Vertical Component Of Ground Motion Research Articles

Soil damping and site dominant vibration period determination, by means of random decrement method and its relationship with the site-specific spectral decay parameter kappa

We use the random decrement method (RDM) to determine the damping (ξ) and dominant period (Td) of soils using earthquake records and ambient noise, and compare the values of ξ with the spectral decay parameter kappa (κ0) previously estimated at several sites in the San Bernardino Valley, northeastern Sonora, Mexico.The dominant period take values from 0.12s at sites on igneous rocks, to 0.17–0.20s at sites on continental deposits (Tertiary conglomerates). The damping varies between 2.8 and 5.7%.We find the empirical relationship: ξ=(0.50±0.178)×κ0, which predominate in the study area, and is comparable with that obtained for stations on conglomerates (ξ=(0.43±0.130)×κ0). We also find a theoretical equation ξ=(Vs/2H)κ0 useful for any region, that depend on the shear wave velocity (VS), the decay parameter (κ0) and the thickness of the soil-layer considered (H).We calculate the thickness of the surface layer at each site and find that it varies from 80- to −150-m. We conclude that: it is possible to obtain ξ and Td simultaneously with the RDM; consistent estimates of ξ and Td can be obtained using the whole record or parts of it; the RDM avoids the calculation of spectral ratios to estimate Td and ξ as well as any other pre-processing of the signal; the vertical component of ground motion reflects the properties of shallow layers at the analyzed sites.

Evaluation of Seismic Performance of Moment Resisting Frames considering Vertical Component of Ground Motion

The design of structures to withstand seismic loading is primarily governed by horizontal ground motion, and the effects of vertical ground motion have long been deemed unimportant or secondary. However, an emerging body of evidence suggests that vertical ground motions have great destructive potential, especially for certain site conditions.Using nonlinear dynamic analyses, the seismic performance of frames is evaluated by simultaneously applying the horizontal and vertical components of earthquake motion. Three special moment frames are subjected to 15 recorded near-field earthquake motions and the results of the analyses are used for performance evaluations, based on FEMA 350 guideline. It is found that, within the elastic range of response, considering the vertical component of ground motion, does not alter the dynamic structural response significantly, however, some tangible differences emerge in nonlinear range of response once the horizontal component of ground motion is accompanied by the vertical component.

Strong ground motion characteristics observed in the 4 September 2010 Darfield, New Zealand earthquake

This manuscript provides a physically-orientated and engineering-focused assessment of the near-source ground motions from the Darfield earthquake that were recorded by 29 strong motion stations in Christchurch city and the surrounding Canterbury Plains. In discussion of the observed ground motions particular attention is given to: (i) source features such as the complexity of the rupture on multiple fault planes and forward directivity effects; (ii) the effects of the Canterbury Plains sedimentary basin on waveguide effects through the region, and basin-edge effects observed around the Port Hills; and (iii) the importance of local site response as evidenced by observations of large high frequency amplification and liquefaction. Additional context is provided by comparison of ground motion amplitudes with empirical prediction models and reconciling ‘outliers’ based on inferred physical mechanisms. The severity of the horizontal and vertical components of ground motion are also emphasised via comparisons with response spectra prescribed for routine seismic design.

Sea level variations at tropical Pacific islands since 1950

The western tropical Pacific is usually considered as one of the most vulnerable regions of the world under present-day and future global warming. It is often reported that some islands of the region already suffer significant sea level rise. To clarify the latter concern, in the present study we estimate sea level rise and variability since 1950 in the western tropical Pacific region (20°S–15°N; 120°E–135°W). We estimate the total rate of sea level change at selected individual islands, as a result of climate variability and change, plus vertical ground motion where available. For that purpose, we reconstruct a global sea level field from 1950 to 2009, combining long (over 1950–2009) good quality tide gauge records with 50-year-long (1958–2007) gridded sea surface heights from the Ocean General Circulation Model DRAKKAR. The results confirm that El Niño-Southern Oscillation (ENSO) events have a strong modulating effect on the interannual sea level variability of the western tropical Pacific, with lower/higher-than-average sea level during El Niño/La Niña events, of the order of ± 20–30 cm. Besides this sub-decadal ENSO signature, sea level of the studied region also shows low-frequency (multi decadal) variability which superimposes to, thus in some areas amplifies current global mean sea level rise due to ocean warming and land ice loss. We use GPS precise positioning records whenever possible to estimate the vertical ground motion component that is locally superimposed to the climate-related sea level components. Superposition of global mean sea level rise, low-frequency regional variability and vertical ground motion shows that some islands of the region suffered significant ‘total’ sea level rise (i.e., that felt by the population) during the past 60 years. This is especially the case for the Funafuti Island (Tuvalu) where the “total” rate of rise is found to be about 3 times larger than the global mean sea level rise over 1950–2009.

Vertical coherency function model of spatial ground motion

Many studies have focused on horizontal ground motion, resulting in many coherency functions for horizontal ground motion while neglecting related problems arising from vertical ground motion. However, seismic events have demonstrated that the vertical components of ground motion sometimes govern the ultimate failure of structures. In this paper, a vertical coherency function model of spatial ground motion is proposed based on the Hao model and SMART 1 array records, and the validity of the model is demonstrated. The vertical coherency function model of spatial ground motion is also compared with the horizontal coherency function model, indicating that neither model exhibits isotropic characteristics. The value of the vertical coherency function has little correlation with that of the horizontal coherency function. However, the coherence of the vertical ground motion between a pair of stations decreases with their projection distance and the frequency of the ground motion. When the projection distance in the wave direction is greater than 800 meters, the coherency between the two points can be neglected.

Seismic response of liquid storage steel tanks with variable frequency pendulum isolator

The seismic response of liquid storage steel tanks (slender and broad) isolated with Variable Frequency Pendulum Isolators (VFPIs) is investigated under normal component of six near-fault ground motions. The continuous liquid mass is lumped as convective mass, impulsive mass and rigid mass. The corresponding stiffness associated with these lumped masses is worked out depending upon the properties of the tank wall and liquid mass. The frictional forces mobilized at the interface of the VFPI are assumed to be velocity independent. The governing equations of motion of liquid storage steel tanks isolated with the VFPIs are derived and solved in the incremental form using Newmark’s step-by-step method assuming linear variation of acceleration over small time interval. For comparative study, the seismic response of liquid storage steel tanks with the VFPIs is compared with that of the same liquid storage steel tanks isolated using the Friction Pendulum Systems (FPSs). In order to measure the effectiveness of isolation system, the seismic response of isolated steel tanks is compared with that of the non-isolated steel tanks. Further, a parametric study has been carried out to critically examine the behaviour of liquid storage steel tanks isolated with VFPIs. The important parameters considered are the friction coefficient of the VFPI, the Frequency Variation Factor (FVF) of the VFPI and the tank aspect ratio. The difference between the liquid storage tanks isolated with the VFPI and the FPS isolators subjected to the harmonic and far-field ground motions was also investigated in this study. Effect of vertical component of ground motions on the behaviour of the VFPI-isolated liquid storage tanks is investigated under triaxial ground excitations by considering the interaction of forces in two orthogonal directions. From these investigations, it is concluded that seismic response, viz. the base shear, the sloshing displacement and the impulsive displacement, of liquid storage steel tanks during near-fault ground motions can be controlled within a desirable range with the installation of the VFPI. Under strong harmonic excitations, the base shear of the VFPI reduces than that of the FPS whereas the sloshing displacement, the impulsive displacement and the isolator displacement of the VFPI exceeds than that of the FPS. It is also found that the isolation by the FPS and VFPI isolators has almost the same effect in the tank to the far-field ground motions. The triaxial ground motions have noticeable effect on the response of the VFPI-isolated liquid storage tanks relative to unilateral ground motion and if ignored, the sliding displacement and base shear will be underestimated.

Spectral Analysis of K-NET and KiK-net Data in Japan, Part II: On Attenuation Characteristics, Source Spectra, and Site Response of Borehole and Surface Stations

Abstract In this study we apply a nonparametric spectral inversion scheme to a data set of accelerograms recorded by the K-NET and KiK-net networks in Japan in order to derive attenuation characteristics, source spectra, and site response. For this purpose, we use a total of more than 67,000 S -wave records from 2178 earthquakes ( M JMA 2.7–8) obtained at 1555 stations at the Earth’s surface and more than 29,000 records from 1826 events recorded at 637 borehole stations at depths of 100 to 3000 m. Attenuation characteristics are investigated in five separate regions, showing that crustal Q depicts lower values in central compared to southern Japan, and a significant frequency dependence is observed in every region. The source spectra follow the ω 2 model with higher stress drops for subcrustal earthquakes as compared with crustal ones. While strong amplification effects dominate the site contributions for the surface sensors, those for the borehole sensors are characterized by smaller variability. Nevertheless, consistent with observations from deconvolution of borehole/surface recording pairs, downgoing wave effects are visible in the site contributions for many borehole stations. Finally, the site amplification functions obtained at the surface are compared with surface-to-borehole (S/B) and horizontal-to-vertical (H/V) spectral ratios, showing that the S/B ratios generally provide better estimates of the horizontal amplification than the H/V ratios due to amplification of the vertical component of ground motion.

Assessment of RC columns subjected to horizontal and vertical ground motions recorded during the 2009 L’Aquila (Italy) earthquake

In the aftermath of recent earthquakes there has been substantial field evidence demonstrating that the collapse of several existing structures was caused by the effects of the vertical component of seismic ground motions. Such field evidence has not yet been supported by comprehensive analytical assessment and experimental tests. The present work focuses on preliminary analyses of the seismic response of reinforced concrete (RC) members subjected to horizontal (HGMs) and vertical (VGMs) ground motions recorded during the 2009 L’Aquila (Italy) earthquake. Normalised axial loads in beam–columns as well as the peak ground acceleration ratios between horizontal and vertical ground accelerations are emphasised as they are considered parameters of paramount importance for the assessment of structural components and systems subjected to combined horizontal and vertical ground motions (HVGMs). Results of extensive parametric nonlinear dynamic analyses carried out on simplified structural models are discussed in detail. The sample models comprise cantilever RC columns and a two-storey, two-bay plane frame designed for gravity loads. The structural response quantities for the performed analyses are expressed in terms of axial loads, axial deformations, bending moment-axial load interaction and shear demand/capacity ratios. It is found that the variation of axial loads is significant in columns under HVGMs, especially in compression. For values of normalised axial loads ( ν ) corresponding to actual RC columns in framed building structures, e.g., normalised axial load ν > 0.10 , the average increase of the compression load ranges between 174% ( ν = 0.20 ) and 59% ( ν = 0.50 ). For high values of normalised axial loads the computed axial load-bending moment pairs lie beyond the threshold interaction curves and, in turn, the RC members may fail. The shear demand-to-supply ratio is also detrimentally affected by the high fluctuations of axial loads in the columns. Net tensile forces were computed for columns with low-to-moderate axial gravity preload. In multi-storey framed buildings, the response of central columns is significantly affected by the HVGMs. Reliable seismic performance assessment of framed systems requires that combined HGMs and VGMs should be accounted for in the analyses. Further experimental and numerical research is needed to formulate efficient mechanical models to evaluate the shear capacity of structural members of existing RC framed buildings under earthquake loading.

Site Effects Assessment in Bishkek (Kyrgyzstan) Using Earthquake and Noise Recording Data

Abstract Kyrgyzstan, which is located in the collision zone between the Eurasian and Indo-Australian lithosphere plates, is prone to large earthquakes as shown by its historical seismicity. Hence, an increase in the knowledge and awareness by local authorities and decision makers of the possible consequence of a large earthquake, based on improved seismic hazard assessments and realistic earthquake risk scenarios, is mandatory to mitigate the effects of an earthquake. To this regard, the Central Asia Cross-Border Natural Disaster Prevention (CASCADE) project aims to install a cross-border seismological and strong motion network in Central Asia and to support microzonation activities for the capitals of Kyrgyzstan, Uzbekistan, Kazakhstan, Tajikistan, and Turkmenistan. During the first phase of the project, a temporary seismological network of 19 stations was installed in the city of Bishkek, the capital of Kyrgyzstan. Moreover, single-station noise recordings were collected at nearly 200 sites. In this study, the site amplifications occurring in Bishkek are assessed by analyzing 56 earthquakes extracted from the data streams continuously acquired by the network, as well as from the single-station noise measurements. A broadband amplification (starting at ∼0.1 and 0.2 Hz), is shown by the standard spectral ratio (SSR) results of the stations located within the basin. The reliability of the observed low-frequency amplification was validated through a time–frequency analysis of denoised seismograms. Discrepancies between horizontal-to-vertical spectral ratio and SSR results are due to the large amplification of the vertical component of ground motion, probably due to the effect of converted waves. The single-station noise results, once their reliability was assessed by their comparison with the earthquake data, have been used to produce the first fundamental resonance frequency map for Bishkek, whose spatial variation shows a good agreement with the presence of an impedance contrast within the Tertiary sedimentary cover.

Parametric vibration of TMD installed in tall building during seismic excitation

AbstractIn this paper shaking‐table tests of a model‐scale building with TMD (tuned mass damper) are conducted. The main aim of the research is to show the influence of kinematical excitation on the dynamic response of the model of tall building with pendulum TMD. The special attention is focused on the functioning of pendulum, which is simultaneously excited by the vertical component of ground motion. There are two kinds of analysis carried out — numerical and experimental. The numerical analysis of the problem is performed with using COSMOS/M system (a FEM numerical model is defined), while experimental analysis is carried out on a physical model‐scale building with TMD. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vector-Valued Probabilistic Seismic Hazard Assessment for the Effects of Vertical Ground Motions on the Seismic Response of Highway Bridges

The vertical ground motion component is disregarded in the design of ordinary highway bridges in California, except for the bridges located in high seismic zones (sites with design horizontal peak ground acceleration greater than 0.6 g). The influence of vertical ground motion on the seismic response of single-bent, two-span highway bridges designed according to Caltrans Seismic Design Code (SDC-2006) is evaluated. A probabilistic seismic hazard framework is used to address the probability of exceeding the elastic capacity for various structural parameters when the vertical component is included. Negative mid-span moment demand is found to be the structural parameter that is most sensitive to vertical accelerations.A series of hazard curves for negative mid-span moment are developed for a suite of sites in Northern California. The annual probability of exceeding the elastic capacity of the negative mid-span moment is as large as 0.01 for the sites close to active faults when the vertical component is included. Simplified approaches based on the distance to major faults or the median design peak acceleration show that there is a large chance (0.4 to 0.65) of exceeding the elastic limit if the current 0.6 g threshold is used for the consideration of vertical ground motions for ordinary highway bridges. The results of this study provide the technical basis for consideration of a revision of the 0.6 g threshold.

Seismic Response of Base-Isolated Benchmark Building with Variable Sliding Isolators

The seismic response of base-isolated benchmark building with variable sliding isolators like variable friction pendulum system (VFPS), variable frequency pendulum isolator (VFPI), and variable curvature friction pendulum system (VCFPS), along with conventional friction pendulum system (FPS), was studied under the seven earthquakes. The earthquakes are applied bi-directionally in the horizontal plane ignoring vertical ground motion component. The shear type base-isolated benchmark building is modeled as three-dimensional linear elastic structure having three degrees of freedom at each floor level. Time domain dynamic analysis of the benchmark building was carried out with the help of constant average acceleration Newmark-Beta method and nonlinear isolation forces was taken care by fourth-order Runge-Kutta method. The base-isolated benchmark building is investigated for uniform isolation and hybrid isolation in combination with laminated rubber bearings through the performance criteria and time history response of important structural response parameters like floor accelerations, base displacement, etc. It is observed that variable sliding isolators performed better than conventional FPS due to their varying characteristic properties which enable them to alter the isolator forces depending upon their isolator displacements thus improves the performance of the structure. The VFPS efficiently controls large isolator displacements and VFPI and VCFPS improve super structural response on the cost of isolator displacement. It is also observed that the hybrid isolation is relatively better in comparison to the uniform isolation for the benchmark building.

Analytical Assessment of the Effect of Vertical Earthquake Motion on RC Bridge Piers

This paper presents analytical assessment of the effect of vertical earthquake ground motion on RC bridge piers. A bridge structure damaged during the Northridge Earthquake and a Federal Highway concept bridge design are examined. The effects of a suite of earthquake ground motion records with different vertical-to-horizontal peak acceleration ratios on the two bridges are presented and the results are compared with the case of horizontal-only excitation. The effects of arrival time interval between horizontal and vertical acceleration peaks are also reported and compared to the case of coincident motion. It is observed that the inclusion of the vertical component of ground motion has an important effect on the response at all levels and components. It is therefore concluded that vertical motion should be included in analysis for assessment and design, especially that there are no particular challenges impeding its inclusion.

Response of the Double Variable Frequency Pendulum Isolator under Triaxial Ground Excitations

Modeling techniques of double variable frequency pendulum isolator (DVFPI) are presented. Seismic responses of the three-dimensional single-story building isolated by DVFPI with different coefficient of friction and initial time period of top and bottom sliding surfaces are investigated under triaxial ground motions and compared with unilateral and bilateral ground excitations. Furthermore, the influence of vertical flexibility on horizontal response is also investigated. It is observed that the triaxial ground motion has noticeable effect on response of the building relative to unilateral ground motion. The error in peak resultant base shear, occurring by neglecting vertical ground motion component, could be significant.

Characterization of shaking intensity distribution and seismic assessment of RC buildings for the Kashmir (Pakistan) earthquake of October 2005

On October 8, 2005, an earthquake of magnitude M w 7.6 shook northern Pakistan particularly the Kashmir region. With nearly 73,000 dead, 70,000 injured, 270,000 buildings destroyed, and 180,000 damaged, the earthquake ranks amongst the worst natural disasters in the history of Pakistan and the Indian subcontinent. In this paper, the shaking intensity distribution of the affected region is reconstructed using the limited ground motion data available. Selection of a suite of records representative of characteristics of the Kashmir earthquake at locations of major damage is undertaken. An ensemble of buildings is collated which represents (i) actual Pakistan reinforced concrete design, (ii) general non-seismic and (iii) code-conforming buildings with different levels of detailing. The buildings are subjected to the selected records, including the vertical component of the earthquake ground motion thought to be significant in this earthquake. Conclusions are drawn with regard to the relative performance of the different types of building investigated, the effect of different levels of design and detailing, and the effect of the vertical earthquake component on damage. It is observed that buildings that are seismically designed to contemporary codes would have survived the earthquake. However, the vertical motion would have caused significant reduction of shear capacity in vertical members. The extensive results reported in the paper are useful for practicing engineers operating in areas of high seismicity where limited seismic design and construction quality control exist, as well as code drafting panels interested in the effect of multi-axial excitation on reinforced concrete buildings.

Response Spectra for Near-Source, Differential, and Rotational Strong Ground Motion

It is shown that the pseudorelative spectral velocity (PSV) of an equiva- lent oscillator, which can be represented by a single degree-of-freedom system for excitation by synchronous horizontal excitation, can be extended to describe the PSV spectra of the same oscillator when excited by simultaneous action of horizontal, vertical, and rocking components of strong ground motion. At short periods, the new spectra are governed by differential ground motion and peak ground velocity, and they depend on the transit time of the waves along the length of the structure and on the oscillator frequency. At long periods, PSV spectral amplitudes tend toward an asymp- tote with amplitude proportional to the maximum rocking angle of ground motion.

Site Amplifications Observed in the Gubbio Basin, Central Italy: Hints for Lateral Propagation Effects

Abstract In this study we investigate the site amplification effects observed in the Gubbio plain, central Italy. The recordings of 140 local earthquakes (1.2≤ Ml ≤4.7) observed by two linear arrays installed along the longitudinal and perpendicular basin axis are analyzed both in the time and frequency domains to determine the spatial variation of the local site effects in the basin. The time series recorded in the Gubbio plain are characterized by locally generated surface waves, which cause a significant increase in both ground-motion duration and amplitude with respect to a nearby reference station on a rock site. In the middle of the basin where the sedimentary cover is thickest (ca 600 m), the peak ground velocity is amplified on average by a factor of 5 with respect to the reference station installed on rock, and the duration is increased by a factor of about 2. The analyses in the frequency domain show that the spectral energy of the basin-generated waves is mainly distributed over the range 0.4–2 Hz and affects both horizontal and vertical components. The site responses estimated by applying two reference site methods show that the overall shape of the amplification functions is similar for the sites located inside the basin with a difference in the amplification levels, which can reach values as high as 30 in the middle of the valley. Amplifications up to a factor of 10 affect the vertical component of ground motion over a broad frequency range for nearly all basin sites with the maximum amplifications occurring at around 1 Hz. Finally, although the horizontal-to-vertical spectral ratios were found to provide a reliable estimation of the fundamental resonance frequency of the site, they fail to estimate the shape of the site transfer function.

Estimation of Earthquake-Induced Crest Settlements of Embankments

Problem statement: Freeboard requirement, a major consideration in the design of embankment dams, is controlled to a great extent by the crest settlements during earthquakes. Approach: The parameters that influence earthquake induced crest settlement had been studied using 152 published case histories on performance of embankment dams during earthquakes. Results: Based on the results a correlation had been proposed for obtaining preliminary estimates of earthquake-induced crest settlements. The correlation used the ratio of the peak horizontal ground acceleration and the yield acceleration as the estimator. Conclusion/Recommendations: The database analysis also indicated that crest settlements are larger where the fundamental periods of the embankment were similar to the predominant periods of the earthquake. Earthquake magnitude and the vertical component of earthquake ground motion, on the other hand, appeared to have a small influence on crest settlement.

Seismic Response of Tall Guyed Masts to Asynchronous Multiple-Support and Vertical Ground Motions

The seismic behavior of tall masts was investigated with special considerations for the realistic modeling of ground motion at the supports. Three existing masts with varying heights of 213, 313, and 607 m were numerically studied under the effects of three earthquake records with all three translational ground motion components. The earthquake excitation was prescribed as a displacement-controlled motion providing the opportunity to consider the asynchronous shaking of the cable ground anchors and mast base. This effect was studied by varying the shear wave velocity of the traveling wave corresponding to different degrees of soil stiffness and observing the general trends of selected response indicators. The three towers showed sensitivity to asynchronous shaking of their ground supports. More severe structural response was obtained for softer soil conditions, and the 607 m mast showed sensitivity even for relatively stiff soils. The effects of considering or excluding the structural damping and the vertical component of ground motion were also separately investigated and both factors were found to affect the response indicators significantly.

A preliminary report on the Great Wenchuan Earthquake

The May 12, 2008 Great Wenchuan Earthquake has resulted in more than 68,858 deaths and losses in the hundreds of billions RMB as of May 30, 2008, and these numbers will undoubtedly increase as more information becomes available on the extent of the event. Immediately after the earthquake, the China Earthquake Administration (CEA) responded quickly by sending teams of experts to the affected region, eventually including over 60 staff members from the Institute of Engineering Mechanics (IEM). This paper reports preliminary information that has been gathered in the first 18 days after the event, covering seismicity, search and rescue efforts, observed ground motions, and damage and loss estimates. The extensive field investigation has revealed a number of valuable findings that could be useful in improving research in earthquake engineering in the future. Once again, this earthquake has shown that the vertical component of ground motion is as significant as horizontal ground motions in the near-source area. Finally, note that as more information is gathered, the numbers reported in this paper will need to be adjusted accordingly.