Spectral Attenuation Relations at Soft Sites Based on Existing Attenuation Relations for Rock Sites

A simple and general method based on well-known random vibration theory is used to compute spectral attenuation relations at soft sites based on existing spectral attenuation relations at rock sites. The method consists of: (1) computation, for given magnitude and distance, of the expected Fourier amplitude spectrum associated with the median rock response spectrum computed with the attenuation relation; (2) inclusion of site effects characterized by a frequency-dependent, linear, or nonlinear transfer function; and (3) computation of the response spectrum at the soft site.     

SEISMIC DESIGN REGULATION CODES: CONTRIBUTION OF K-NET DATA TO SITE EFFECT EVALUATION

The purpose of this study is to compare the site effect section of building codes (EC8 and UBC97) with the set of data provided by the Kyoshin network. In order to obtain a set of site coefficients and spectral shapes, we have first deduced an attenuation law for both horizontal and vertical motion. Site conditions are represented by the shear velocity averaged over the upper 30 m (V _s ³⁰). Our site classification (4 categories similar to those proposed in the new ECS and the UBC97) is based on borehole investigations at every station. This classification has permitted to distinguish clearly four response spectra which demonstrates the efficiency of V _s ³⁰ as characterising site conditions. Our law is then used to test site coefficients and spectral shapes of building codes ECS and UBC97. Concerning spectral shapes and site coefficients, our results are found to be in good agreement with EC8 and UBC97 only if category B (400<V _s ³⁰<800 m/s) is taken as reference. We also conclude that a site which is characterised as “rock” on geological criteria can not generally be classified in category A (V _s ³⁰>800 m/s). This suggests that classification in category A should be based only on field measurements. Concerning vertical motion, our analysis of the K-NET data shows that the ratio av/ah (vertical peak ground acceleration over horizontal peak ground acceleration) is between 0.50 and 0.68.     

EVALUATION OF ACCIDENTAL ECCENTRICITY IN BUILDINGS DUE TO ROTATIONAL COMPONENT OF EARTHQUAKE

This study adopts a random procedure in the evaluation of the effect of the rotational component of earthquake on the accidental eccentricity of symmetric and asymmetric buildings. The spectral density function of the rotational component of earthquake acceleration (about the vertical axis) is obtained on the basis of the spectral density function of the horizontal component of earthquake acceleration. The rotational component of an earthquake can increase the response of the structure. The degree of the increase is highly dependent upon the dynamic characteristics of the system and the rotational component of the earthquake. To bring this increase under consideration, seismic codes represent a parameter referred to as accidental eccentricity, as a part of the design eccentricity. The purpose of the present study is to estimate the value of this increase and to make appropriate suggestions based on frequency domain analysis.     

Site Classification Assessment for Estimating Empirical Attenuation Relationships for Central-Northern Italy Earthquakes

The aim of this article is to investigate the ground motion attenuation of the most industrialized and populated regions of Italy, evaluating the capability of different approaches to estimate site dependent models. The 5.2 local magnitude earthquake on November 24, 2004 shocked the areas of Northern Italy producing damage of about 215 million euros. The data set, including 243 earthquakes of local magnitude up to 5.2, has been collected in the period December 2002–October 2005 by 30 three-component seismic stations managed by Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Milano (INGV-MI). Empirical attenuation relationships have been estimated for horizontal peak ground velocity (PGHV), acceleration (PGHA), displacement (PGHD), and for response spectral acceleration (SA) for periods between 0.1 and 1.5 s. To estimate suitable attenuation models, in particular for sites characterized by thick sedimentary geological formations, a soil discrimination based on EU8 code can lead to wrong evaluations. On the contrary, a classification based on H/V spectral ratios of seismic ambient noise (NHV) allows the models to fit better real and predicted data and to reduce the uncertainties of the process. For each receiver, NHV have been strengthened by additional H/V spectral ratio of earthquake data (EHV), calculated considering different portions of the analysed signals. In order to validate the PGHA attenuation relationship for greater magnitudes, accelerometric records, relative to Central-Northern Italy strong motions occurring in the last 30 years, have been collected and superimposed to our attenuation curves.     

Ground-Motion Prediction Equations Based on Data from the Himalayan and Zagros Regions

This study derives ground-motion prediction equations for the horizontal elastic response spectral acceleration for 5% damping for application to the Indian Himalayas. The present equations include a consideration of site category (rock/soil) and style-of-faulting (strike-slip/reverse). Due to a lack of near-field data from India, additional strong-motion data have been included from the Zagros region of Iran, which has comparable seismotectonics to the Himalayas (continental compression). A set of 201 records from 16 earthquakes were used within the regression. The derived model predicts similar ground motions to previously published equations for the Himalayan region but with lower standard deviations.     

Inelastic Demand Spectra of Bi-Linear Models for Capacity Spectrum Method and Response of Simple Structures under Near-Source Records

A very useful tool for the preliminary design of structures is the elastic demand spectrum that can be used in the capacity spectrum method. A pseudo-acceleration relationship has to be assumed when constructing a demand spectrum. This assumption results in large errors for long period structures with large damping ratios and the conventional demand spectra require a substitute elastic structure. In the present study, the conventional demand spectra are extended to bi-linear models. Pseudo-acceleration is still assumed but results in acceptably small errors, when a constant viscous damping coefficient for a single-degree-of-freedom (SDF) structure is calculated from the tangent stiffness and the damping ratio is set at 5% in both elastic and yield phases. For nonlinear structures, tangent stiffness dependency of damping force could be acceptable because energy absorption is primarily the result of structural nonlinear deformation. To extend the conventional demand spectra to a bi-linear model, effective period calculated from the secant stiffness has to be used. The use of effective period introduces no approximation because the peak displacement of the SDF structure is computed from nonlinear analysis in the time domain. The method presented in this study is also valid if damping coefficient proportional to initial elastic spectra is used. In this case, the pseudo-acceleration is defined as the base shear coefficient that is required to produce the peak displacement of the SDF structure in a static manner. We present demand spectra of bi-linear models for a number of near-source records from large earthquakes, and spectral ratios of two horizontal components. The effects of different types of ground motion on the response reduction factor due to inelastic deformation are investigated.     

Vector-valued Intensity Measures Incorporating Spectral Shape For Prediction of Structural Response

Vector-valued ground motion intensity measures (IMs) are developed and considered for efficiently predicting structural response. The primary IM considered consists of spectral acceleration at the first-mode structural period along with a measure of spectral shape which indicates the spectral acceleration value at a second period. For the IM to effectively predict response, this second period must be selected intelligently in order to capture the most relevant spectral shape properties. Two methods for identifying effective periods are proposed and used to investigate IMs for example structures, and an improvement in the efficiency of structural response predictions is shown. A method is presented for predicting the probability distribution of structural response using a vector IM while accounting for the effect of collapses. The ground motion parameter ε is also considered as part of a three-parameter vector. It is seen that although the spectral shape parameter increases the efficiency of response predictions, it does not fully account for the effect of ε. Thus, ε should still be accounted for in response prediction, either through informed record selection or by including ε in the vector of IM parameters.     

Nonlinear Structural Performance of a Historical Brick Masonry Inverted Dome

ABSTRACT

Traditional domes are obtained by double curvature shells, which can be rotationally formed by any curved geometrical plane figure rotating about a central vertical axis. They are self-supported and stabilized by the force of gravity acting on their weight to hold them in compression. However, the behavior of inverted domes is different since the dome is downward and masonry inverted domes and their structural behaviors in the literature received limited attention. This article presents a nonlinear finite element analysis of historical brick masonry inverted domes under static and seismic loads. The brick masonry inverted dome in the tomb of scholar Ahmed-El Cezeri, town of Cizre, Turkey, constructed in 1508 is selected as an application. First, a detailed literature review on the masonry domes is given and the selected inverted dome is described briefly. 3D solid and continuum finite element models of the inverted masonry dome are obtained from the surveys. An isotropic Concrete Damage Plasticity (CDP) material model adjusted to masonry structures with the same tensile strength assumed along the parallel and meridian directions of the inverted dome is considered. The nonlinear static analyses and a parametric study by changing the mechanical properties of the brick unit of the inverted masonry dome are performed under gravity loads. The acceleration records of vertical and horizontal components of May 1, 2003 Bingöl earthquake (Mw = 6.4), Turkey, occurred near the region, are chosen for the nonlinear seismic analyses. Nonlinear step by step seismic analyses of the inverted dome are implemented under the vertical and horizontal components of the earthquake, separately. Static modal and seismic responses of the inverted masonry dome are evaluated using mode shapes, minimum and maximum principal strains and stresses, and damage propagations.     

Impact of Vertical Ground Excitation on a Bridge with Footing Uplift

Ground motions recorded in the epicentral region of an earthquake often have a strong vertical component with dominant high frequencies. Damage to bridges in near-source regions due to strong vertical ground motion has been reported. The beneficial effects of footing uplift on structural performance in form of reduction of seismic response of structural members have been confirmed in previous research. The uplift of bridge piers has been utilised in a very limited number of bridge structures, e.g., the South Rangitikei railway bridge in New Zealand. However, the near-fault seismic behaviour of bridges with footing uplift has been even less addressed. In this study shake table investigations were carried out on the response of a single-span bridge model with footing uplift subjected to simultaneous vertical and horizontal excitations. Near-fault ground motions recorded in the Canterbury earthquake sequences of 2010 and 2011 were used. The experimental results show that inclusion of vertical ground motions produce stronger axial force in the pier and larger bending moment in the deck. Concurrent horizontal and vertical excitations may also cause more frequent footing uplift than the solely horizontal excitations.     

Time-Domain Spectral Matching of Earthquake Ground Motions using Broyden Updating

Time-domain spectral matching of an earthquake ground motion consists of iteratively adding sets of wavelets to an acceleration history until the resulting response spectrum sufficiently matches a target spectrum. The spectral matching procedure is at its core a nonlinear problem because the addition of a wavelet often causes shifting in the time of peak response or creation of a larger second peak at a different time. A modification to existing time-domain spectral matching algorithms is proposed using Broyden updating for solving the set of nonlinear equations. Three wavelet bases are evaluated and the corrected tapered cosine wavelet is selected. The proposed algorithm is then tested and compared with other methods that are commonly used for spectral matching. The results show that the proposed algorithm is able to match the target spectrum while reasonably preserving the spectral nonstationarity, energy development, and the frequency content of the original time histories.     

FINITE FAULT MODELLING OF NEAR-FIELD ROCK MOTIONS IN THE NEW MADRID SEISMIC ZONE

Due to lack of strong motion records, point-source and finite-fault models have been used to simulate far-field motions at Memphis and St. Louis Cities from earthquake events in the New Madrid Seismic Zone. However, near-field rock motions and their associated uncertainties have never been studied within this zone. The objectives of this study are to develop a simple procedure to account for the uncertainty effect of earthquake source parameters, to analyze the sensitivity of near-field rock motions to input source parameters, and finally, to generate rock motions at two sites located within 11 km from the southwestern segment (strike-fault) and a third site bove the Reelfoot Rift (reverse fault) using a well-validated finite-fault simulation program; FINSIM. An equal-weight logic tree was developed to ensure that the assumed uncertainties are within physical, geological, and seismological constraints. For each site, 100 acceleration time histories with various combinations of parameter uncertainties were respectively simulated for an earthquake of M _w 7.0, 7.5, and 8.0 from each of the two faults. Their average spectral accelerations were in good agreement with those derived from the attenuation relation-ships representative to the Central and Eastern United States. Numerical simulations indicated that spectral accelerations are sensitive to the slip velocity, depth to top of fault, fault strike, slip distribution, and hypocentre location along the strike.     

Effect of the joint type on the bearing capacity of a multi–drum column under static load

The results of an experimental study on the effect of the joints between the blocks on the ultimate bearing capacity of a multi-drum column loaded to centric vertical force and horizontal force in the middle of its height are shown. The column is approximately 2.5 m high, with one hinge at the top and another hinge at the bottom. Four types of joints between the blocks were considered: a dry joint (column C-DJ) and joints with stone powder (column C-SPJ), lead (column C-PBJ), and epoxy (column C-EPJ). The applied vertical and horizontal forces, horizontal displacement, vertical strains, and horizontal circumferential strains in the middle of the column height, as well as shortening of the column, were measured. Under axial compression, ratios between the ultimate load bearing capacities of tested columns were C-EPJ: C-DJ: C-SPJ: C-PBJ = 1: 0.68: 0.59: 0.51. The bearing capacity of the tested columns with regard to the horizontal force depended on the level of the applied centric compression force. Columns with soft joints (C-PBJ, C-SPJ) had the largest shortening and the largest horizontal displacements for the equal forces.     

NEW EMPIRICAL RESPONSE SPECTRAL ATTENUATION LAWS FOR MODERATE EUROPEAN EARTHQUAKES

This paper presents response spectral attenuation laws used in the new French Safety Rule, which is the reference for nuclear safety studies in France. Attenuation laws were derived from 965 horizontal and 485 vertical components from a two-step inversion method and accounts for geometrical spreading, anelastic attenuation and geological site condition. The datasets are mainly constituted of European strong motion records (83%) recently collected and homogeneously processed. In order to complete the distribution data beyond magnitude 6, a few American records were added, representing 17% of the datasets. The magnitude type and source-to-site distance definitions chosen to derive the laws are tested with respect to other definitions. These parametric tests induce a conservative law, for some magnitude and distance ranges of interest. The residual values between observed and predicted spectral accelerations are studied and do not exhibit any bias. The inferred laws are in good agreement with classical strong motion attenuation laws.     

Discoveries of Neolithic prehistoric sites at Pleistocene carbonate rock shelters on the east coast of the UAE

Three newly discovered prehistoric sites on the east coast of the United Emirates (UAE) are described. All are located on surfaces of Pleistocene carbonates or rock shelters that are generally rare along the eastern coast of the Gulf of Oman. Aqqah 1 (Le Meridien al Aqqah Beach Resort), the most important and best preserved of these sites, is a partially collapsed rock shelter with an exposed section, lithic finds and marine molluscs. Deriving an exact date from the material present is difficult because of a lack of comparanda. A bifacial fletched arrowhead made of yellow jasper and the lithic debris of five different raw materials as well as an undecorated ceramic fragment might suggest a date in the Late Neolithic or Early Bronze Age. The presence of many marine bivalves and snails with operculae, which differ from recent coastal species, indicates the collection and consumption of living molluscs by the prehistoric population of Aqqah. Nearby burials may be related to the rock shelters.     

Rocking Soil-Structure Systems Subjected to Near-Fault Pulses

Seismic performance of rocking soil-structure systems subjected to near-fault pulses is investigated considering foundation uplifting and soil plasticity. An extensive parametric study is conducted including medium-to-high-rise buildings with different aspect ratios based on shallow raft foundation at stiff-to-rock sites. Mathematical directivity and fling pulses are used as input ground motion. The superstructure is assumed to have three different boundary conditions: (a) fixed-base, (b) linear soil-structure interaction (SSI), and (c) nonlinear SSI. Evidently, the prevailing pulse period T_p is a key parameter governing nonlinear SSI effects. The normalized acceleration response spectra reveal that despite beneficial effects of foundation uplifting and soil yielding in most cases, there are some minor regions in which the response accelerations are amplified. In addition, more slender buildings significantly benefit from uplifting and soil yielding when subjected to short- and medium-period directivity pulses compared to squat structures. However, response amplifications with respect to fixed-base structures are considerable in case of slender structures subjected to medium- or long-period directivity pulses. So that neglecting the SSI effects on seismic performance of rocking structures with shallow foundations, as mostly assumed in common practice, may give rise to inaccurate estimations of force demands against near-fault pulselike ground motions. Furthermore, the envelope of residual foundation tilting θ_r is limited to 0.015 rad, in case of directivity pulses.     

SITE AMPLIFICATION FACTOR IN ANCHORAGE,ALASKA BASED ON LONG PERIOD MICROTREMORS

Long period microtremors with periods ranging from 0.5 to 10 seconds were measured in the Anchorage metropolitan area. Two horizontal components of motion were recorded at 81 sites uniformly distributed throughout the basin with spatial resolution of about 2 km. Recording at each site was done for 300 seconds with a sampling rate of 20 Hz. Repeated measurements were performed at a bedrock reference site simultaneously with the measurements in the field. The measurements were completed in six days. In addition, multiple recordings were obtained concurrently at the reference bedrock site and a sediment site. Based on these measurements the Fourier spectra were calculated for each of the site. Ground motion amplification is determined in terms of spectral ratio of horizontal spectral amplitudes at a sediment site and the reference bedrock site. Mean spectral ratio contours were evaluated for different period bands. The results show that for period band 3 to 5 seconds the spectral ratio contours agree well with the ground failure susceptiblity map of Anchorage.     

Simultaneous observations of E and F region electric field fluctuations at the magnetic equator

Simultaneous daytime observations of E region horizontal irregularity drift velocities in the equatorial electrojet and F region vertical plasma drifts were made on a few magnetically quiet days at the magnetic equatorial station of Trivandrum (dip 0.5°N). Measurements of the electrojet irregularity velocities by VHF backscatter radar and the F region vertical plasma drifts by HF Doppier radar are used to deduce the daytime East-West electric fields in the E and F regions, respectively. The fluctuating components of the electric fields are separated and subjected to power spectral analysis. The E and F region electric field fluctuations are found to be well correlated; the estimated correlation coefficient is in the range of 0.52–0.8. The fluctuation amplitudes are of the order of 15% over the background for the E region and 25% for the F region. The spectral analysis reveals dominant components in the range of 30–90 min with F region components stronger than those of the E region by a factor of about 1.5 on the average. The F region electric fields during daytime being coupled from the low latitude E region, the good correlation observed between the E and F region perturbations suggests that the electric fields in the E region at low and equatorial latitudes are coherent for the temporal scales of the order of few tens of minutes. The spectral characteristics are such that the commonly occurring medium scale gravity waves could possibly be the source for the observed fluctuations in the E and F region electric fields.     

Equatorial spread-F and neutral atmospheric turbulence : a review and a comparative anatomy

A number of satellite and rocket plasma density spectra obtained during equatorial spread-F conditions are presented and discussed in the light of similar measurements in the neutral atmosphere. We discuss this comparison in some detail and find both distinct similarities and subtle differences. The horizontal spectral measurements show a peak at an outer scale quite similar to the scale of the undulations caused by gravity wave interactions with the ionosphere. This feature is similar to a buoyancy subrange but it is easy to show that the amplitudes of the plasma fluctuations are too large to be directly driven by the neutral atmosphere. At intermediate scales the plasma fluctuations have a one-dimensional horizontal spectrum with a power law well described by a ($\text{−5}\text{3}$) slope. Once again it can be shown that the neutral fluid cannot be similarly structured at 400 km altitude due to the high viscosity coefficient. A plasma cascade process seems to be operating but it is not at all clear how the spectrum is formed. Furthermore, vertical power spectra seem to run the gamut in spectral form with slopes (n) varying in the range from −1 to −3. So the horizontal spectra are near universal in form, while the vertical spectra are quite variable.     

Observational evidence of a saturated gravity wave spectrum in the mesosphere

Five vertical profiles of scalar horizontal winds have been measured at high resolution (25m) in the range from 80–95 km during the last salvo of the MAC/SINE campaign in the summer 1987 at Andenes, Northern Norway (69.3°N). Our purpose in this study is to examine the consistency of the motion spectrum with the saturated spectrum of gravity waves proposed by Smith S. A., Fritts D. C. and Van Zandt T.E., (1987, J. atmos. Sci. 44, 1404). An analysis of vertical wavenumber spectra of the five horizontal wind profiles is presented and it is found that (a) the average slope of the five vertical wavenumber spectra is −3.0 ± 0.2 for wavelengths in the range from 6.4 km to 100 m. The slope is considerably steeper than the vertical wavenumber spectra of the horizontal velocity discussed in the literature, (b) the average vertical wavenumber spectrum shows that there is excellent agreement between the observed spectrum and the saturated spectrum in both slope and amplitude, suggesting that saturation processes do indeed act to control spectral amplitudes at large wavenumbers, and (c) a dominant vertical wavelength of 6.4 km is found in the mesosphere. Taken together, our observations provide further support for the saturated spectrum theory.     

QUICK ESTIMATES OF SOFT SEDIMENT THICKNESSES FROM AMBIENT NOISE HORIZONTAL TO VERTICAL SPECTRAL RATIOS: A CASE STUDY IN SOUTHERN ITALY

An approach devoted to quickly assess the thickness of soft sedimentary cover in areas of unknown subsurface morphology is applied in this study. In particular, soil thickness (h) is derived by combining estimates of the resonance frequency (f _r ) relative to soft sediments with the local shear-wave velocity (V_s) profile. For this purpose, (f _r ) values are assessed from horizontal to vertical (H/V) spectral ratios of seismic noise recordings and the (V _s ) profile is obtained by considering information from shallow seismic surveys. Results obtained for a Quaternary sedimentary basin in Southern Italy are discussed. Since in the investigated area only weak independent constraints are available, special emphasis is given to the assessment of uncertainties involved in this estimate of soil thickness.