Effects Of Near-fault Ground Motions Research Articles

Nonlinear Modeling and Analysis of R.C. Framed Buildings Located in a Near-Fault Area

Structural damage to r.c. buildings located in a near-fault area has been observed during strong ground motions, with long-duration horizontal pulses and high values of the ratio between the peak value of the vertical acceleration and the analogous value of the horizontal acceleration. The design provisions of current seismic codes are generally not very accurate for assessing the structural effects of near-fault ground motions. In the present work, six- and twelve-storey r.c. spatial frames are designed according to the provisions of the Italian seismic code, considering horizontal and vertical seismic loads in a high-risk seismic region and assuming low and high ductility classes. A lumped plasticity model based on the Haar-Kàrmàn principle is used to describe the inelastic behavior of the r.c. frame members. In particular, the lumped plasticity model for a column includes a piecewise linearization of the bounding surface of the axial load-biaxial bending moment elastic domain, at the end sections where inelastic deformations are expected. Moreover, the lumped plasticity model for a girder takes into account the potential plastic hinges along the span, due to the vertical ground motion, so avoiding the computational effort required by the sub-discretization of the frame member. The nonlinear dynamic response of the test structures is studied with reference to the horizontal and vertical components of near-fault records. The occurrence of a directivity effect at arbitrary orientations is checked rotating the horizontal components of the selected motions, rather than considering only fault-normal and fault-parallel orientations.

Effects of near-fault ground motions in seismic performance evaluation of a symmetric arch dam

Near-fault ground motion effects in comparison with far-fault ground motions on seismic performance of a symmetry concrete arch dam are investigated. Various sets of near-fault and far-fault ground motions with approximately identical peak ground acceleration were used for analyses of coupled system. For this purpose the Karaj arch dam was selected as case study and two different options were considered for dam-foundation interaction. In the first case, foundation rock was assumed rigid, and in the second case foundation rock was simulated as a massless medium. The behavior of the dam and foundation was expressed in terms of displacement, and the reservoir was modelled based on fluid elements. The seismic performance evaluation of the arch dam was performed using demand-capacity ratios obtained from linear analyses.

Amplification Effect of Near-Fault Vertical Ground Motions on Horizontal Deformation Response of Structures

In this paper, the amplification to structural non-linear displacement response caused by near-fault vertical ground motions is investigated quantitatively. Based on a set of 30 acceleration records of near-fault ground motions, two typical concrete frame structures, as numerical examples, are examined by means of nonlinear dynamic time-history analysis at two different stages: (1) through the application of horizontal components of acceleration records, (2) through application of the combined horizontal and vertical components at the same time. The numerical results show that the maximum floor-displacements of two concrete frames are limitedly amplified by the vertical effect of near-fault ground motion. On the other side, the maximum inter-story drifts are amplified obviously, and the magnification caused by the vertical effect is about 15%~20% on average. Hence it is evident that the amplification of near-fault vertical effect on horizontal deformation response could not be neglected, and worth serious consideration in future seismic design and evaluation.

Semi-active fuzzy control for seismic response reduction of building frames using variable orifice dampers subjected to near-fault earthquakes

In this paper, a fuzzy-rule-based semi-active control of building frames using semi-active variable orifice dampers (VODs) is presented. Additionally, the consequences of well-known characteristics of near-fault ground motions, forward directivity and fling step, on the seismic response control is investigated. The membership functions and fuzzy rules of the fuzzy controller were optimized by genetic algorithm. To illustrate the efficiency of the proposed fuzzy controller strategy in application and effects of near-fault ground motions, numerical simulation for a 10-story building frame equipped with VODs is presented. The VODs are installed in all storeys to prevent damage to the structure from severe earthquakes. The controlled response of the frame was compared with results obtained by controlling the frame by the classical clipped-optimal control method based on linear quadratic regulator theory. Results revealed that the fuzzy logic controller is capable of improving the structural responses and VODs are promising for reducing seismic responses during near-fault earthquakes.

Investigation of effect of near-fault motions on substandard bridge structures

The objective of this study was to investigate the effects of near-fault ground motions on substandard bridge columns and piers. To accomplish these goals, several large scale reinforced concrete models were constructed and tested on a shake table using near- and far-field ground motion records. Because the input earthquakes for the test models had different characteristics, three different measures were used to evaluate the effect of the input earthquake. These measures are peak shake table acceleration, spectral acceleration at the fundamental period of the test specimens, and the specimen drift ratios. For each measure, force-displacement relationships, strains, curvatures, drift ratios, and visual damage were evaluated. Results showed that regardless of the measure of input or response, the near-fault record generally led to larger strains, curvatures, and drift ratios. Furthermore, residual displacements were small compared to those for columns meeting current seismic code requirements.

Effects of near-fault ground motions and equivalent pulses on multi-story structures

The focus of this paper is the structural response of multi-story structures to near-fault ground motions, and whether structural response is dominated by the ground motion pulses present in forward-directivity ground motions. Also considered is whether simplified pulses are capable of representing the effects of these pulses on structural response. Incremental Dynamic Analysis was employed to assess the effects of forward-directivity pulses on the response of near-fault multi-story structures. Three different generic multi-story shear buildings were subjected to fifty four near-fault ground motions including ordinary and forward-directivity records. The Maximum Story Displacement Ductility Demand was selected as the Engineering Demand Parameter. Results showed that pulse-like forward-directivity ground motions impose a larger ductility demand to the structure compared to ordinary ground motions. Moreover, the response of the structures to forward-directivity motions shows higher scatter than the response to ordinary ground motions when correlated with simple intensity measures such as PGA or spectral acceleration at the first mode period. The only intensity measure that appears to be valid for both ordinary and forward-directivity ground motions is the peak ground velocity. The structural response to the forward directivity ground motions was reproduced using an equivalent pulse model based on the modified Gabor Wavelet pulse. It is shown that when the ratio of pulse period to the fundamental structural period falls in a range of 0.5–2.5, the equivalent pulse model appropriately represents the structural response to forward-directivity ground motions. The simplified pulse parameters can be predicted using existing relationships and can be incorporated into probabilistic seismic hazard analysis to develop a seismic reliability analysis. Finally, the effects of damping ratio and P - Δ were investigated for forward-directivity ground motions. The effect of variations in the damping ratio on the ductility demand was insignificant while P - Δ -effects on the ductility demand are significant.

Discussion on “Damping coefficients for near-fault ground motion response spectra”

This discussion is based on the paper by Hubbard and Mavroeidis [1]. In this paper, the authors have presented an interesting study on the effect of near-fault ground motions on the damping coefficients examining single degree of freedom (SDOF) systems. This discussion presents some comments on the results and the conclusions of that paper, which imply that some aspects need further clarification and/or improvement.

Risk assessments of long-span bridges considering life-cycle cost concept and near-fault ground motion effect

The influence of the near-fault ground motion on the response of long-span bridges must be considered as a critical factor for seismic design because the response indicates different aspects from existing earthquake characteristics. Also, it is important to note that the safety index for the risk assessment of long-span bridges is determined based on the minimum expected life-cycle cost E(LCC). In this study, earthquake characteristics are analyzed by creating elastic and inelastic response spectrums with actual measurement records (Chi-Chi earthquake records) and then the numerical analysis of the long-span bridge in Namhae, Korea is performed according to the increase and reduction of the member stiffness based on the standard design., the reliability evaluation of the long-span bridge considering aleatory uncertainties is performed on the basis of the combined results of static analysis and seismic response analysis. Also, the minimum LCC is estimated based on failure probabilities by the different alternative design. Because of epistemic uncertainties, the results of reliability evaluation and the LCC of optimal design are selected as random variables; the safety index, failure probability and expected minimum LCC are re-evaluated with regard to critical percentage values for a risk-averse design of the long-span bridge, and are presented graphically using cumulative percentages. It is, therefore, expected that this study will provide the basic information for the risk assessment and optimal design method in performing seismic design of the long-span bridge considering earthquake characteristics.

Experimental Study of Reinforced Concrete Bridge Columns Subjected to Near-Fault Ground Motions

Near-fault ground motions in earthquakes are very destructive due to their high-velocity pulse. Although the California Department of Transportation (Caltrans)has developed seismic design criteria, the effectiveness of these provisions has not yet been studied. In this study, shake table tests were conducted to investigate the effects of near-fault ground motions on the seismic performance of bridge columns that are designed for near-fault earthquakes. Four large-scale reinforced concrete circular columns with different initial periods were tested. The design spectra included the current Caltrans spectrum and a new spectrum developed in this study. The most distinct measured column response was the relatively large residual displacements even under moderate levels of motion. The shake table test results revealed that it is necessary to control residual displacement at the design stage if the bridge is near an active fault. The data also showed that the plastic hinge length in sufficiently confined columns subjected to near-fault earthquakes is comparable to that of columns experiencing far-field motions. The column designed based on Caltrans near-fault provisions showed a 36% reduction of the residual displacement compared to a column designed for far-field earthquakes.

A Study on the Response of Multi-Storey Buildings to Near-Fault Ground Motions

This paper mainly emphasizes two objectives. The first objective is a comparative study on the inelastic seismic response of multi-storeyed buildings to fault normal and fault parallel components of near-field ground motions. The study has shown that it is necessary to consider the effect of near-fault ground motions separately and make provisions for design in the codes of practice accordingly. The second objective of this study is to reduce the response of the bottom storey in multi-storeyed buildings to the fault normal component of near-field ground motions through an expendable top storey concept.

주철근 겹침이음된 RC교각의 근단층지반운동에 대한 진동대 응답과 분석

The near-fault ground motion (NFGM) is characterized by a single long period velocity pulse of large magnitude. NFGM`s have been observed in recent strong earthquakes, Izmit Turkey (1999), Kobe Japan (1995), Northridge USA (1994), etc. These strong earthquakes have caused considerable damage to infrastructures because the epicenter was close to the urban area, called as NFGM. Extensive research for the near-fault ground motion (NFGM) have been carried out in strong seismic region, but limited research have been done for NFGM in low or moderate seismic regions because of very few records. The purpose of this study is to investigate and analyze the effect of near-fault ground motions on reinforced concrete (RC) bridge piers with lap-spliced longitudinal reinforcing steels. The seismic performance of four RC bridge piers under near-fault ground motions was investigated on the shake table. In addition, a RC bridge pier is subjected to pseudo-dynamic loadings. Test results showed that large residual displacements were observed in RC bridge piers under NFGM. RC specimens on the shake table failed at relatively low displacement ductility, compared with the displacement ductility of RC bridge pier subjected to pseudo-dynamic loadings.

Effect of Near-Fault Vertical Ground Motions on Seismic Response of Highway Overcrossings

Results of comprehensive nonlinear response history analyses on a range of configurations representing typical highway overcrossings subjected to combined effects of vertical and horizontal components of near-fault ground motions are reported. Current seismic design guidelines in California neglect the vertical components of ground motions for peak ground accelerations less than 0.6 g and provide rather simplistic measures to account for vertical effects when they need to be incorporated in the design. Results from the numerical simulations show that the vertical components of ground motions cause significant amplification in the axial force demand in the columns and moment demands in the girder at both the midspan and at the face of the bent cap. Axial capacity of the columns and moment capacity of the girder at the face of the bent cap were generally found to be sufficient to resist the amplification in the respective demands due to vertical effects. However, midspan moments in negative bending due to vertical motions are found to exceed the capacity of the girder. The amplified midspan moments lead to yielding of the top reinforcement resulting in average peak strains on the order of 1%. It is concluded that seismic demand analysis of ordinary highway bridges in general and overcrossings in particular should incorporate provisions for considering the adverse vertical effects of near-fault ground motions.

Effect of supplemental damping on LRB and FPS seismic isolators under near-fault ground motions

Numerical simulations are performed to assess the effects of near-fault ground motions on base-isolated buildings that consist of either lead-rubber (LRB) or friction-pendulum system (FPS) bearings in addition to supplemented viscous dampers. While LRB and FPS isolation systems have been applied for a number of years, the addition of supplemental damping devices is being currently considered for strong ground motions to reduce the isolator displacements. However, the main problem in this case is that the addition of damping may increase both internal deformation and absolute accelerations of the superstructure and thus may defeat many of the gains for which base isolation is intended. In the present paper, a detailed and systematic investigation on the performance of LRB and FPS isolation systems, provided with supplemental viscous damping under the effect of near-fault ground motions, has been carried out by using commercial finite element software.

Near-fault ground motion effects on the nonlinear response of dam-reservoir-foundation systems

Ground motions in near source region of large crustal earthquakes are significantly affected by rupture directivity and tectonic fling. These effects are the strongest at longer periods and they can have a significant impact on Engineering Structures. In this paper, it is aimed to determine near-fault ground motion effects on the nonlinear response of dams including dam-reservoir-foundation interaction. Four different types of dam, which are gravity, arch, concrete faced rockfill and clay core rockfill dams, are selected to investigate the near-fault ground motion effects on dam responses. The behavior of reservoir is taken into account by using Lagrangian approach. Strong ground motion records of Duzce (1999), Northridge (1994) and Erzincan (1992) earthquakes are selected for the analyses. Displacements, maximum and minimum principal stresses are determined by using the finite element method. The displacements and principal stresses obtained from the four different dam types subjected to these nearfault strong-ground motions are compared with each other. It is seen from the results that near-fault ground motions have different impacts on the dam types.

Study on inelastic displacement ratio spectra for near-fault pulse-type ground motions

In displacement-based seismic design, inelastic displacement ratio spectra (IDRS) are particularly useful for estimating the maximum lateral inelastic displacement demand of a nonlinear SDOF system from the maximum elastic displacement demand of its counterpart linear elastic SDOF system. In this study, the characteristics of IDRS for near-fault pulse-type ground motions are investigated based on a great number of earthquake ground motions. The influence of site conditions, ratio of peak ground velocity (PGV) to peak ground acceleration (PGA), the PGV, and the maximum incremental velocity (MIV) on IDRS are also evaluated. The results indicate that the effect of near-fault ground motions on IDRS are significant only at periods between 0.2 s–1.5 s, where the amplification can approach 20%. The PGV/PGA ratio has the most significant influence on IDRS among the parameters considered. It is also found that site conditions only slightly affect the IDRS.

Near-Fault Ground Motion Effects on Reinforced Concrete Bridge Columns

Characteristics of near-fault ground motions warrant special consideration due to their severe and impulsive effects on structures. These characteristics are unique compared to far-field ground motions, upon which nearly all seismic design criteria are based. The objectives of this study were to explore the shake table response of reinforced concrete, to investigate near-fault ground motion effects on reinforced concrete bridge columns subjected to near-fault ground motions, and to provide a framework for the evaluation of bridge columns near active faults. Two large-scale columns were designed and tested under a near-fault ground motion on a shake table at the University of Nevada, Reno. One column represented the current California Department of Transportation far-field design, and the other was based on the American Association of Highway and Transportation Officials provisions. The most unique measured response characteristic in both columns was the large residual displacements even under moderate motions. A new hysteresis model was developed to capture this effect and was incorporated in an analytical model. Based on this finding, a framework for the evaluation of reinforced concrete bridge columns with respect to the control of residual displacement was proposed.

Effect of near-fault earthquake on bridges: lessons learned from Chi-Chi earthquake

The objective of this paper is to describe the lessons learned and actions that have been taken related to the seismic design of bridge structures after the Chi-Chi, Taiwan earthquake. Much variable near-fault ground motion data was collected from the rupture of Chelungpu fault during the Chi-Chi earthquake, allowing the seismic response of bridge structures subjected to these near-fault ground motions to be carefully examined. To study the near-fault ground motion effect on bridge seismic design codes, a two-level seismic design of bridge structures was developed and implemented. This design code reflects the near-fault factors in the seismic design forces. Finally, a risk assessment methodology, based on bridge vulnerability, is also developed to assist in decisions for reducing seismic risk due to failure of bridges.

Near-Fault Ground Motion Effects on Simple Structures

Ground motion records obtained from sites at known locations relative to earthquake faults were used to study near-fault (NF) ground shaking effects on the inelastic response of simple structures. The structures were idealized as single-degree-of-freedom oscillators with periods ranging from 0.05 to 3.0 s, a damping ratio of 2%, and target ductilities of 2, 4, and 6. The severity of inelastic response was described by a relationship between the oscillator period, the target ductility, and the lateral force reduction factor obtained from analysis. It was found that the inelastic demands of medium and longer period oscillators responding to NF strike-normal shaking increased for sites close to the fault as the distance along the fault from the epicenter increased. The inelastic response of short-period oscillators was not affected as much by NF shaking. A methodology for obtaining average and design level demands for structures at specific locations relative to the epicenter and fault is described. A design...