Near-fault Ground Motions Research Articles

Nonlinear seismic analysis of irregular base isolated structures using a new modified lateral load pattern

Nowadays, seismic isolation systems are increasingly used by engineers for retrofitting existing structures and designing new ones. Nevertheless, limited studies have been conducted on the performance evaluation of the nonlinear static analysis methods for estimating the seismic demands of the base isolated structures. The current study proposes a new modified lateral load pattern for seismic analysis of base isolated structures using modified capacity spectrum method (MCSM). Moreover, this study investigates the applicability of the modal pushover analysis (MPA) and modified capacity spectrum methods for assessing the base isolated structures with setback irregularity. To this end, two types of steel moment resisting structures including 3-story (low-rise) and 9-story (medium-rise) with different irregularities in elevation are considered. All the structures are isolated by three types of lead rubber bearing (LRB) isolators with various stiffnesses. In order to consider the effect of ground motion, two types of near-fault and far-fault ground motions are selected. To evaluate the performance of the MCSM and MPA methods, the structural seismic demands estimated by these methods are compared with their benchmark values obtained by the nonlinear time history analysis method. The obtained results show the superiority of the modified capacity spectrum method and the effectiveness of the new modified lateral load pattern presented for this method.

Evaluation of ground motion intensity measures for time-history dynamic analysis of isolated bridges

The ground motion intensity measure (IM) is an important part of probabilistic seismic demand analysis and is used to predict the seismic response of structures. In this study, two typical isolation bridges are selected as cases, and 80 far-fault and 40 near-fault ground motions are used as inputs for the time-history analysis. By performing a comparison with 19 common IMs, it is proved that the IM based on the geometric mean of the response spectrum is an optimal choice for isolated bridges. The effects of response spectrum type and period range on this form of IM are investigated. The results show that the response spectrum type has a slight effect on the IM based on the geometric mean of the response spectrum, and the period range has a greater impact on the IM. The effects of the upper and lower limits of the period range of this IM are illustrated. The applicability of the IM to the far-fault ground motions and the near-fault ground motions is finally discussed. It can be concluded that when only near-fault ground motions are considered, the period range of the IM should be appropriately increased.

Effects of pulse-like ground motions on tunnels in saturated poroelastic soil for obliquely incident seismic waves

It has been widely recognised that near-fault ground motions can be distinctly different from far-fault ground motions, in terms of their amplitudes and spectral characteristics, and there are a number of studies investigating the effects of near-fault ground motions on the structural response. However, only a few studies focus on underground tunnels, and most of them consider vertically incident seismic waves, neglecting the wave passage effect which is critical for long tunnels. This paper investigates the consequences of near-fault pulse-like ground motions on the seismic tunnel response, with an example of a tunnel embedded in saturated poroelastic soil. The input motions are represented by obliquely incident P1 waves, and the wave passage effect along the longitudinal direction is considered by a 2.5D modelling technique. The seismic tunnel response for near-fault pulse-like and far-fault ground motions is compared. Additionally, artificial ground motions, which have consistent acceleration response spectra with the near-fault pulse-like ground motions, but without velocity pulses, are generated to gain further insight into the contribution of velocity pulses. It is shown that the near-fault pulse-like ground motions can noticeably increase the tunnel internal forces, due to large seismic energy associated with the velocity pulses. For pulse-like ground motions with similar acceleration response spectra, the fling-step velocity pulse can be more detrimental to the tunnel than the forward-directivity velocity pulse. Moreover, the amplification effect of the near-fault pulse-like ground motions on the tunnel internal forces tends to be more prominent for large vertical angles of incidence, highlighting the significance of considering the oblique incidence case for seismic design.

Implications of Arch Warp Altitudes on an Ancient Masonry Bridge under Ground Movements

Although only a few of the ancient masonry arch bridges (MABs) are in fairly good condition today, many ancient arch bridges are still in use. Over time, the condition of the masonry bridges declines and the safety requirements change. Therefore, it is important to examine these bridges under different influences. The strengthening of MABs is generally not essential. The major cause of damage to MABs is their insufficient width and height, and thus, it is not the safety but the usability that has restricted the life-time of the MABs. Therefore, in this investigation, the effect of the arch height on the static and dynamic behavior of a single-span MAB was investigated. For this aim, the Ancient Tokatlı Bridge, built in Karabük, Türkiye, was selected for investigation under near-fault (NF) and far-fault (FF) ground motions (GMs). To observe the altitude of the arch warp on the ancient MAB, first, the finite element model (FEM) was utilized, using ANSYS and SAP 2000. Furthermore, to constitute the arch warp’s influence on a MAB, the FEM was remodeled considering the different arch warps between 7.0 and 9.0 m. Moreover, GMs were applied to the FEM to investigate the effect of dynamic behavior. Under these GMs, stresses and strains (compression and tensile) were observed and compared with each other. Consequently, at the end of these investigations, it was observed that the maximum motions were reduced, while the height of the one-span MAB was increased under NF and FF GMs, and this was also true for the contrary situations. The compression stresses were not observed to be hazardous at the point of destruction, while the altitude of the one-span MAB increased.

Reliability assessment for hybrid solar tower under near-fault pulse-like ground motions using Kriging surrogate and subset simulation

Concentrated solar towers are commonly designed as high-rise hybrid structures, which exhibit properties of non-uniform mass and rigidity along the height of the tower, and consequently are vulnerable to strong seismic ground motions. This study presents a comprehensive analysis of the seismic performance of a hybrid solar tower, which is one of the tallest hybrid solar towers in the world. The hybrid solar tower includes a reinforced concrete tube at the bottom and a steel-truss tube at the top and is subjected to near-fault pulse-like ground motions. A simulation technique that utilizes the stochastic point-source model and a velocity pulse model was introduced to generate stochastic pulse-like ground motions. The stochastic response of the hybrid solar tower to near-fault pulse-like ground motions was computed to estimate the tower’s reliability. To enhance the efficiency of reliability estimation, an algorithm combining the Kriging surrogate and subset simulation (K-SS) was presented. The aggregate response of the tower was found to be significantly more damaging over the border of the reinforced concrete tube and the steel-truss tube than below the border. The study found that the hybrid solar tower has a reliability of 90.53% when subjected to stochastic near-fault pulse-like ground motions with a peak ground acceleration of 0.62 g. The findings of this study contribute to the understanding of the seismic performance of high-rise hybrid solar towers under near-fault pulse-like ground motions.

Vertical effects of near-fault ground motions and the optimal IMs for seismic response of continuous girder bridges with FPB isolators

The characteristics of vertical components in the near-fault region were investigated based on 121 pairs of typical near-fault ground motions. A weak positive correlation between the vertical and horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) is shown. Basically, the ratios of vertical to horizontal peak ground acceleration and velocity of the near-fault ground motions comply with the lognormal distribution, and the vertical ground motions are significantly stronger at lower periods. Fifteen scaler horizontal and nine scaler vertical IMs are compared to select the optimal scaler IMs of horizontal and vertical seismic demands of an isolated girder bridge with friction pendulum bearings (FPB isolators). It is found that acceleration-related scalar IMs perform better for vertical seismic demands, while velocity-related scaler IMs perform better for horizontal seismic demands. The optimal vector-valued IM, consisting of scalar horizontal and vertical IMs in the forms of IMh,IMv, for horizontal seismic demands were proposed. The optimal vector-valued IMs show an apparent improvement in efficiency compared with the corresponding scaler IMs when vertical components are considered. Furthermore, the probabilistic seismic demand models (PSDMs) for horizontal seismic demands with optimal vector-valued IM were adopted to assess the seismic demands under a given intensity of vertical and horizontal ground motions. It is identified that the vertical effects significantly influence the shear force of piers while slightly influencing the curvature of piers and bearing displacements.

Effects of response spectrum of pulse-like ground motion on stochastic seismic response of tunnel

It is widely accepted that near-fault pulse-like ground motions potentially cause more severe damage to structures than ordinary ground motions. However, the effects of stochastic pulse-like ground motions on underground structures are not effectively considered. The impacts of response spectra of pulse-like ground motions on seismic response are also unclear. Hence, this study utilizes an underground tunnel to investigate the effects of response spectra of pulse-like ground motions on the stochastic seismic response by combining a pulse-like ground motion simulation method and a finite element method. The combination procedure enables stochastic seismic response analysis under spectrum-compatible pulse-like/ordinary ground motions. The finite element model considers the nonlinear dynamic properties of soil and soil–structure interaction. Monte Carlo simulations are carried out to quantify uncertainties of stochastic tunnel response. Results show that pulse-like ground motions cause larger bending moments for tunnel lining even when they match the same target spectrum of ordinary ground motions. Furthermore, the bending moment of tunnel lining is significantly amplified when the spectral acceleration of pulse-like ground motion contains bulges or multiple peaks in the ranges of 1 s–6 s. Therefore, the seismic risk of tunnels may be significantly underestimated in the near-fault regions without considering pulse-like ground motions and their spectral acceleration characteristics.

Seismic responses of liquid storage tanks subjected to vertical excitation of near-fault earthquakes

This study aims to evaluate the effect of vertical seismic component on dynamic responses of liquid storage tanks, with a special focus on near-fault earthquakes. A shaking table test was performed on a 1:25 scaled steel tank, which was loaded by both far- and near-fault ground motions with horizontal unidirectional, vertical unidirectional, and combined horizontal and vertical bi-directional excitations. Seismic responses, including sloshing height and hydrodynamic pressure were recorded and compared. Subsequently, a refined finite element (FE) model of the scaled tank model was established by utilizing the structured Arbitrary-Lagrange-Eulerian (S-ALE) solver and Fluid-Structure Interaction (FSI) algorithm, which was verified by comparison with the test results. Afterward, the modeling methodology was further applied to a prototype large-scale liquefied natural gas (LNG) inner tank for seismic analysis. In addition, seismic responses of the LNG inner tank were compared with the recommendations given in seismic design codes for cylindrical tanks. The experimental and numerical findings showed that both the convective and impulsive motions of liquid was amplified by vertical excitations, especially under near-fault earthquakes. The peak sloshing heights under near- and far-fault earthquakes were increased by 14–142% and 8–56% due to the existence of vertical action, respectively. The LNG inner tank was more vulnerable to inelastic buckling due to the presence of vertical seismic component. Moreover, the effect of vertical excitations of near-fault earthquakes on hydrodynamic pressure and tank stresses was underestimated by the current seismic design codes.

Effects of Near-Fault Ground Motions on Civil Infrastructure

Near-fault earthquakes (NFEs), characterized by high peak ground velocity (PGV) and long period pulses, show different properties from far-field ones [...]

Effect of foundation embedment ratio in suppressing seismic-induced vibrations using optimum tuned mass damper

Tuned mass damper (TMD), which is used in structures to mitigate the dynamic responses caused by environmental loads, is tuned according to the dominant frequency of the structures. The dominant frequency of structure built on soil with low stiffness is significantly affected by the soil-structure interaction (SSI). In previous studies, only the surface foundation case is considered to investigate the design parameters of TMD considering SSI effects and the embedment depth of the foundation is neglected in these studies. Thus, this paper investigates the optimization of TMD for mitigation of responses of a high-rise building considering various depths of embedment and soil properties. The optimum parameters of TMD are investigated using five different optimization algorithms (i.e., simulated annealing, sequential quadratic programming, genetic algorithm, pattern search, and particle swarm optimization). The objective function minimizes the peak acceleration amplitude at the top of the structure. The effectiveness of optimum TMD is verified to use 20 near-fault pulse-like ground motions. The comparisons show that the embedment ratio of the foundation and the soil properties significantly affect the optimization of TMD.

Predictive model for seismic displacements of flexible sliding block subjected to near-fault pulse-like ground motions

A post investigation of landslides induced by strong earthquake events in recent years shows that near-fault pulse-like ground motions have a considerable impact on slopes. Accurate evaluation of the stability of slopes under near-fault earthquakes has become a crucial issue in seismic-prone areas. Permanent displacement is widely recognized as an effective index for evaluating the stability of slope. Based on the coupled analysis, an empirical earthquake-induced permanent displacement prediction model was developed. A displacement database of 318 pulse-like ground motions from 50 earthquake events was established. The proposed predictive model is a function of the maximum seismic coefficient-time history (kmax), maximum velocity coefficient of velocity-time history of the k-time history (kv-max), yield acceleration (ky), and period ratio (ratio of the natural period of the slope to the mean period of input motion, Ts/Tm). The dynamic responses (kmax and kv-max) are represented by two different functions. At small period ratios (Ts/Tm≤1.8), it is a function of peak ground acceleration (PGA), peak ground velocity (PGV) and ky. At large period ratios (Ts/Tm > 1.8), it is a function of PGV, Ts and ky. It successfully captured the velocity pulse and long period characteristics of pulse-like ground motions. Compared to the existing model, it is found that the model is more accurate when considering pulse-like ground motions. This prediction model can be used not only for the preliminary evaluation of slope stability but also for probabilistic seismic demand analysis of flexible slopes considering pulse-like ground motion in near-fault regions.

Optimum tuned tandem mass dampers for suppressing seismic-induced vibrations considering soil-structure interaction

In recent studies, it has been stated that tuned tandem mass dampers (TTMD) will be a practical mechanical solution to reduce the dynamic responses of structures under seismic excitations. However, since TTMD is tuned based on the dominant frequency of the superstructure, ignoring the soil-structure interaction (SSI), it may cause a notable deterioration in the control performance of the control devices. Thus, a methodology for the optimum design of TTMD positioned on the top story of a high-rise building due to near-fault pulse-like ground motions considering potential SSI effects has been investigated in this paper. Firstly, the TTMD is located on the top story of a well-known 40-story shear building, and the equations of motion for soil-structure-TTMD interaction are derived. Based on the sequential quadratic programming (SQP), the tuning parameters of TTMD are then found for different support conditions in the frequency domain. The efficiency of SQP is also compared with two meta-heuristic algorithms (i.e., simulated annealing and pattern search algorithms). The effectiveness of optimum TTMD is tested using 30 near-fault pulse-like ground motions using time-domain analyses. Numerical results showed that the optimum TTMD is more effective in suppressing the maximum displacement and inter-story drift of the building, considering SSI. It is also found that the frequency content of the earthquake, as well as the pulse period of the earthquake and soil type, has a significant effect on the control performance of the TTMD.

Seismic evaluation of composite moment frames subjected to near-fault ground motions

This paper investigates the seismic behavior of composite special moment frames (CSMF) under near-fault ground motions. For this purpose, two factors, response modification and deflection amplification are chosen. 3-, 9- and 20-story frames of SAC joint venture are modeled in OpenSees software. The nonlinear behavior of the panel zone was modeled using a 2D joint element and a tri-linear relation. FEMA p695 near-fault ground motions are applied in Incremental Dynamic Analysis (IDA). Ultimately, the obtained results are compared to the values recommended by ASCE provisions. The average results indicate that the ASCE provision can appropriately predict the response modification factor; however, in the case of deflection amplitude, IDA results may not show good agreement with ASCE/SEI 7 suggested value. Also, by comparing the results of composite special moment frames with steel moment frames from literature, it has been revealed that, unlike ductility, overstrength increases significantly when concrete filled tube columns are employed in special moment frames.

Seismic fragility assessment for highway bridges incorporating multi-level shape memory alloy cable dampers

An innovative multi-level shape memory alloy (SMA) cable damper is proposed for bridges in near-fault seismic regions. This study commences with the introduction of a constitutive material model for simulating SMA cables. The proposed model not only considers the self-centering hysteretic property but also accounts for the strength degradation and residual strain accumulation effects. Subsequently, the layout and working mechanism of the proposed multi-level SMA cable damper is presented. A seismic fragility analysis framework involving Box-Cox transformation and Bayesian inference is subsequently presented. The Box-Cox transformation is utilized to establish the nonlinear probabilistic seismic demand models (PSDMs), and the Bayesian-based logistic regression is employed to formulate fragility functions. Finally, a three-span continuous reinforced concrete (RC) girder bridge incorporating the multi-level SMA dampers is selected as an example to demonstrate the proposed framework. A total of 160 near-fault ground motions, containing pulse and non-pulse motions, are applied along the longitudinal direction of the bridge. To investigate the influences of different parameter values of the SMA cable damper on fragility functions, various parameters are generated with respect to a central composite design philosophy. Based on the seismic fragility analysis, it is found that the SMA isolation damper reduces the damage probabilities of the bearing and pier for all four damage states. In addition, the failure probability of the bearing decreases with the increase of the axial stiffness of the SMA damper, which is opposite to the pier. Moreover, the optimum designed SMA damper can significantly reduce the damage probability of the case study bridge, and the parameter values of the damper should be carefully selected to achieve the desired seismic performance.

Superelastic Conical Friction Pendulum Isolator for Seismic Isolation of Bridges under Near-Fault Ground Motions

To improve the seismic performance of bridges, this study develops a novel superelastic conical friction pendulum isolator (SCFPI) by incorporating an improved conical friction pendulum isolator (CFPI) with shape memory alloy (SMA). The flat sliding function designed for the improved conical friction pendulum isolator is to enhance the adaptability under service loadings. The analytical model of the novel SCFPI is derived and the numerical model is developed within the OpenSees platform to investigate the hysteretic behavior under cyclic loadings. A cost-effective design method is proposed to design the SCFPI system for example bridges. The response mitigation efficacy of the novel SCFPI system is investigated using the optimum design parameters under near-fault ground motions. Several case studies are conducted to demonstrate the effectiveness of the novel SCFPI system and cost-effective design method. Results indicate that the cost-effective design method is particularly effective for parameter optimization of the novel SCFPI, which achieves effective mitigation for the displacement responses of the isolator and superstructure and also effectively controls the seismic force increment in piers. Case studies indicate the reliable re-centering capacity and mitigation efficacy of the SCFPI system for bridges under near-fault ground motions. The findings of this study contribute to upgrading structural resilience and promoting seismic applications as a reliable technical guidance.

The Effect of Soil Characteristics On NFGM and FFGM Ground Motion Response Spectra

This study aims to achieve the following goals: 1- To distinguish between the inelastic responses of buildings under earthquakes with NFGM and FFGM. 2- To inspect the effect of soil shear velocity on the response spectra. Several earthquake events, with different characteristics, are brought from the PEER website for “Pacific Earthquake Engineering Research” and analyzed using PRISM software to achieve these goals. The research found that the acceleration, velocity, and displacement response of the selected near-fault ground motions on the structure has a higher effect than the far- field ground motion response in both types of soils and this difference is displayed more noticeably in long periods (periods after 0.2 sec). And when comparing responses of the two types of soils (the soft soil type and the rock type) it shows that the geological and geotechnical aspects of the soil deposits majorly affect the response spectra of the free field surface motions

Seismic response mitigation of prefabricated industrial equipment structural frames through a hybrid isolation system

For reducing the excessive acceleration response of the critical equipment in a type of industrial equipment structural frames under strong earthquakes, a hybrid isolation system (HIS), incorporating linear natural rubber (LNR) and lead rubber (LR) bearings, is proposed. The efficiency and reliability of the HIS are verified through a series of full-scale shaking table tests on two prefabricated industrial moment resisting frames (MRFs), with respect to a corresponding fixed-base system (FBS). The structural dynamic characteristic and seismic performance of the two types of MRFs, i.e., reinforced concrete structure (RCS) and composite concrete-filled steel tube structure (CCFSTS), are examined both with the HIS and FBS subjected to a range of seismic intensities. The test results demonstrate the superior performance of the HIS in controlling transient acceleration response under near-fault, far-field and artificial ground motions. The average peak acceleration reduction coefficients (PARCs) of the HIS range between 75.60% and 79.51% in the two considered MRFs along two horizontal directions, with a strict satisfaction on the peak acceleration response for normal operation of the critical equipment. Also, the hysteretic behavior of the HIS is stable and controllable during the earthquake excitations. The peak transient displacement of the HIS is satisfactory, and the residual deformation is negligible even under the maximum considered earthquake (MCE) excitations. Besides, components of the two MRFs are undamaged during the whole test. Thus, with the capacity of protecting the acceleration-sensitive equipment and structural components, the proposed HIS possesses a great prospect in enhancing the structural resilience.

Seismic response characteristics and whiplash effect mechanism of continuous rigid-frame bridges subjected to near-fault ground motions

Seismic response characteristics and whiplash effect mechanism of continuous rigid-frame bridges subjected to near-fault ground motions

Experimental Study of Dynamic Responses of Special Tunnel Sections under Near-Fault Ground Motion

Data surveys show that near-fault ground motion does great damage to tunnel structures, especially the portal section and fault zone. In this paper, a series of shaking table model tests of near-fault tunnels were conducted and the surrounding-rock fault-zone-lining model of the near-fault tunnel was established. Accelerometers and strain gauges were arranged at specific locations, and the experimental process of earthquake occurrence was simulated by inputting seismic waves of different working conditions, which obtained the characteristics of stress, damage and deformation of the tunnel model. The tested results showed that the acceleration response of the tunnel portal section was close to the wave shape of the inputted seismic wave, and the acceleration response of the arch shoulder, arch waist and arch foot was more prominent. The internal force of lining at the arch shoulder and arch foot was greater than that at the arch crown, and the peak internal force appeared at the arch foot. The internal force and the maximum or minimum principal stress of the lining under impulse ground motion were larger than those under non-impulse ground motion. Additionally, the surrounding rock had a filtering effect on the high-frequency band of seismic waves. Meanwhile, when the geological characteristics of the fault zone were poor, and the tensile damage first appeared at the arch foot, the compressive damage appeared at the junction of the surrounding rock and fault zone. This study will offer a practical guidance for tunnel engineering earthquake damage.

Seismic performance improvement of continuous rigid-frame bridges with hybrid control system under near-fault ground motions

To improve the seismic performance of continuous rigid-frame bridges (CRFBs) in near-fault earthquakes, this study developed a hybrid seismic control system (HSCS) combining a three-dimensional hybrid seismic isolation bearing (3D-HSIB) with a longitudinal fluid viscous damper (L-FVD). The hybrid numerical model of the HSCS and the nonlinear numerical model of a CRFB prototype were simulated in OpenSees. The study comparatively analyzed the seismic responses of non-controlled bridges and HSCS-controlled bridges under near-fault ground motions, evaluated the seismic control effectiveness of the HSCS, and investigated the effects of vertical excitations on the seismic performance of the CRFB. The numerical results show that the proposed HSCS effectively reduces the three-directional seismic responses of the CRFB. When the PGA is 0.4 g, the seismic isolation ratios of displacement, internal force, and pounding force all exceed 0.4. The hysteresis behavior of the HSCS can be fully utilized under three-directional excitations with pulse-like effects. The girder-end uplift behavior under vertical excitation can be effectively controlled by the proposed 3D-HSIB with a seismic isolation ratio of at least 0.6. When vertical excitation exceeds a certain level, the bearing is tensioned upward while axial tension force may also occur in main piers. Furthermore, the seismic responses under pulse-like near-fault ground motions are significantly larger than those under non-pulse near-fault ground motions. This work may provide a useful reference for the seismic analysis and design of CRFBs.