Near-field Motions Research Articles

Performance of seismic shear panels under near-field motions

The purpose of this study is to describe the structural response of steel frames with different stiffness and geometrical characteristics subjected to near-field earthquakes. Such actions, in fact, may be responsible of high levels of damage, as they are characterized by pulses of high amplitude. Thus, the effects on the structures are quite different from the ones induced by far-field earthquakes. In detail, the main aim is to evaluate the performance of such frames equipped with passive devices: two hysteretic dampers are considered which during the seismic action are mainly interested by shear stresses. Nevertheless, the main characteristics of their hysteretic behavior are different and, consequently, for a correct use the comparison of the performance is an important tool. Thus, the response of the protected frames is evaluated varying the characteristics of the near-field motions.

Online tuning of fractional order fuzzy PID controller in smart seismic isolated structures

This paper aims at designing an adaptive fractional order fuzzy proportional–integral-derivative controller for seismic control of smart base-isolated structure by means of variable friction dampers (VFD). One main challenge occurs when large displacement of the isolator happens due to near-field motions. To overcome this challenge, a solution is to use VFDs. However, the floor accelerations of the superstructure can increase because of sudden changes in the damper friction force of VFDs. Therefore, a suitable control strategy is desired to handle the displacement of isolator without any increase in superstructure acceleration responses during both far-field and near-field earthquakes. First, a sub-level fractional order fuzzy PID (FOFPID) controller is designed to reduce the isolator displacement without significant increase in roof acceleration based on a multi-objective optimization algorithm. Using an adaptive strategy, the fuzzy rule weights of the FOFPID controller are then tuned on-line based on information sensed from both the earthquake and the building responses. Considering nine important performance criteria and several real-data earthquakes, numerical studies are carried out for a benchmark base-isolated structure equipped with VFDs. Simulation results show the superior performance of the proposed controller in mitigation of seismic responses of base-isolated structure against various types of earthquakes in comparison with other controllers presented in previous researches.

Near-field non-radial motion generation from underground chemical explosions in jointed granite

Near-field non-radial motion generation from underground chemical explosions in jointed granite

Influence of Bidirectional Near-Fault Excitations on RC Bridge Piers

AbstractLateral load-resisting structural members, such as piers, are often analyzed under one component base excitation. The influence of bidirectional shaking is accounted for by using a simplified 30% rule. During seismic shaking, piers are often subjected to bidirectional ground motion, which leads to a complex combination of biaxial moments. In this backdrop, a RC bridge pier has been analyzed under a set of bidirectionally applied near-field motions with forward directivity (FD) and fling-step characteristics. Bidirectional interaction under near-fault motion is observed to substantially amplify damage, particularly for a stiff system (30% and 45%–50% under FD and fling motions, respectively). A complex combination of bidirectional load paths may have also contributed to increased vulnerability. Additional studies have shown that pulses with fling-step characteristics may often be more detrimental than FD, at least for the first mode-dominated systems. The roles of important ground-motion parameters...

Effects of higher modes and MDOF on strength reduction factor of elastoplastic structures under far and near-fault ground motions

Abstract In seismic codes, the force strength reduction factor is proposed to transform elastic to inelastic strength. The ductility reduction factor, R μ , plays a key role on R factor if no overstrength is present. The R μ is determined by SDOF systems. But the higher mode (HM) and Multi-Degree-of-Freedom (MDOF) effects need to be considered to extract the R factor . These effects are studied by proposing α v-MDOF via at least 1764 Nonlinear dynamic Analysis of 2D-frames. Also a sensitivity study has been performed on R μ and α v-MDOF . Results obtained from studies conducted on the frames, indicate that HM and MDOF effects have a considerable influence on the base shear. Also the α v-MDOF is typically higher than unity both for ordinary and for near-field earthquakes and it is affected by period, span number and ductility level. These effects are remarkable for near-field motions. Finally, a simplified practical expression is proposed to estimate the α v-MDOF .

Steel braced frames with dual-core SCBs and sandwiched BRBs: Mechanics, modeling and seismic demands

A new steel dual-core self-centering brace (DC-SCB) is developed to have a flag-shaped re-centering hysteretic response under cyclic loads. Axial deformation capacity of the brace is doubled by serial deformations of two sets of tensioning elements arranged in parallel. In this paper, the mechanics of the new brace is first explained by considering a fabrication error of the member length, followed by testing one DC-SCB to evaluate its cyclic performance. The member length fabrication error, which is even 1/3000 times the brace length, significantly decreases the initial axial stiffness of the brace but does not affect the overall hysteretic behavior. Finite element analysis is conducted on the specimen to verify the mechanics and hysteretic responses observed in the test. Finite element analyses are also performed on other 16 DC-SCBs to evaluate how tensioning element types, initial post-tensioned force, and friction force affect the cyclic performance of the brace. Additionally, three braced frames of varying heights are designed using two bracing members, DC-SCBs and sandwiched buckling-restrained braces (BRBs). Nonlinear time history analyses are conducted on these braced frames to obtain seismic demands under both design and maximum considerable levels of earthquake motions and near-field motions. Self-centering braced frames generally exhibit smaller peak interstory drifts and residual drifts than buckling-restrained braced frames.

Near-field motion of the Haiyuan fault zone in the northeastern margin of the Tibetan plateau derived from InSAR permanent scatterers analysis

The Haiyuan fault zone is a major discontinuity in the northeastern margin of the Tibetan plateau. A magnitude 8.5 earthquake occurred there in 1920. Both geological investigations and GPS measurements show that this fault zone is still highly active, with a slip rate of 3 to 10 mm/year , exhibiting a large range of variance in both space and time. We attempt to use the permanent scatterers interferometric (PSI) synthetic aperture radar technique to better detect the near-field motion of this fault zone. We process and analyze 38 scenes of ENVISAT/ASAR images from two neighboring descending orbits using the PSI method. The results show a remarkable velocity gradient of about 5 mm/year across the central segment of the fault zone and a rate of about 5 to 6 mm/year on its eastern segment. The motion senses are consistent with a left-lateral strike slip. The motion histories of most PS points show a stable linear variation trend in time series. In addition to these motion features that agree with those from geological, GPS and other observations, the dense PS analysis also reveals spatially continuous variations of crustal motion around the fault zone.

Elastic floor response spectra of nonlinear frame structures subjected to forward-directivity pulses of near-fault records

This article presents the statistical characteristics of elastic floor acceleration spectra that represent the peak response demand of non-structural components attached to a nonlinear supporting frame. For this purpose, a set of stiff and flexible general moment resisting frames with periods of 0.3-3.6 sec. are analyzed using forty-nine near-field strong ground motion records. Peak accelerations are derived for each single degree of freedom non-structural component, supported by the above mentioned frames, through a direct-integration time-history analysis. These accelerations are obtained by Floor Acceleration Response Spectrum (FARS) method. They are statistically analyzed in the next step to achieve a better understanding of their height-wise distributions. The factors that affect FARS values are found in the relevant state of the art. Here, they are summarized to evaluate the amplification and/or reduction of FARS values especially when the supporting structures undergo inelastic behavior. The properties of FARS values are studied in three regions: long-period, fundamental-period and short-period. Maximum elastic acceleration response of non-structural component, mounted on inelastic frames, depends on the following factors: inelasticity intensity and modal periods of supporting structure; natural period, damping ratio and location of non-structural component. The FARS values, corresponded to the modal periods of supporting structure, are strongly reduced beyond elastic domain. However, they could be amplified in the transferring period domain between the mentioned modal periods. In the next step, the amplification and/or reduction of FARS values, caused by inelastic behavior of supporting structure, are calculated. A parameter called the response acceleration reduction factor (<TEX>$R_{acc}$</TEX>), has been previously used for far-field earthquakes. The feasibility of extending this parameter for near-field motions is focused here, suggested repeatedly in the relevant sources. The nonlinearity of supporting structure is included in (<TEX>$R_{acc}$</TEX>) for better estimation of maximum non-structural component absolute acceleration demand, which is ordinarily neglected in the seismic design provisions.

Elastic floor response spectra of nonlinear frame structures subjected to forward-directivity pulses of near-fault records

This article presents the statistical characteristics of elastic floor acceleration spectra that represent the peak response demand of non-structural components attached to a nonlinear supporting frame. For this purpose, a set of stiff and flexible general moment resisting frames with periods of 0.3- 3.6 sec. are analyzed using forty-nine near-field strong ground motion records. Peak accelerations are derived for each single degree of freedom non-structural component, supported by the above mentioned frames, through a direct-integration time-history analysis. These accelerations are obtained by Floor Acceleration Response Spectrum (FARS) method. They are statistically analyzed in the next step to achieve a better understanding of their height-wise distributions. The factors that affect FARS values are found in the relevant state of the art. Here, they are summarized to evaluate the amplification and/or reduction of FARS values especially when the supporting structures undergo inelastic behavior. The properties of FARS values are studied in three regions: long-period, fundamental-period and short-period. Maximum elastic acceleration response of non-structural component, mounted on inelastic frames, depends on the following factors: inelasticity intensity and modal periods of supporting structure; natural period, damping ratio and location of non-structural component. The FARS values, corresponded to the modal periods of supporting structure, are strongly reduced beyond elastic domain. However, they could be amplified in the transferring period domain between the mentioned modal periods. In the next step, the amplification and/or reduction of FARS values, caused by inelastic behavior of supporting structure, are calculated. A parameter called the response acceleration reduction factor ሺ௔௖௖ ሻ , has been previously used for far-field earthquakes. The feasibility of extending this parameter for near-field motions is focused here, suggested repeatedly in the relevant sources. The nonlinearity of supporting structure is included in ሺ௔௖௖ ሻ for better estimation of maximum non-structural component absolute acceleration demand, which is ordinarily neglected in the seismic design provisions.

Change in response of bridges isolated with LRBs due to lead core heating

This study focuses on the response of seismic isolated bridges subjected to near-field ground motions with distinct pulse type behavior in terms of maximum isolator displacements (MIDs) and maximum isolator forces (MIFs) transferred to the substructure. The employed isolation systems are composed of lead rubber bearings (LRBs) with bi-linear force-deformation relations that consider cycle-to-cycle deterioration in the yield strength of the LRBs due to heating of the lead core. MIDs and MIFs with due consideration of cycle-to-cycle deterioration are compared with that of non-deteriorating ones. Bounding analyses are also performed for comparison purposes. Nonlinear response history analyses are conducted with two bins of ground motions recorded at different soil conditions to investigate the effect of ground motion characteristics. Results indicate that MIDs are overestimated by lower bound analyses when seismic isolated bridges are subjected to near-field motions with high velocity pulses especially for the bearings with higher Q/ W ratios.

Dynamic Response of High Steep Rock Slopes Based on Wenchuan Near-Field Seismic Motion Characteristics

Near-field seismic motion characteristics are analyzed in accordance with records of the 2008 Ms8.0 Wenchuan Earthquake measured at Wolong Station, upon which the determination of seismic load is introduced. Dynamic response features, such as acceleration, displacement and stress, of high steep rock slopes on the banks of Zipingpu Reservoir at a variety of locations resulting from horizontal seismic force are analyzed with a numerical analysis routine. The dynamic amplification factor on the slope top is determined, leading to a characterization of the mode of failure of the high steep slope. Analyses show that the dynamic amplification factor at the top of the slopes is about 1.34; however, dynamic response deformation features and stress state at different positions on the slope vary. Earthquake damage of the high steep rock slopes consists mainly of partial avalanche of the rock mass at the top of the slopes by joint cutting. Field investigations after the earthquake have partially confirmed the numerical analysis results presented in this paper.

Using a Fractal Analysis and Polarization Method for Phase Identification in Three-Component Seismograms

This study presents the automatic P-wave and S-wave arrivals picking algorithm which is essentially based on the fractal dimension and polarized method. With an estimate of the spectral exponent γ in a 1/f process, an interval that indicates the preferred intersection containing both noise and the P-wave is well-detected by corresponding to the minimum absolute spectral exponent γ value along the trace. Based on the different properties of background noise and deterministic signal, the fractal dimension technique can detect the position of the P-wave. The place where the fractal dimension value changes suddenly within the intersection interval indicates the location of arrival of the P-wave. Testing that adds various levels of noise to the seismic signal shows the method can prove able to tolerate noise to a signal-to-noise (S/N) ratio 1.5. Based on the P-wave arrival, the polarized P-wave could be detected by a genetic algorithm (GA) with the strength of polarization and phase difference between the vertical and horizontal components as constraints. Using the first arrival phase as the basis phase, this study combines a polarization filter including rectilinearity functions, linear polarization, phase difference and directionality with GA to detect polarized S-wave of seismograms. Finally, the technique was applied to teleseismic data and near-field motion to verify the accuracy and wide applicability of this method. To conclude, this proposed method, an efficient and brand-new method of associating signal processing technique with physical wave motion properties, may be of importance in finding P-wave and S-wave phase arrivals accurately using three-component seismograms.

Comparison of Strong Ground Motion from Several Dislocation Models*

Summary To examine the effects of different earthquake sources, ground motion near the rupture for several dislocation models has been calculated. Quite different dislocation models of the rupture can give very similar displacements at stations located only one fault dimension from the fault in an infinite homogeneous elastic space. In particular: (1) models with fault motions and rupture geometry quite different from Haskell's propagating ramp can often have near-field motions very similar to those from a ramp model; and (2) quite similar near-field motions are found, for different ramp models in which the rupture velocity and rise time are varied together over rather large ranges (e.g. a factor of three in rise time). We infer that there is considerable ambiguity in interpreting near-field displacement data in terms of a model of fault motion. The ambiguity may be reduced by using acceleration data, although this entails a considerable increase in computer time for solving forward problems.

Ground deformation at Soufrière Hills Volcano, Montserrat during 1998–2000 measured by radar interferometry and GPS

We examine the motion of the ground surface on the Soufrière Hills Volcano, Montserrat between 1998 and 2000 using radar interferometry (InSAR). To minimise the effects of variable atmospheric water vapour on the InSAR measurements we use independently-derived measurements of the radar path delay from six continuous GPS receivers. The surfaces providing a measurable interferometric signal are those on pyroclastic flow deposits, mainly emplaced in 1997. Three types of surface motion can be discriminated. Firstly, the surfaces of thick, valley-filling deposits subsided at rates of 150–120 mm/year in the year after emplacement to 50–30 mm/year two years later. This must be due to contraction and settling effects during cooling. The second type is the near-field motion localised within about one kilometre of the dome. Both subsidence and uplift events are seen and though the former could be due to surface gravitational effects, the latter may reflect shallow (< 1 km) pressurisation effects within the conduit/dome. Far-field motions of the surface away from the deeply buried valleys are interpreted as crustal strains. Because the flux of magma to the surface stopped from March 1998 to November 1999 and then resumed from November 1999 through 2000, we use InSAR data from these two periods to test the crustal strain behaviour of three models of magma supply: open, depleting and unbalanced. The InSAR observations of strain gradients of 75–80 mm/year/km uplift during the period of quiescence on the western side of the volcano are consistent with an unbalanced model in which magma supply into a crustal magma chamber continues during quiescence, raising chamber pressure that is then released upon resumption of effusion. GPS motion vectors agree qualitatively with the InSAR displacements but are of smaller magnitude. The discrepancy may be due to inaccurate compensation for atmospheric delays in the InSAR data.

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.

A hybrid analysis method for linear dynamic soil-structure interaction in time and frequency domain

A hybrid analysis method in time and frequency domains for linear soil-structure interaction is presented. First, the time domain solution of the system with Rayleigh damping excited by a short time impulse is obtained by the decoupling numerical simulation technique of near-field wave motion. Then, the corresponding frequency domain solution can be got by Fourier transform. According to the relationship between damping value and dynamic response of a system, the solution of the system with complex damping can be got by Taylor expansion. The hybrid method makes the best of decoupling and explicit algorithm in time domain, and increases the calculation efficiency for linear soil-structure interaction analysis.

Transmitting boundary and radiation conditions at infinity

Relationship between the radiation conditions at infinity and the transmitting boundary for numerical simulation of the near-field wave motion has been studied in this paper. The conclusion is that the transmitting boundary is approximately equivalent to the radiation conditions at infinity for a large class of infinite media. And the errors of the approximation are of the same order of magnitude as those of the finite elements or finite differences in numerical simulation of wave motion. This result provides a sound theoretical basis for the transmitting boundary used in the numerical simulation of the near-field wave motion and gives a complete explanation for the major experiences accumulated in applications of the transmitting boundary to the numerical simulation.

3-D SIMULATION OF NEAR FIELD MOTION IN IRREGULAR GROUND

3-D SIMULATION OF NEAR FIELD MOTION IN IRREGULAR GROUND

Surface wave interactions with systems of oscillating bodies and pressure distributions

Theoretical descriptions of arrays of wave-power devices have been given previously for systems consisting of either oscillating bodies or oscillating pressure distributions. A number of devices have an oscillating water column placed within a floating structure, thus there is a need for a general theory describing composite systems of both oscillating bodies and pressure distributions. Such a theory is presented here. The states of the oscillators in the system are described by the complex amplitudes of the air-pressure distributions within floating or submerged chambers, and of the velocity components for oscillating bodies. By analogy with electric circuit theory, the radiation coupling between all oscillators is represented by a partitioned matrix composed of the radiation admittance matrix for the pressure distributions, the radiation impedance matrix for the oscillating bodies, and a radiation coupling matrix between the bodies and the pressure distributions. Using potential theory of an ideal fluid some reciprocity relations involving these matrices are derived. A little-known relationship between the added mass matrix and the energy of the near-field motion due to oscillating bodies is generalised to include pressure distributions. Previously, only the proof of the result for a single oscillating body has been published. By joining eigenfunction expansions on the common boundaries of rectangular domains, numerical results are obtained for a two-dimensional system where a pressure distribution is trapped between two rigidly connected vertical barriers which are able to oscillate in the surge mode. It is shown that this system absorbs all of the incident wave power, provided optimum values of the complex oscillation amplitudes can be achieved.

Parametric study of near-field ground motions for oblique-slip and dip-slip dislocation models

abstractThe near-field motion on the surface of a uniform half-space for oblique-slip and dip-slip faults has been studied by the use of a dislocation model. The fault is modeled by an infinitely long buried dislocation of finite width; rupture propagates horizontally along the fault and past the observation points with a constant rupture velocity lower than the Rayleigh wave velocity. In addition to those parameters which control peak amplitudes near a vertical, strike-slip fault (depth of the top of the fault, horizontal rupture velocity), the dip of the fault plays an important role. The slip direction and the angle between the rupture front and the down-dip direction of the fault also become increasingly important in determining amplitudes of peak ground motions as the dip of the fault decreases from vertical to shallow angles. In some regions near a thrust fault, peak amplitudes are significantly greater than the largest values near a vertical, strike-slip fault.