscillators exhibited ductility levels as high as 6. Based on the results, a new simplified equation as a function of the period of vibration normalized by the predominant period of ground motion records, the level of inelastic deformation, post-yield stiffness ratio and damping ratio for structures built on soft soil, has been proposed using nonlinear regression analysis.

Dynamic Response of a Combined Isolation Based Mega-Substructure under Bidirectional Near-Fault Ground Motions

A typical megaframe structure has a high lateral stiffness and is excellent for high-rise structures. However, this high stiffness can lead to poor seismic response of a structure. Seismic isolation technology is a mature and cheap vibration control method that is used for vibration reduction in megaframes. This paper introduces a megaframe structure based on substructure combined isolation. The structure consists of two parts. The main body is a megaframe, and the substructure is the subframe with the combined isolation layer arranged at the bottom of the subframe. The seismic performance of this structure system was evaluated by performing shaking table tests of two megaframe model structures. The responses of the deformation, acceleration, and shear of the structure were measured. The dynamic behaviors of the structure with or without the combined isolation layer when exposed to single and bidirectional near-fault and far-fault ground motions with different peak values were investigated. The results showed that the combined isolation layer can reduce the bidirectional seismic response of the main frame and subframe. The acceleration, base shear, and displacement responses had similar vibration reduction trends for the two model structures, and the structural responses under bidirectional earthquake were generally greater than that under a single directional earthquake. The near-fault pulse effect increased the seismic response of the structure. The increase of the predominant period of ground motion also increased the seismic response of the structure.

Analysis of seismic collapse mechanism of concrete frame structures subjected to near-fault ground motions

The characteristics of near-fault ground motions are significantly different from far-field ground motions. There are also great differences in the damage of concrete structures caused by these two kinds of ground motions. Earthquake damage indicates that the damage of concrete frame structures subjected to near-fault ground motions is more serious and collapse mechanism is more complicated. In this paper, the traditional alternative path method (APM) in the analysis of collapse was improved. Collapse mechanism of reinforced concrete frame was studied by numerical simulation method. A typical ten-story reinforced concrete frame model was established. Three harmonic waves and three practical near-fault ground motions were selected as acceleration time history input. The results show that different damaged parts of structure caused by strong earthquake have little effect on the final collapse mode when the structure is under free vibration. The failure mode of columns subjected to different ground motions is almost the same. Simple harmonic excitation whose period is close to the predominant period of ground motion can approximately simulate the failure mode of structure caused by earthquake.

Inelastic displacement ratios of fully self-centering controlled rocking systems subjected to near-source pulse-like ground motions

Modern self-centering controlled rocking (SC-CR) systems are capable of reducing residual drift after severe earthquakes by swaying on their bases and concentrating damage in energy dissipation devices. The present research determines constant-strength (CR) spectra for SC-CR systems subjected to far-field and near-field pulse-like (NF-pulse) ground motions. SC-CR systems are simulated as single degree of freedom models with a flag shape behavior. Nonlinear time-history analyses are conducted using 91 NF-pulse and 44 far-field records to derive the CR spectra. Each point of CR spectrum of a SC-CR system is the ratio of maximum inelastic to maximum elastic displacement. An extensive statistical study is carried out to examine the effects of ground motion and modeling parameters on the CR spectra. It is found that the influences of pulse period, predominant period of ground motion, and modeling parameters on inelastic displacement ratios (CR ratios) of SC-CR are significant, while other effects such as the source-to-site distance, earthquake magnitude, and site classes are not as much important. Finally, using two-stage regression analyses, formulas are developed for determining un-normalized (code-compliant) and normalized CR spectra of SC-CR systems subjected to far-field and NF-pulse ground motions.

Bound Direct Displacement-Based Seismic Design of Buildings

Limitations of probabilistic model appear when information is insufficient. Convex set model, which much less information is needed, is employed to model uncertainties of target displacement, peak acceleration and predominant period of ground motion. A new bound direct displacement-based seismic design method is proposed. The interval of seismic design results is obtained by considering the uncertainties of design variables and the relative risk of any design scheme within the interval is also simultaneously provided. The results of a four-storey RC building show that the base shear and beam sectional area by Xue's design scheme are close to the lower bound of the bound design method, and the column sectional area and the weight of steel bar are just over the middle value to the upper bound. The sensitivity analysis indicates that the base shear is more sensitive to target displacement.

Correlation of Site Characteristic Period with Predominant Period of Ground Acceleration by a Simplified Model

Site characteristic period is an important index in seismic zonantion and seismic resistance design. The characteristics of the ground motions are conceptually related with site characteristics. A simplified single-layer model is used for simulating horizontal strata under seismic loading to calibrate the correlation of site characteristic period with the predominant period of ground motion. The analytical results show that the inputted motions were amplified considerably by the site typically for the period components which were close to the site characteristic periods; with the shear wave velocity of the soil layer decreases with respect to the initial shear wave velocity, the predominant period of ground motion increases; the site characteristic periods are consistent with the predominant periods of the ground motion.

Seismic vulnerability assessment of multi-degree-of-freedom systems based on total input energy and momentary input energy responses

In energy-based seismic evaluation and design, input energy as a seismic demand parameter and hysteretic energy are two important factors. Previous studies for seismic assessment and design based on energy approach have been generally limited to single-degree-of-freedom (SDOF) structures. The purpose of this paper is to find a suitable energy-based parameter for estimation of the damaging potential of ground motion in reinforced concrete frames of multi-degree-of-freedom (MDOF) systems. In this regard, 40 common reinforced concrete frames subjected to four different earthquakes have been analyzed. The results indicate that maximum momentary input energy is a more appropriate parameter than maximum total input energy for estimation of structural damage in short-duration earthquakes or those in which the major damage to structures happens within a short duration of ground motion. In addition, in earthquakes with a wide frequency range, the predominant period of ground motion in Fourier spectrum is shown to be the period corresponding to maximum input energy.

BRACED FRAMES WITH HYSTERETIC DISSIPATIVE DEVICES: SEISMIC RESPONSE AND DESIGN CRITERIA

The seismic behaviour of elastic braced frames with hysteretic dissipative devices is investigated. The analysis is carried out using a simplified single degree of freedom model that is deduced on the basis of an equivalence criterion between one-storey and multi-storey braced frames. The criterion allows one to characterise the structure by means of a few global parameters, such as the fundamental period of vibration, the ratio between the fundamental period and that of the correspondent unbraced frame, and the behaviour factor. The combined effect of design values of structural parameters and the energy frequency content of the input is stressed by evaluating the power spectral density of the stationary phase of the response. Consistent energy, displacement and strength response spectra are shown. The results elucidate the different behaviour of short and medium period structures with respect to the predominant period of ground motion. For each of the two classes, design criteria and guidelines for choosing the optimal values of the structural parameters are summarised. Four design examples are presented in order to demonstrate the effectiveness of the proposed criteria.