Frequency Content Of Ground Motion Research Articles

An uncoupled approach for estimating seismic-induced pore water pressures in liquefiable sandy soils

Proper evaluation of seismic-induced excess pore water pressures in saturated sandy soils is still an open issue, which can be tackled with fully coupled to uncoupled approaches. The former are more accurate but computationally onerous, while the latter require seismic demand and pore pressure build-up to be computed in two successive steps, typically employing simple constitutive assumptions.Starting from the work by Seed et al. (1975), this paper presents a novel uncoupled procedure to compute excess water pressures developing in a 1D soil column under partially drained conditions, when subjected to horizontal seismic excitations. Fundamental modifications are introduced to account for: non-uniform distribution of equivalent loading cycles; soil stiffness degradation; and modification of the frequency content of ground motion due to pore pressure build-up.The approach was implemented in Matlab via the Finite Difference Method and validated against both fully-coupled Finite Element analyses and one centrifuge test. An extensive parametric study was also performed for a two-layer soil column, by varying the thickness and hydraulic conductivity of the shallow layer, as well as the seismic input. The good agreement with both numerical and experimental data demonstrates that key features of liquefaction are well-captured by the proposed uncoupled approach.

Mean period prediction models for Mexican interplate and intermediate-depth intraslab earthquake ground motions

The aim of this paper is to predict the mean period, Tm, as a measure of the frequency content of interplate and intermediate-depth intraslab earthquake ground motions recorded at rock sites due to subduction earthquakes in Mexico. For this purpose, both Ground Motion Prediction Models (GMPMs), which use a parametric regression, and Support Vector Machine (SVM) regression, a type of machine learning algorithm used for regression analysis, are employed in this investigation. Therefore, two databases which include 484 and 300 earthquake ground motions recorded during interplate and intraslab historical Mexican earthquakes were assembled in this study. It is shown that Tm estimates depend on the type of earthquake (i.e., interplate or intraslab). While the SVM regression yields the lowest total standard deviation in predicting Tm, the GMPM still yields an acceptable total standard deviation similar to that reported in other studies. A comparison of the GMPM for intraslab earthquakes in Mexico with another GMPMs for predicting Tm recently introduced in the literature showed differences, which suggests that GMPMs for predicting Tm should be developed for specific regions.

Effect of site specific soil characteristics on the nonlinear ground response analysis and comparison of the results with equivalent linear analysis

Damages occurring during earthquakes may vary depending on the ground conditions in which the earthquake waves pass, the magnitude of the earthquake, the focal depth of the shaking and as well as the structural characteristics. Regional seismicity, ground movement and behavior of local soil conditions are important in earthquake-resistant building designs. Local site conditions consist of the layering, bedrock depth, and dynamic and topographic characteristics of soil that alter the bedrock waves through the soil profile during an earthquake. The change occurs in terms of amplitude, frequency and the time when the peak happens during the wave propagation. This initiates a big difference between the surface and bedrock motion. The behavior of the soil under cyclic loads resulting from seismic action is non-linear. This study aims to demonstrate the effects of strong ground motion by taking into account the nonlinear behavior of the soil layers. In addition, the results obtained from the equivalent linear and non-linear methods were compared. The results of the study showed that the characteristics of soil layers and strong ground motion (frequency content and duration) significantly affect the field response analysis and generally larger spectral parameters (about %20) have been obtained with the equivalent linear method compared to the nonlinear behavior. Finally, empirical models to estimate the soil amplification reflect different compared to the site specific analysis.

Double distance dependence in high-frequency ground motion along the plate boundary in Northern Chile

In subduction zones, the forearc crust transitions from a highly fractured wedge near the trench to a less fractured basement near the volcanic arc. Here we study the role of wedge integrity on the frequency content of strong ground motion produced by subduction earthquakes in Northern Chile. Our data includes aftershocks from the 2014 Mw 8.1 Iquique earthquake and intermediate-depth seismicity, including the 2005 Mw 7.6 Tarapacá earthquake. We focus on the S-wave spectral decay parameter κ obtained from strong ground motion in the frequency band 2−40Hz, paying particular attention to how κ varies with hypocentral distance. We report two distinct trends of κ versus hypocentral distance. One trend applies to events near the trench, where we observe a rapid increase of κ with hypocentral distance. Another trend applies to events near the bottom of the megathrust and greater depths, where we observe a relatively slow increase of κ with hypocentral distance. We interpret this difference as resulting from a heterogeneous tectonic structure with lateral variations in anelastic attenuation (i.e., attenuation seems stronger in the fractured wedge near the trench). Our results improve our understanding of the role of high-frequency ground motion in seismic hazard analysis near the Chilean coastline. In addition, our results stress the need for a better characterization of anelastic attenuation in the Chilean subduction zone, specially close to the highly fractured wedge near the trench.

Slosh Dynamics of the Seismically Excited Chamfered Bottom Tank with Bottom-Mounted Internal Object

The inherent energy dissipation ability of the sloshing liquid is effective in vibration control of the supporting structure. Due to the advancement in the construction techniques in structural and mechanical fields, liquid containers of different geometrical configurations were required to avoid tank interference with rigid structural or mechanical components. The quantity of liquid that participates in the connective mode determines the effectiveness of the tuned liquid damper (TLD). To reduce the impulsive liquid mass and to increase the participation of liquid mass in the convective mode, a chamfered bottom tank with a bottom-mounted internal object is proposed in this study. A 2D finite element model developed employing the potential flow theory is used for the present numerical investigation. The effect of the frequency content of ground motion on the slosh dynamics of the chamfered bottom tank has been investigated under six different earthquake ground motions classified based on frequency contents. Also, the dynamic impulsive and convective components of base shear force and hydrodynamic pressure are quantified successfully. A parametric study has been performed to quantify the influence of the bottom-mounted object on the seismic response by altering the object’s height. The developed numerical model in this study can be utilized for tuning and designing structure-coupled chamfered bottom TLDs as well as the irregular liquid containers with internal submerged components.

The effect of the ground motion frequency content on seismic performance and fragility curve of special concentric braced frames

Several articles have investigated the effect of various seismic parameters on the response of the structure. However, there has been limited discussion on the impact of seismic frequency content on the brace failure modes and the collapse mechanism of this system. This study investigated the effect of this parameter, considering three special concentric bracing frames of 2, 4, and 6 stories. For the frame dynamic analysis, the open system for earthquake engineering simulation software (OpenSees) is used. To evaluate the effect of frequency content, 60 earthquake records with high, medium and low frequency content are selected from the pacific earthquake engineering research center ground motion database, considering the peak ground acceleration divided by the peak ground velocity ratio. Incremental dynamic analysis is conducted to draw fragility curves and examine the response of structures.The results indicated that the collapse capacity of structures decreased as the frequency content was reduced. The percentage of reduction in collapse capacity under earthquakes with high to the low frequency content in 2, 4 and 6 story structures were 68, 67 and 71, respectively. In fact, reducing the frequency content increased the probability of occurrence of low cycle fatigue mode in the brace.

Analytical approach for seismic analysis of onshore wind turbines considering soil-structure interaction

The aim of this study is to present an efficient analytical approach, based on a combination of transfer matrix formulations and modal superposition, to perform seismic response analysis of onshore wind turbines (WT) incorporating the effects of soil-structure interaction (SSI). The WT tower and rotor-nacelle assembly (RNA) are idealised as a segmented Timoshenko beam-column and lumped mass, respectively. SSI is taken into account using elastic springs at the bottom of the tower to model soil flexibility. The mass of the foundation is also considered in the mathematical model as a lumped mass at the base of the tower. As the serviceability limit state tends to be the governing design condition for WTs, the soil behaviour is idealised as linear-elastic with equivalent linear stiffness. Firstly, natural frequencies and mode shapes of the WT are obtained using the transfer matrix method (TMM), which is based on analytically-calculated state vectors of beam-column members, and state vectors of joints at the top and bottom of the tower. The entire vibrating structure is simplified into an equivalent single-degree-of-freedom (SDOF) system using mode shapes calculated from TMM. The seismic responses are derived using the Newmark-Beta method and modal superposition. The application of TMM for free vibration analysis is validated with reference to results from existing literature. Additionally, the accuracy of the method for calculation of seismic responses is verified using finite-element software. Three different ground models are considered, namely stratum over half-space, stratum over rigid base, and elastic half-space; and the effect of each on the free vibration and seismic responses are examined in terms of displacement and base shear time-histories of the WT with varying foundation geometries. The effects of varying soil properties on the natural frequencies and seismic responses of the WT are also investigated. Finally, the dynamic responses of the model subjected to different seismic time-histories are presented comparatively, to highlight the significant effects of the frequency content of ground motion records.

Effect of Frequency Content of Earthquake Ground Motions on Structures with Varying Dimension

An earthquake is a catastrophe which is always a topic of concern for structural designers and civil technocrats. Not only it causes physical damage to the structures but also successful to impart psychological disorder in human minds for long-run. In order to mitigate the devastating effects of earthquake on society, it is eminent to study the dynamic characteristics in detailed manner i.e. to examine how the structure responds owing to the seismic characteristics. The various ground motion characteristic includes time-duration and velocity, frequency content and amplitude, displacement, incremental velocity and incremental displacement, peak ground accelerations (PGAs), etc. Out of these, effect of frequency content and maximum amplitude value of earthquake ground motions on the seismic response of structures is often underestimated. This paper investigates the effect of frequency content and maximum amplitude value on the seismic response of structures which is dominant of all characteristics. The study proceeds with recording time-history of acceleration obtained from conventional unidirectional harmonic shake table. Further, FFT (Fast Fourier Transform) analysis of applied time-history is done and the effect of frequency content and the maximum amplitude value of applied time-history on the seismic responses of structures is investigated and studied. For this, structures with varying dimensions (height, length, and width) are modelled and time-history analysis of structural models has been carried out in SAP2000v19. Seismic responses of analysed structures are represented in the form of fundamental natural frequency, storey displacement, and base shear. It is reported that out of many earthquake indices, frequency content and amplitude of earthquake ground motions are the most dominating. It is reported that the resonance phenomena occurs for the less height structure and thus it shows maximum displacement and base shear responses. Also, as height, length, and width of structure increases; displacement, base shear, and fundamental time-period of structure increases.

Influence of ground motion frequency content on the nonlinear response and seismic performance of RC moment frame structures

Several parameters may affect the response of structures to seismic excitation. These include features of the structures, properties of imposed ground motion and geological specifications of building location. This research has been conducted to investigate the effects of the frequency content of earthquakes as an important property of ground motion on the seismic performance and nonlinear response of reinforced concrete moment frame (RCMF) structures. To this end, forty-two earthquake records have been selected from the stations on site classes A and B based on ASCE7-10. These input excitations have been classified into three groups based on PGA to PGV ratios namely: Low, Intermediate, and High frequency motions. Three RCMFs with 2, 6, and 10 stories have been designed according to ACI318-14 and ASCE7-10. Finally, utilizing OpenSEES platform, based on the FE method a numerical model has been built to perform incremental dynamic analysis (IDA). After interpreting the results of the IDA, fragility curves have been obtained as a function of PGA for pre-defined damage states. The results showed that the associated PGA with the median of the fragility curve is in ascending order Low, Intermediate and High frequency motions. Accordingly, it is concluded that in the same PGA level, Low frequency motions are more probable to reach a damage threshold than Intermediate and High frequency motions. Similarly, Intermediate frequency motions is more capable reaching the damage threshold in comparison to High frequency motions.

Unravelling a 2300 year long sedimentary record of megathrust and intraslab earthquakes in proglacial Skilak Lake, south‐central Alaska

AbstractSeismic hazards in subduction settings typically arise from megathrust, intraslab and crustal earthquake sources. Despite the frequent occurrence of intraslab earthquakes in subduction zones and their potential threat to communities, their long‐term recurrence behaviour is barely studied. Sedimentary sequences in lakes may register ground shaking from different seismic sources. This study investigates two long sediment cores (13 m and 16 m) from Skilak Lake, a proglacial lake in south‐central Alaska, to evaluate whether different seismic sources leave a distinct imprint. The sedimentary record shows a continuously varved sediment sequence, occasionally interrupted by turbidites, slump deposits and tephra beds. Turbidites and slump deposits were objectively identified using a statistical outlier analysis on varve thickness. The earthquake origin of these deposits was ascertained by resemblance with deposits induced by instrumentally recorded earthquakes (for example, 1964 ce Mw 9.2 megathrust and 1954 ce Mw 6.4 intraslab earthquakes) and correlation with multiple coeval landslide deposits on sub‐bottom profiles. The Skilak Lake record chronicles 19 earthquakes with moderate to very high confidence level in the past 1350 years. The sedimentary evidence of instrumentally‐recorded intraslab and megathrust earthquakes within the past 70 years demonstrates that not only megathrust earthquakes, but also past intraslab events are recorded. Although reported seismic intensities at Skilak Lake are comparable for the 1964 ce megathrust and the 1954 ce intraslab earthquakes, the long duration and low frequency content of seismic ground motion during megathrust earthquakes facilitate the triggering of multiple, voluminous landslides and the generation of megaturbidites. In contrast, the shorter duration and higher frequency source spectrum of intraslab earthquakes may only induce surficial slope remobilization and the generation of thinner turbidites. This study demonstrates that the sedimentary record of Skilak Lake has the potential to decipher multiple seismic sources, which opens possibilities for a comprehensive seismic hazard analysis for south‐central Alaska.

Optimization and performance assessment of tuned mass damper inerter systems for control of buildings subjected to pulse-like ground motions

Optimal tuned mass damper inerter (TMDI) systems in controlling displacement demands on structures affected by pulse-like near fault ground motions is presented. Simplified mathematical models of three buildings and many recorded near-fault ground motions, all containing a dominant velocity pulse, are used to make this investigation valid for a wide range of structural vibration periods and ground motion frequency content. For each of these ground motions, optimal parameters of TMDIs are estimated by minimizing structural displacement demand. Time history analysis is used for structural response simulation and Genetic Algorithm is used for minimizing the structural displacement demands. The performance or lack of it of the TMDIs are investigated and explained in relation to important properties of ground motions such as the frequency and oscillatory nature of the dominant pulse contained in the ground motion. The results indicate that properly optimized TMDIs can effectively control structural response under certain circumstances. When the structure resonates with the ground motion pulse, the less impulsive the ground motion, the more effective is the control scheme. An interesting finding is that TMDIs can be very effective in controlling response of structures which are not resonating with the pulse but nevertheless experience high demands due to other frequency components of near-fault ground motions. It is found that TMDI solutions obtained by minimizing the H2 norm of the elastic transfer function are not optimal when the fundamental period of vibration of the structure is lower than the period of the dominant pulse carried by the ground motion. Such solutions also result in unnecessarily high damping in the TMDI dashpot, without any benefit in controlling structural response.

Hazard-consistent seismic losses and collapse capacities for light-frame wood buildings in California and Cascadia

We evaluate the seismic performance of modern seismically designed wood light-frame (WLF) buildings, considering regional seismic hazard characteristics that influence ground motion duration and frequency content and, thus, seismic risk. Results show that WLF building response correlates strongly with ground motion spectral shape but weakly with duration. Due to the flatter spectral shape of ground motions from subduction events, WLF buildings at sites affected by these earthquakes may experience double the economic losses for a given intensity of shaking, and collapse capacities may be reduced by up to 50%, compared to those at sites affected by crustal earthquakes. These differences could motivate significant increases in design values at sites affected by subduction earthquakes to achieve the uniform risk targets of the American Society of Civil Engineers (ASCE) 7 standard.

Improvement of energy damage index bounds for circular reinforced concrete bridge piers under dynamic analysis

AbstractAn improvement of energy damage index bounds for circular reinforced concrete (RC) bridge piers under dynamic time history analysis is presented in this study. The energy‐based damage model has been defined as a ratio of hysteresis dissipation energy to input energy for performance evaluation of circular RC bridge pier systems. Based on the energy balance equation, the mentioned damage model can consider all energy aspects, including kinematic energy, strain energy, hysteresis energy, damping energy, and input energy. Additionally, the effect of ground motion duration, amplitude, and frequency content is appropriately considered in this damage model. For this purpose, the damage limit state bounds for two empirical RC bridge piers are statistically evaluated under the 22 far‐field ground motion sets of FEMA‐P695 for simulation of strong‐earthquake motion and consideration of record‐to‐record variability. First of all, appropriate validation between empirical and numerical fiber element models is implemented to estimate accurate results. Then, damage data at each performance level through strain limit states are collected by developing energy incremental dynamic analysis curves. Statistical results on collected damage data are presented in all performance levels to determine damage index bounds by the convolution of fragility curve distribution. Hence, five damage index bounds at five performance‐level are suggested, including very slight, slight, moderate, extensive, and complete damage. Finally, the effect of the other damping ratios on the modified energy damage model is carried out by using the beta coefficient in the formula. In the end, the modified energy damage model is compared with other damage indices to investigate the efficiency of the proposed damage model.

IDA-based seismic fragility of high-rise frame-core tube structure subjected to multi-dimensional long-period ground motions

Due to resonant effects, a lot of high-rise structures could be severely damaged under long-period seismic excitations. In this study, the seismic fragility method based on incremental dynamic analysis (IDA) is employed to quantitatively evaluate the damage levels of high-rise structures under long-period ground motions. Firstly, the frequency contents of special long-period ground motions are analyzed. Then, the finite element models of high-rise frame-core tube structures are established and verified. Lastly, the IDA curve clusters, quantile curves, probabilistic demand models, exceeding probability curve and seismic fragility matrix of the structure under multi-dimensional long-period ground motions are researched respectively. The analytical results show the amplitude contents of long-period ground motions are mainly distributed in the narrow low-frequencies band. Long-period earthquake is more likely to cause structural collapse than ordinary earthquake under large earthquake intensity, and the seismic fragility analysis based on a certain exceeding probability needs to consider the adverse effect of the increased horizontal translational components. The research conclusions will provide urgently needed theoretical basis for seismic design in areas with hidden long-period earthquake damage, and it also promote the development of seismic design methods and the improvement of seismic structural measures for high-rise structures.

Influence of extent of remedial ground densification on seismic site effects via 2-D site response analyses

Remedial ground densification techniques are widely employed in seismically active regions to mitigate geotechnical hazards and reduce the potential for damage to structures. These techniques can substantially modify the local soil conditions and the seismic site response characteristics of the profile. This study provides a quantitative insight into the local site amplification in remedially densified soils. Parametric site response analyses are performed using various subsoil conditions and extents of ground densification. The free-field site responses obtained with and without ground densification are compared using two-dimensional (2-D) finite element models. The results show that the frequency content of ground motions propagating in densified sites is modified, with a shift of soil response harmonics towards higher frequencies. When a densified crust is implemented with unimproved soft soil layers underneath, the PSAs at the ground surface are generally reduced, at frequencies between 5 and 10 Hz. However, the levels of de-amplification decrease as the depth of ground densification increases. In particular, the densification of the full depth of liquefiable soil layers leads to higher PSAs at the ground surface. The results presented provide a better understanding of the geometry and density of the improved zones that can lead to the amplification of ground surface motions in the frequency range of interest for building response, prior to investigating more complex problems that include soil-structure interaction.

Effects of Earthquake Magnitude, Distance, and Site Conditions on Spectral and Pseudospectral Velocity Relationship

ABSTRACT The spectral velocity (SV) is necessary information in the seismic design of structures with supplemental velocity-dependent dampers, and it is conventionally approximated by the pseudospectral velocity (PSV), which is available in seismic codes. Because of the significant approximation error, it is important to clarify the relationship between the two spectra to establish a suitable formulation to relate SV to PSV. Recent studies have point out that this relationship is influenced not only by the oscillator period and damping ratio but also by earthquake characteristics (Papagiannopoulos et al., 2013; Samdaria and Gupta, 2018). To clarify the seismological effects, in this study, an approach to relate SV to PSV based on the random vibration theory is proposed, and it is verified by comparing its results with those of traditional time-series analysis. The effects of earthquake magnitude and distance as well as site conditions on the relationship between the two spectra are explored based on the proposed approach as well as statistical analysis of recorded seismic motions. It is found that the SV approaches the PSV with increasing magnitudes at long oscillator periods but performs oppositely at short oscillator periods. The demarcation range beyond which the opposite trend is observed varies from (0.07–0.24) to (0.12–0.87) s using the proposed approach and considering the regions of central and eastern North America. The range varies from (0.1–0.15) to (0.3–0.7) s based on the results obtained by the statistical analysis of seismic records in Japan. The observed phenomena were theoretically explained, and the seismological effects were found to be governed by the ground-motion frequency content.

The influence of the frequency content of ground motion on the nonlinear dynamic response and seismic vulnerability of historical masonry towers

Observations made on the response of historical masonry towers during past earthquakes indicate that in addition to the intensity of the ground shaking, the frequency content of shaking also affects the seismic performance of these monuments. To evaluate this phenomenon, the influence of the mean period of the ground motion (Tm), as a frequency content indicator, on the seismic behavior of the towers is assessed. To this end, first, the vulnerability of four towers with different aspect ratios and vibration periods (Ts) are evaluated by means of the Incremental Dynamic Analysis (IDA). For this purpose, 37 ground motion records, corresponding to the stations located in sites with different types of soil, are utilized. The nonlinear time history analyzes of the towers are carried out using the OPENSEES software by means of an Equivalent Beam Element with fiber sections. In order to investigate the effects of the frequency content of the ground motion on the seismic response of the towers, for every tower, the variation of the PGA of the ground motion and the induced internal force in the tower at the point of failure are plotted against the period ratio (Tm/Ts). According to the analysis results, it is found that the failure PGA increases as the period ratio becomes smaller. It is also noted that the induced shear in the tower exhibits a similar trend.

Seismic design of RC frame structures based on energy-balance method

The force- and displacement-based design methodologies have been widely used in current seismic design practice. In both methodologies, inelastic behavior is determined indirectly since the seismic demands are derived from elastic acceleration response spectra. In this study, a novel energy-based design (EBD) approach is proposed in the determination of both seismic demand and dissipation capacities of reinforced concrete (RC) frame members through their inelastic behavior directly. In the case of frame-type RC structures, the distribution of demand plastic energy throughout the structure is achieved. The proposed EBD is then accomplished by comparing the demand plastic energy with the energy dissipation capacity of members while integrating performance-based design guidelines. A cantilever column and a benchmark frame, both from the literature, are analyzed in verification of the proposed methodology. Comparisons are made between force, displacement and EBD methodologies in terms of soil condition, member, system and ductility performance requirements. Member and global performance targets are found to be comparable in displacement and EBD methodologies. However, owing to lack of ductility considerations in the prior design methodologies, the proposed EBD showed more reliable result in the determination of both cross-sectional geometry and its rebar configuration. The proposed methodology provides better understanding of damage state limits of RC members by incorporating the plastic energy spectrum, which includes cyclic action, duration, and frequency content of ground motions, as well as inherent ductility in both demand and capacity determinations. The sensitivity analyses between code compliant-design and soft-story-mechanism buildings showed a distinct difference in the variations of seismic energy among the members and their corresponding damage distributions. EBD methodology provides better in depth understanding of the seismic design in terms of demand and capacity of member and overall structural system, and their corresponding performance.

Evaluation of pulse effect on frequency content of ground motions and definition of a new characteristic period

Evaluation of pulse effect on frequency content of ground motions and definition of a new characteristic period

INFLUENCE OF NUMERICAL SOIL MODEL TYPE ON DYNAMIC RESPONSE OF REINFORCED CONCRETE FRAME STRUCTURES

In engineering practice, numerical models of structures are typically supported using nondeformable media. However, the compliance and stratification of the underlying soil influence the seismic response of the structure. Its influence on structures subjected to dynamic loads has been investigated using various approaches to soil foundation modeling. This study was conducted for 70 different cases. Three actual soil profiles and two structures were analyzed, and the soil was modeled in three ways. Dynamic analysis was performed using five records of previous earthquakes. Finally, the relationship between the magnitude of the top displacement of the structure and the frequency content of ground motions was established.