Seismic Amplification Factors Research Articles

Seismic amplification factors for low plasticity medium-stiff soil deposits

The seismic response of low plasticity medium-stiff soil deposits has been investigated through non-linear one-dimensional analyses carried out with reference to a set of 30 soil profiles, with bedrock depths varying in the range 20–60 m. More than 300 horizontal acceleration time histories recorded on almost flat rock outcropping sites have been used as input motions with amplitude, energy and frequency content varying in wide intervals, which should lead to outcomes of general validity. In the modelling of soil hysteretic behaviour, the backbone curve was described by a modified hyperbolic model calibrated against well-established stress-dependent relationships describing the variation of shear modulus and damping with the strain level. The analysis results show that the stratigraphic amplification factor is significantly influenced by the peak outcrop acceleration and bedrock depth while it is almost unaffected by magnitude sorting of the data; accordingly, original empirical predictive models have been proposed to estimate the stratigraphic amplification factor as a function of the peak outcrop acceleration and bedrock depth. Also, it has been demonstrated that, regardless of the amplitude of the input motion and whatever is the sorting of data with reference to magnitude and bedrock depth, a direct proportionality exists between the stratigraphic and the spectral amplification factors, implying an almost constant value of the spectral shape ratio. Finally, the relevance of using stress-dependent shear modulus and damping curves has been pointed out to allow a reliable evaluation of the stratigraphic and spectral amplifications.

Seismic amplification factor and dynamic response of soil-nailed walls

Seismic amplification factor and dynamic response of soil-nailed walls

Italian seismic amplification factors for peak ground acceleration and peak ground velocity

ABSTRACT Ground motion modification over large areas is generally evaluated by focusing on source effects disregarding local lithostratigraphic site conditions. Hence, amplification maps of peak ground acceleration and peak ground velocity are proposed to improve the forecast of ground motion on a national scale. Topological information about litho-type successions and soil mechanical behaviour were retrieved from the Italian database of seismic microzonation and more than 30 million of seismic site response analyses were performed to quantify the amplification factors (i.e. the ratio between expected ground motion at the site of interest and that at the outcropping engineering bedrock). The maximum value of the amplified peak ground acceleration on the Italian territory results in about twice as much as the value expected at the outcropping of the engineering bedrock. Finally, damage scenario maps based on the amplified ground motion could be produced as a supporting tool for urban planning and emergency system management.

Seismic Microzonation of the Pompeii Archaeological Park (Southern Italy): Local Seismic Amplification Factors

Pompeii Archaeological Park is the best laboratory for the study of the seismic site effects on cultural heritage: the ancient site was destroyed and buried by the 79 AD Vesuvian eruption and, furthermore, it was also affected by the 62–63 AD strong earthquake. Large sectors of the city were reconstructed after this earthquake while other parts were still under reconstruction when the fall-out and pyroclastic density currents of the eruption buried the Roman city. In order to evaluate the distribution of the damage and reconstructions due to the earthquake, detailed mappings of the structures were carried out using multidisciplinary approaches. In addition, analyses of the topographical features, subsoil stratigraphies, and geophysical surveys, responsible for local seismic amplification (site effects), allow us to define the sectors of the ancient city where the Amplification Factors (AFs) were the main ones responsible for damage. Selected areas and examples of compromised and reconstructed buildings show that the ancient topography and subsoil features (both lithological and seismic) are the main AFs. In particular, the damages caused by the 62–63 AD earthquakes seem to be mainly due to topographical factors such as steep scarps and slopes, ridges, peaks, and terraces, as well as to the major thickness of the soft sediments (loose volcanoclastic layers, paleosols, weathered lavas, and anthropogenic infillings) located over the well-lithified lavas. It is not uncommon to also have the combination of these two factors. For the first time, this multidisciplinary approach allows us to draw a seismic microzonation map for one of the most important archaeological sites of the world.

Estimation of the moment magnitude and local site effects of a postulated Late Bronze Age earthquake: Mycenaean citadels of Tiryns and Midea, Greece

Previously interpreted archaeological and geological field data from the Argive Basin, Greece, have been used to hypothesized that the nearby Late Bronze Age Mycenaean citadels of Tiryns and Midea, which settled on bedrock, might record synchronized co-seismic structural damage due to earthquake ground-shaking at ca. 1190 BCE, the end of the Mycenaean palatial period. However, from a quantitative archaeoseismological perspective, this hypothesis has flaws: (a) it overlooks that several structures excavated in the undeformed Holocene sediments lack evidence of co- seismic structural damage and damage is only documented for sturdier structures within the citadels, (b) it leaves unexplained the moment magnitude of the ‘causative’ surface-rupturing earthquake, and (c) disregards the role of local site effects on surface ground-motion. The plausibility of the previously proposed earthquake hypothesis is tested by estimating the earthquake magnitude of the alleged ancient earthquake and its local site effects at and around the citadels, assuming that the earthquake truly occurred. A retrospective geotechnical site microzonation coupled with the calculation of seismic amplification factors, surface ground-motions, and earthquake intensity account for the local site effects assessment. The present numerical modeling results indicate that the citadels and contemporaneous adjacent structures of the peasants had a lower and higher seismic hazard, respectively. Seismic amplification factors, earthquake-induced ground shaking, and seismic intensity are indeed lower for Tiryns than for Midea. Therefore, these results refute the idea of seismically induced destruction of the citadels and challenge the plausibility of the earthquake hypothesis. The previously archaeologically documented destruction patterns unlikely represent physical evidence of co-seismic damage by the archaeologically proposed earthquake. Other explanations ought to be sought to elucidate the interpreted destruction pattern seen in these Mycenaean centers.

Seismic Amplification by Deep Subsurface and Proposal of a New Proxy

Codes of practice and ground motion prediction equations involve ground structure proxies to account for seismic amplification. Although the ground consists of both shallow and deep subsurface, proxies are mainly related to the shallow subsurface as it is shallow subsurface information that is mostly available. However, as deep subsurface seismic amplification is not negligible, it may not be appropriate to use shallow subsurface proxies. In this study, the relationship between shallow and deep subsurface seismic amplification factors is discussed on the basis of S-wave velocity profile data from Japanese KiK-net strong-motion observation system stations. The correlation between typical proxies such as the average S-wave velocity of the top 30m of the ground surface and the seismic amplification factor was examined. Although there was a negative correlation between the two, the degree of the correlation was weak. A new proxy showing stronger correlations with the seismic amplification factor is proposed and its effectiveness is demonstrated.

New 2019 Risk-Targeted Ground Motions for Spectral Design Criteria in Indonesian Seismic Building Code

Indonesia has followed development of new seismic design criteria in the new seismic building codes, from hazard-based in the former SNI-03-1726-2002 to the current risk-based SNI-1726-2012. The major changes in SNI-1726-2012 are using Risk-Targeted Maximum Considered Earthquake (MCER) Spectral Response Acceleration maps. Five years later (2017), the seismic hazard maps have been updated adopting the most recent data and current state of knowledge in probabilistic and deterministic seismic hazard assessment methodologies. To establish the New 2019 Risk Targeted Ground Motion (RTGM) of spectral acceleration (Ss and S1), and risk coefficients (CRS and CR1), for both short (T=0.2s) and 1-second (T=1s) periods, respectively have been developed based on the 2017 Indonesian hazard maps. The RTGM was calculated as the spectral value resulting in 1% probability of building collapse in 50 years through numerical integration of hazard curves and structural capacity. The log-normal standard deviation (?) of the structural capacity envelope has been revised from 0.70 to 0.65. This paper presents the new resulted RTGM maps. Furthermore, the paper also presents revision of seismic amplification factors for 0, 0.2, and 1 second periods (FPGA, Fa, and, Fv) to generate ground surface maximum and design spectra associated with the siteclassifications.

Evaluation of two-stage scaling for physical modelling of soil−foundation−structure systems

Several factors, such as the payload capacity of a geotechnical centrifuge, performance of an in-flight shake table and a maximum achievable centrifugal acceleration, limit the size of a structure that can be modelled to study soil−foundation−structure interaction (SFSI) effects using centrifuge modelling. The two-stage scaling method is an innovative technique to model larger prototypes in currently available centrifuge facilities. In this study, two sets of dynamic centrifuge experiments were conducted to evaluate the applicability of the method for SFSI studies. Two physical models, representing a target structure placed on dry sandy soil, were designed based on the scaling factors of conventional centrifuge modelling at 50g and the two-stage scaling method at 25g. The models were excited under different realistic earthquake motions. The results showed good agreements for the estimation of flexible-base natural frequencies of the physical models. The performance of the method for estimation of seismic amplification factors, kinematic incoherence parameters and foundation impedance functions was fair. Poor and fair agreements were observed for the estimation of seismic soil settlement recorded beneath the physical models and free field, respectively. Overall, the method can be used for SFSI studies in the centrifuge modelling while considering its strengths and weaknesses.

Assessment of seismic amplification factor of excavation with support system

Retaining walls have been used in many construction projects such as for road and inclined surfaces protection. The damage caused by an earthquake depends on the fundamental frequency, amplitude and the duration of the seismic motion. These parameters strongly depend on the seismic properties of the layers that are near the surface. In the study of retaining walls, in addition to the influence of soil, the influence of topography is also important. In the present study, site response analysis is performed by using finite element software PLAXIS to obtain the effect of various factors such as embedded length of the sheet pile, underground water table, length and angle of the nail, shear wave velocity of soil on site effect and dynamic response. Moreover, for better understanding of the effect of the above parameters, the stability analysis was performed by using shear reduction method. The results show that an increase in the embedded length of the sheet pile and the length of nailing causes an increase in the amplification factor. Moreover, for shear-wave velocity in the range of 200–600 m/s, the amplification factor increases with increase of the shear-wave velocity due to the decrease of nonlinear behavior.

Seismic Stability of Gravity Retaining Structures by Pseudo-Dynamic Method

An analytical expression of a gravity retaining wall's seismic stability against sliding and overturning is proposed in this article. The derivation, aiming at the cohesionless soil with inclined backfill surface and nonvertical wall back, is based on limit equilibrium analysis and the pseudo-dynamic method. The variations of the sliding and overturning stability safe factors with the horizontal seismic acceleration are investigated for different seismic amplification factors, soil friction angles, wall friction angles, vertical seismic acceleration coefficients, wall back inclination angles, and backfill surface inclination angles. The results indicate that the soil friction and horizontal seismic action significantly impact the seismic stability. The increase of vertical earthquake action changes the curvature of stability factor curves. The wall friction and back inclination strengthen the gravity retaining wall's resistance to sliding and overturning failure while the backfill surface inclination plays a negative role in the seismic stability. We also found that the seismic stability safe factors calculated by the proposed method are larger but more reasonable than those by the Mononobe-Okabe method.

Evaluation of effect of soil characteristics on the seismic amplification factor using the neural network and reliability concept

Peak ground acceleration (PGA) is one of the most important parameters in structural seismic design. Since many variables such as soil characteristics and attenuation relations are probabilistic parameters, the PGA cannot be determined using deterministic approaches. In this paper, by using the concept of reliability, effects of soil characteristics on PGA have been investigated for Kermanshah (Iran) as the case study. Several deterministic analyses were performed using the PLAXIS software, and the amplification factor of soil was obtained. From the obtained results an artificial neural network (ANN) was trained according to the input–output pairs and these emulators were used as an alternative for PLAXIS software. As the next step for effective parameters on PGA, a great number of input data according to the probability density function, average, and standard deviation were generated. According to these data, PGA was determined on the ground by using ANN, and reliability was calculated according to Monte Carlo simulation. In fact, reliability was calculated by comparing the PGA in deterministic and probabilistic approach. Finally, sensitivity analyses for the effect of soil characteristics on PGA were performed.

Seismic Site Response Analysis for Ottawa, Canada: A Comprehensive Study Using Measurements and Numerical Simulations

The surficial geology of the city of Ottawa primarily consists of soft soil sediments with low shear‐wave velocities (![Graphic][1] ) underlain by hard bedrock with very large shear‐wave velocities (![Graphic][2] ). Earthquake recordings show unusually large seismic amplification values for weak motion. These unusually large seismic amplification factors were reconfirmed with the earthquake spectral ratio method using two stations, the horizontal‐to‐vertical earthquake spectral ratio method using a single station, and the horizontal‐to‐vertical spectral ratio technique using background noise. These findings were the motivation for carrying out an extensive site response analysis, using finite element modeling (FEM), as a part of the seismic microzonation studies for the city of Ottawa. The FEM results confirmed the large amplification ratios for weak‐motion recordings. FEM analysis was also carried out using a selection of strong‐motion time series for the study area. The combined effect of the soil–bedrock acoustic impedance contrast and the level of ground shaking on the variation of soil amplification factors for the fundamental frequency were investigated. The maximum value of the soil amplification factor for the fundamental frequency increased with increasing impedance contrast ratios until the soil/bedrock acoustic impedance contrast ratio reached values that were usually greater than 12; however, the change in peak amplification was much less with subsequent increases in the contrast ratio beyond that value. As expected, the value of the soil amplification factors for the fundamental frequency decreased with increasing peak ground acceleration (PGA) of the input motion due to nonlinear soil damping. Finally, for the Ottawa region, a mathematical model is suggested for soil amplification factors at the fundamental frequency, as a function of the soil/bedrock acoustic impedance contrast ratio and the PGA of the input motion. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Evaluating the effective spectral seismic amplification factor on a probabilistic basis

All contemporary seismic Codes have adopted smooth design acceleration response spectra, which have derived by statistical analysis of many elastic response spectra of natural accelerograms. The above smooth design spectra are characterized by two main branches, an horizontal branch that is 2.5 times higher than the peak ground acceleration, and a declining parabolic branch. According to Eurocode EN/1998, the period range of the horizontal, flat branch is extended from 0.1 s, for rock soils, up to 0.8 s for softer ones. However, from many natural recorded accelerograms of important earthquakes, the real spectral amplification factor appears to be much higher than 2.5 and this means that the spectrum leads to an unsafe seismic design of the structures. This point is an issue open to question and it is the object of the present study. In the present paper, the spectral amplification factor of the smooth design acceleration spectra is re-calculated on the grounds of a known "reliability index" for a desired probability of exceedance. As a pilot scheme, the seismic area of Greece is chosen, as it is the most seismically hazardous area in Europe. The accelerograms of the 82 most important earthquakes, which have occurred in Greece during the last 38 years, are used. The soil categories are taken into account according to EN/1998. The results that have been concluded from these data are compared with the results obtained from other strong earthquakes reported in the World literature.

Empirical Formula for Calculating Maximum Acceleration of Concrete-Faced Rockfill Dam

On the condition of different dam heights and different shapes of river valley, 3-D dynamical responses of concrete-faced rockfill dams subjected to different seismic intensity input are analyzed based on equivalent-linear model in this paper. The results show that on the condition of the same dam height and the base acceleration excitations unchanged, the position of the maximum acceleration on the axis of the top moves from the middle to the riversides symmetrically with the increase of the river valley width. For the narrow river valley, the position of the maximum acceleration which occurs near the riversides to the wide valley is on the middle of the axis at the top. The result negates the application of 2-D dynamical computation for wide valleys, and shows that for the seismic response of high concrete-faced rockfill dams, the seismic amplification factor along the axis should be given, except for that along the dam heights. Statistical analysis is also made to the seismic coefficient, and an empirical equation for calculating the maximum acceleration of the dam is provided, which provide an reference for the seismic stability analysis of rockfill dams using pseudo-static method.

Estimates on seismic amplification factors of reactor components

Employing an averaging technique we obtain estimates on seismic amplification factors for different components in nuclear reactors.