Strong Motion Duration Research Articles

A parametric study of strength reduction factors for elasto-plastic oscillators

Strength reduction factors (SRFs) continue to play a key role in obtaining design forces from elastic design spectra (via response modification factors) in ductility-based earthquake-resistant design. Despite several years of sustained research efforts, it has not been conclusively shown how SRF for a given singledegree-of-freedom structural system depends on various source and site parameters. A parametric study is carried out here for the explicit dependence of SRF spectrum (describing variation of SRF with system period for a given ductility demand) on strong motion duration, earthquake magnitude, geological site conditions, and epicentral distance in case of (non-degrading) elasto-plastic oscillators. For this, scaled response spectra are considered for different combinations of earthquake magnitude, site conditions and epicentral distance, and SRF spectra are generated from 1274 accelerograms recorded in western USA after making those compatible with each of these spectra. It is shown that there is no clear and significant dependence of SRF spectrum on strong motion duration. Further, the parametric dependence on earthquake magnitude, site conditions, and epicentral distance broadly conforms to the trends reported by earlier investigations. In particular, this study confirms that the dependence of SRF spectra on earthquake magnitude should not be ignored.

A State-of-Knowledge Review of the Influence of Strong-Motion Duration on Structural Damage

The important role played by the duration of ground shaking in the response of saturated soil deposits is universally acknowledged, but no such consensus exists regarding the degree of influence that duration exerts on structural damage. There are several hundred papers in the literature that link structural damage to parameters related either directly or indirectly to the duration of strong ground motion. The conclusions of these studies differ widely with regard to the influence of strong-motion duration on structural demand. This paper provides a summary and critical review of the literature on this subject. It is found that studies employing damage measures related to cumulative energy usually find a positive correlation between strong-motion duration and structural damage, while studies employing damage measures using maximum response generally do not find strong correlations between duration and damage.

Displacements, damage measures and response spectra obtained from a synthetic accelerogram processed by causal and acausal Butterworth filters

The aim of this study is to investigate the reliability of strong motion records processed by causal and acausal Butterworth filters in comparison to the results obtained from a synthetic accelerogram. For this purpose, the fault parallel component of the Bolu record of the Duzce earthquake is modeled with a sum of exponentially damped sinusoidal components. Noise-free velocities and displacements are then obtained by analytically integrating the synthetic acceleration model. The analytical velocity and displacement signals are used as a standard with which to judge the validity of the signals obtained by filtering with causal and acausal filters and numerically integrating the acceleration model. The results show that the acausal filters are clearly preferable to the causal filters due to the fact that the response spectra obtained from the acausal filters match the spectra obtained from the simulated accelerogram better than that obtained by causal filters. The response spectra are independent from the order of the filters and from the method of integration (whether analytical integration after a spline fit to the synthetic accelerogram or the trapezoidal rule). The response spectra are sensitive to the chosen corner frequency of both the causal and the acausal filters and also to the inclusion of the pads. Accurate prediction of the static residual displacement (SRD) is very important for structures traversing faults in the near-fault regions. The greatest adverse effect of the high pass filters is their removal of the SRD. However, the noise-free displacements obtained by double integrating the synthetic accelerogram analytically preserve the SRD. It is thus apparent that conventional high pass filters should not be used for processing near-fault strong-motion records although they can be reliably used for far-fault records if applied acausally. The ground motion parameters such as ARIAS intensity, HUSID plots, Housner spectral intensity and the duration of strong-motion are found to be insensitive to the causality of filters.

지반 운동과 구조물 특성에 따른 구조물의 에너지 요구량

에너지 설계법은 지진에 의해 누적된 손상과 구조물의 이력거동에 의한 영향을 직접적으로 고려할 수 있기 때문에 현행 내진설계 기준보다 더 합리적이다. 그러나 지반운동과 구조물 특성에 따른 에너지 응답에 대한 관련 연구자들의 합의가 아직 도출되지 않고 있다. 따라서 본 연구에서는 에너지 요구에 대한 지진하중과 구조물 특성의 영향을 다른 지반조건에서 계측된 100개의 지진기록을 이용하여 평가하고 기존 연구결과와 비교하였다. 해석 결과에 따르면 연성비와 지반조건은 입력에너지에 상당한 영향을 주는 것으로 나타났다. 입력에너지에 대한 이력에너지비는 연성비, 감쇠비와 강한 지진파의 지속시간에 많은 영향을 받았지만 지반조건에 따른 변화는 작았다. The energy-based seismic design method Is more rational in comparison with current seismic design code in that it can directly account for the effects of cumulative damage by earthquake and hysteretic behavior of the structure. However there are research results that don't reach a consensus depending on the ground motion characteristic and structural properties. For that reason in this study the influences of ground motion characteristics and structural properties on energy demands were evaluated using 100 earthquake ground motions recorded in different soil conditions, and the results obtained were compared with those of previous works. Results show that ductility ratios and sue conditions have significant influence on input energy. The results show that the ratio of hysteretic to input energy is considerably influenced by the ductility ratio, damping ratio, and strong motion duration, while the effect of site condition is insignificant.

Scaling of strength reduction factors for degrading elasto-plastic oscillators

The inelastic (design) spectra characterizing a seismic hazard are generally obtained by the scaling-down of the elastic (design) spectra via a set of response modification factors. The component of these factors, which accounts for the ductility demand ratio, is known as the strength reduction factor (SRF), and the variation of this factor with initial period of the oscillator is called an SRF spectrum. This study considers scaling of the SRF spectrum in the case of an elasto-plastic oscillator with strength and stiffness degradation characteristics. Two models are considered: one depending directly on the characterization of source and site parameters and the other depending on the normalized design spectrum characterization of the seismic hazard. The first model is the same as that proposed earlier by the second author, and is given in terms of earthquake magnitude, strong-motion duration, predominant period, geological site conditions, ductility demand ratio, and ductility supply-related parameter. The second model is a new model proposed here in terms of the normalized pseudo-spectral acceleration values (to unit peak ground acceleration), ductility demand ratio and ductility supply-related parameter. For each of these models, least-square estimates of the coefficients are obtained through regression analyses of the data for 956 recorded accelerograms in western U.S.A. Parametric studies carried out with the help of these models confirm the dependence of SRFs on strong-motion duration and earthquake magnitude besides predominant period and site conditions. It is also seen that degradation characteristics make a slight difference for high ductility demands and may lead to lower values of SRFs, unless the oscillators are very flexible. Copyright © 2004 John Wiley & Sons, Ltd.

Effect of peak ground velocity on deformation demands for SDOF systems

SUMMARY The eect of peak ground velocity (PGV) on single-degree-of-freedom (SDOF) deformation demands and for certain ground-motion features is described by using a total of 60 soil site records with source- to-site distances less than 23 km and moment magnitudes between 5.5 and 7.6. The observations based on these records indicate that PGV correlates well with the earthquake magnitude and provides useful information about the ground-motion frequency content and strong-motion duration that can play a role on the seismic demand of structures. The statistical results computed from non-linear response history analyses of dierent hysteretic models highlight that PGV correlates better with the deformation demands with respect to other ground motion intensity measures. The choice of PGV as ground motion intensity decreases the dispersion due to record-to-record variability of SDOF deformation demands, particularly in the short period range. The central tendencies of deformation demands are sensitive to PGV and they may vary considerably as a function of the hysteretic model and structural period. The results provided in this study suggest a consideration of PGV as a stable candidate for ground motion intensity measure in simplied seismic assessment methods that are used to estimate structural performance for earthquake hazard analysis. Copyright ? 2005 John Wiley & Sons, Ltd.

The effective number of cycles of earthquake ground motion

The seismic response of any system that accumulates damage under cyclic loading is dependent not only on the maximum amplitude of the motion but also its duration. This is explicitly recognized in methods for estimating the liquefaction potential of soil deposits. Many researchers have proposed that the effective number of cycles of the ground motion is a more robust indicator of the destructive capacity of the shaking than the duration. However, as is the case with strong-motion duration, there is no universally accepted approach to determining the effective number of cycles of motion, and the different methods that have been proposed can give widely varying results for a particular accelerogram. Definitions of the effective number of cycles of motion are reviewed, classified and compared. Measurements are found to differ particularly for accelerograms with broad-banded frequency content, which contain a significant number of non-zero crossing peaks. The key seismological parameters influencing the number of cycles of motion and associated equations for predicting this quantity for future earthquakes are identified. Correlations between cycle counts and different duration measures are explored and found to be rather poor in the absence of additional parameters. Copyright © 2004 John Wiley & Sons, Ltd.

The Influence of Strong-Motion Duration on the Seismic Response of Masonry Structures

The influence of strong-motion duration on the response of saturated soils is clearly recognised and accounted for in the assessment of liquefaction potential. The degree to which duration of shaking influences damage to structures, however, remains a topic of debate, with resolution of the issue complicated by the variety of definitions of duration and the variety of structural behaviours, as well as the difficulty of decoupling the specific effect of duration from other features of the ground motion. A suite of seven structural models with strength and stiffness degrading characteristics, designed to reflect the seismic behaviour of masonry structures commonly encountered in many parts of Europe, are analysed using a suite of almost 500 strong-motion accelerograms. Correlations are explored between the damage, measured in terms of the strength degradation, and a range of strong-motion parameters, demonstrating that Arias intensity and spectral acceleration at the fundamental initial period of the structure are both reasonably good damage indicators for such structures. A significantly improved correlation is obtained by using the elastic spectral accelerations averaged over a period range from the initial period of the structure to a value approximately three times greater, reflecting the stiffness degradation as the shaking progresses. The scatter in the correlation is shown to be partially explained by differences in duration.

Rockburst induced ground motion—a comparative study

While rockbursts from underground copper mining in Western Poland normally produce surface peak ground accelerations (PGA) and velocities of 0.05–0.1 g and 1–3 cm/s, occasionally these peak motions may exceed 0.15 g and 10 cm/s, respectively. These larger motions are of considerable concern and an investigation has been undertaken to define the nature of these larger induced ground motions. This paper compares these rockburst motions with low intensity earthquakes. Various strong motion parameters such as PGA, peak ground velocity (PGV) and displacements as well as strong motion duration, Arias intensity, Fourier and response spectra are compared with those from earthquakes. It is concluded that although short duration is the most obvious parameter that differentiates rockbursts from earthquakes, in fact their high dominant frequencies, which result in high PGA/PGV ratios differentiate them the most. Two types of rockburst-induced ground motions are indicated in this paper: typical—with 3–6 months return period and characteristic, high frequency content—as well as rare events similar to shallow, low intensity earthquakes.

Performance-based seismic design of structures: a direct displacement-based approach

In this paper, a performance-based seismic design procedure, which is directly associated with pre-quantified performance criteria, is presented by employing a displacement-based approach. A lower bound of yielding displacement of the structure to satisfy these performance criteria is proposed. The approach is general in employing either equivalent elastic or inelastic response spectrum. Non-linear time history analysis verified that this approach is applicable to control the target displacement to the performance acceptable limit. Its flexibility in considering special effect such as near-fault or strong motion duration and simplicity in a proposed multiple performance objectives design are demonstrated.

INFLUENCE OF IRAN'S EARTHQUAKE GROUND MOTION PARAMETERS ON DESIGN SPECTRA USING DETERMINISTIC AND PROBABILISTIC APPROACHES

This paper summarises the results of comprehensive statistical study of response and design spectra in which a total of 106 Iran's horizontal acceleration components recorded during various earthquakes was considered. Special emphasis is given to the influence of effective duration of strong motion, soil condition and damping ratio of structure on elastic response and design spectra. The study included both deterministic and probabilistic approaches- The records with peak ground accelerations greater than 0.05 g from 23 stations in 47 seismic events are divided into four soil categories in accordance with Iranian seismic code. For each soil group, mean and mean plus one standard deviation spectral ordinates were computed for: effective duration of strong motion and damping ratio of structures and also a design spectra for different soil conditions has been constructed by deterministic and probabilistic approaches and results have been compared with code provisions (i.e. NEHRP, UBC, EC8 and seismic code of Iran). The influence of effective duration of strong motion has been studied by definition based on energy of accelerogram. The paper also presents an evaluation of the damping ratios on the response spectra. Results and conclusions from this study can be directly used for the evaluation of the present Iranian seismic code in order to construct new design spectra and also for the evaluation of seismic design provisions.

Liquefaction damage and related remediation in Wufeng after the chi‐chi earthquake

The strong motion and long duration of the Chi‐Chi earthquake that occurred in the latter part of 1999 in Taiwan caused severe soil liquefaction damage in Wufeng, an area beside the western foothills of the island. This study investigates areas of soil liquefaction and consequent damage to public and/or private residential houses in Wufeng after the Chi‐Chi earthquake. Remediation methods for foundations of damaged buildings in Wufeng are synthesized, compared, and analyzed. Four major remedial methods are specified: (1) reconstructing ground slabs with reinforced concrete; (2) increasing the size and depth of the footings; (3) constructing a mat foundation after the tilted building has been lifted and leveled, by removing soils under the structure, and (4) lifting and leveling the building with compaction grouting. Three remedial cases are introduced and studied in detail. Recommendations for reconstructing buildings in areas susceptible to liquefaction are also suggested.

Wavelet‐based characterization of design ground motions

AbstractWith the recent emergence of wavelet‐based procedures for stochastic analyses of linear and non‐linear structural systems subjected to earthquake ground motions, it has become necessary that seismic ground motion processes are characterized through statistical functionals of wavelet coefficients. While direct characterization in terms of earthquake and site parameters may have to wait for a few more years due to the complexity of the problem, this study attempts such characterization through commonly available Fourier and response spectra for design earthquake motions. Two approaches have been proposed for obtaining the spectrum‐compatible wavelet functionals, one for input Fourier spectrum and another for input response spectrum, such that the total number of input data points are 30–35% of those required for a time‐history analysis. The proposed methods provide for simulating ‘desired non‐stationary characteristics’ consistent with those in a recorded accelerogram. Numerical studies have been performed to illustrate the proposed approaches. Further, the wavelet functionals compatible with a USNRC spectrum in the case of 35 recorded motions of similar strong motion durations have been used to obtain the strength reduction factor spectra for elasto‐plastic oscillators and to show that about ±20% variation may be assumed from mean to 5 and 95% confidence levels due to uncertainty in the non‐stationary characteristics of the ground motion process. Copyright © 2002 John Wiley & Sons, Ltd.

Influence of earthquake source parameters and damping on elastic response spectra for Iranian earthquakes

In this paper the influence of effective duration of strong motion, soil condition, magnitude and shape of accelerogram time history on the elastic response spectra has been investigated. A total of 106 of Iran's horizontal acceleration components of strong motion records are selected. These accelerograms are categorized based on their earthquake parameters and soil condition of recording station and the influence of different soil conditions are plotted in graphs. The influence of damping ratio on the response spectra is also investigated. Analytical results show that the influence of soil condition is significant for the shape of response spectra. The influence of effective duration of strong motion has been studied by definition based on the energy of the accelerogram. It can be seen that an increase in effective duration causes a reduction in the slope of response spectra in the long period part and increases the spectral values. The effect of damping on response spectra is represented by a special coefficient which depends on the period and damping of the structure and soil condition.

SEISMIC HAZARD IN GREECE BASED ON DIFFERENT STRONG GROUND MOTION PARAMETERS

The available Greek strong ground motion records to date are used in order to study the duration of strong-motion in Greece, covering magnitudes between 4.5 and 6.9 and distances from 1 km to 128 km. An attenuation relation of strong-motion duration is calculated and compared to earlier existing similar relations proposed for Greece and Japan. Furthermore, the seismic hazard for the area of Greece is assessed, using the strong-motion parameters of duration and peak ground acceleration. The results are presented in the form of a map according to which Greece is classified in four different categories of equal seismic hazard.

Seismic Hazard in Greece Based on Different Strong Ground Motion Parameters

The available Greek strong ground motion records to date are used in order to study the duration of strong-motion in Greece, covering magnitudes between 4.5 and 6.9 and distances from 1 km to 128 km. An attenuation relation of strong-motion duration is calculated and compared to earlier existing similar relations proposed for Greece and Japan. Furthermore, the seismic hazard for the area of Greece is assessed, using the strong-motion parameters of duration and peak ground acceleration. The results are presented in the form of a map according to which Greece is classified in four different categories of equal seismic hazard.

Correlation study between seismic acceleration parameters and damage indices of structures

This study describes numerically, the interdependency between several seismic acceleration parameters and diverse structural damage indices. Peak ground motion, spectral and energy parameters are used for characterising the seismic excitation. On the other hand both, structural and nonstructural damage is considered, expressed by the modified Park/Ang overall structural damage index (OSDI), the maximum interstory drift (ISD) and the maximum floor acceleration. After the numerical evaluation of several seismic parameters, nonlinear dynamic analyses are conducted to furnish the structural damage status. The degree of the interrelationship between the seismic parameters and the damage indices is provided by correlation coefficients. The investigation is carried out for a reinforced concrete plane frame system designed after Eurocodes 2 and 8 (EC2, EC8) and the aim is to determine the characteristics of the accelerograms that exhibit the strongest influence on structural and nonstructural damages. The numerical results have shown, that peak ground motion seismic parameters provide poor or fair correlation with the OSDI, whereas the spectral and energy parameters provide good correlation. Furthermore, the central period and the strong motion duration after Trifunac/Brady exhibit poor correlation with the OSDI. All these results give reason to recommend the spectra and energy related seismic intensity parameters as reliable descriptors of the seismic damage potential.

Duration of strong ground motion during Mexican earthquakes in terms of magnitude, distance to the rupture area and dominant site period

AbstractA study of the duration of strong ground motion using accelerometric data of subduction and normal‐faulting Mexican earthquakes is presented. Duration is obtained based on the time between 2.5 and 97.5 per cent of the Arias intensity. An expression to predict this duration in terms of the magnitude, distance to the rupture area and site period is proposed and compared with predictions available in the literature. The effect of large duration for very distant sites and the contribution of soft soils to the duration of strong ground motion are widely discussed. We have found that large magnitude not only yields long duration at the source, but also proportionally longer duration with distance and with dominant site period compared to small magnitude. The duration obtained from the regression is used as a parameter to obtain input and hysteretic energy and on the use of damage models available in the literature. Finally, duration is used together with the random vibration theory to predict response spectra. Copyright © 2001 John Wiley & Sons, Ltd.

Seismicity, seismotectonics and seismic hazard of Italy

A first generation of probabilistic seismic hazard maps of the Italian country are presented. They are based on seismogenic zoning deriving from a kinematic model of the structural tectonic units and on an earthquake catalogue with the foreshock and aftershock events filtered out. The following ground motion parameters have been investigated and mapped using attenuation equations based on strong-motion recordings of Italian earthquakes: peak ground acceleration and velocity; Arias intensity; strong motion duration; and the pseudovelocity and pseudoacceleration spectral values at 14 fixed frequencies both for the vertical and the largest horizontal component. A Poissonian model of earthquake occurrence is assumed as a default and the hazard maps are presented in terms of ground motion values expected to be exceeded at a 10% probability level in 50 years (return period 475 years) according to the requirement of Eurocode 8 for the seismic classification of national territories, as well as in terms of exceedance probabilities of selected ground motion values. Finally, as a tentative study, the use of hybrid methods (implementing both seismogenic zones and structures), renewal processes (including earthquake forecasting) and the influence of site effects (as the basis for the planning of earthquake scenarios) were explored.

Scaling of ductility and damage-based strength reduction factors for horizontal motions

The conventional approach of obtaining the inelastic response spectra for the aseismic design of structures involves the reduction of elastic spectra via response modification factors. A response modification factor is usually taken as a product of (i) strength factor, RS, (ii) ductility factor, Rμ, and (iii) redundancy factor, RR. Ductility factor, also known as strength reduction factor (SRF), is considered to primarily depend on the initial time period of the single-degree-of-freedom (SDOF) oscillator and the displacement ductility demand ratio for the ground motion. This study proposes a preliminary scaling model for estimating the SRFs of horizontal ground motions in terms of earthquake magnitude, strong motion duration and predominant period of the ground motion, geological site conditions, and ductility demand ratio, with a given level of confidence. The earlier models have not considered the simultaneous dependence of the SRFs on various governing parameters. Since the ductility demand ratio is not a complete measure of the cumulative damage in the structure during the earthquake-induced vibrations, the existing definition of the SRF is sought to be modified with the introduction of damage-based SRF (in place of ductility-based SRF). A parallel scaling model has been proposed for estimating the damage-based SRFs. This model considers damage and ductility supply ratio as parameters instead of ductility demand ratio. Through a parametric study on ductility-based SRFs, it has been shown that the hitherto assumed insensitivity of earthquake magnitude and strong motion duration may not be always justified and that the initial time period of the oscillator plays an important role in the dependence of SRF on these parameters. Further, the damage-based SRFs are found to show similar parametric dependence as observed in the case of the ductility-based SRFs. Copyright © 2000 John Wiley & Sons, Ltd.