Ground Motion Excitation Research Articles

Characteristics of high-frequency attenuation in the Dead Sea Basin

In this study, we derive the characteristics of high-frequency attenuation and excitation of ground motion for the Dead Sea Basin (DSB) area by regressing the peak amplitudes of narrowband-filtered velocity seismograms measured around the shear wave arrivals. We analyzed about 2000 seismograms from 43 local earthquakes in the magnitude range of MW = 0.9–4.5 that occurred in and around the DSB. The regional crustal attenuation is modeled with a frequency-independent piece-wise continuous linear geometrical spreading function and a frequency-dependent quality parameter Q. Our analysis exhibits that S wave attenuation in the DSB has irregular behavior with the effects of arrivals of supercritical reflections. For distances r ≤ 20 km, the geometrical spreading is 1/r; for distances r ≥ 40 km, the geometric attenuation is r−0.4; and for distances 20 < r <40 km, it is r−0.5. The quality parameter Q is modeled as Q = 68f0.5. The excitation is modeled using the proposed propagation model, increasing stress drop and a high-frequency roll-off parameter κ = 0.03 s. To model the theoretical excitation, we use stress drop values ∆σ = 3 ΜPa, ∆σ = 4 ΜPa, ∆σ = 8 ΜPa, and ∆σ = 9 MPa for earthquakes of MW = 3, MW = 3.3, MW = 4, and MW = 4.5 and site amplification factor of 3. Applying the extended coda normalization method to earthquakes in the DSB area provides quality factors for S and P waves of Qs =80f0.81 and QP = 41f0.84 and QS/QP ratio varying in the range of 1.7–2.9. Anisotropy of seismic wave attenuation in the DSB area is observed. The main direction of anisotropy N20° (110°) is determined based on the model of two orthogonal components producing maximum separation of the attenuation function values.

Seismic damage identification by fitting the nonlinear and hysteretic dynamic response of monitored buildings

The identification of hysteretic degrading systems exposed to nonstationary loading is a paramount research topic, especially in the case of structures subjected to ground motion excitations. In this paper, the data recorded by a masonry building are used to detect the presence of seismic damage. To this aim, a parametric nonlinear identification is performed by adopting a Bouc–Wen-type multiple oscillator model. Starting from the results of the identification process, a damage index based on the degrading stiffness matrix is defined. The damage index provides preliminary information to be used in post-event management. Finally, the identification procedure is applied to a monitored masonry structure, the Town Hall of Pizzoli (L’Aquila), which is part of the Italian Seismic Observatory of Structures (OSS). The records measured by accelerometers permanently installed on the building are used for the detection of damage occurred during a seismic event.

High-resolution monitoring of the initial development of cracks in experimental masonry shear walls and their reproduction in finite element models

Induced seismicity in the north of the Netherlands has recently exposed unprepared, unreinforced masonry structures to considerable earthquake hazard. While the ultimate-limit state capacity of the structures is vital to assess the individual’s risk, their behaviour during more frequent, lighter earthquakes leading to ‘light damage’, has shown to be strongly linked to economical losses and societal unrest. To study the effect of these repeated light earthquakes, the behaviour of masonry structures, typical of the region, needs to be investigated when subjected to multiple small cycles. In particular, insight into the potential aggravation of the light damage is sought. Hence, shear walls of replicated clay brick masonry have been tested in the laboratory and exposed to a high number of low in-plane drift cycles. The experiments show, under inspection with high-resolution Digital Image Correlation, how stair-case, diagonal cracks initially distribute in multiple cracks running in joints at the centre of the walls, but later localise into a single, visible, wider crack. The tests are used to establish an interval over which light damage can be expected for these types of walls.Furthermore, this initial distribution of strains in ‘bands’ and subsequent localisation, previously observed only with computational finite element models and which could not be verified experimentally, is compared and reproduced with improved models both at the composite continuum scale (macro-model) and the brick-to-brick micro scale. The models are calibrated based on the quasi-static, cyclic experimental results and later used for extrapolation in dynamic nonlinear settings to assess the influence of natural ground motion excitations.

Performance of torsionally eccentric RC wall frame buildings designed to DDBD under bi-directional seismic excitation

The seismic performance of a class of reinforced concrete eccentric wall frame buildings under bi-directional ground motion excitation is investigated. For this purpose, four and eight-storey RC buildings comprising structural walls, flat-plates and gravity columns, having different strength and stiffness eccentricity and torsional restrain, are designed in accordance with the Direct Displacement Based Design (DDBD) method for damage control. The structures are analysed under static and dynamic inelastic response, considering ten ground motion record pairs, three intensity levels and different angles of incidence. Several global and local engineering demand parameters, namely, inter-storey drift, slab rotation, member plastic rotations and wall internal forces are evaluated and compared with the DDBD limits. It is shown that ground motion directionality effects are small, and the average drift and wall moment profiles are within DDBD predictions and, often, considerably lower than DDBD demanded. However, wall shear force and local inelastic demands are underpredicted by both DDBD and static inelastic analyses and, more significantly so, for the torsionally flexible structures.

Assessment of seismic vulnerability of continuous bridges considering soil-structure interaction and wave passage effects

Identical ground motion excitations at all supports are typically assumed in practice for the design and seismic analysis of structures. Such assumption is customarily made despite the fact that ground motions do vary among supports especially for extended-in-plan structures such as long continuous bridges, tunnels, dams, pipelines, etc. This paper focuses on studying the effects of non-synchronized motion (due to the difference in the ground motion arrival time at different bridge supports) on the seismic performance of continuous box girder bridges, and the severity of such effects whether soil-structure interaction, SSI, is ignored or considered in the analysis. The study is carried out on a nine-span bridge with a total length of 430 m supported on deep (piled) foundation embedded in sandy soils. Three different sand soil profiles are investigated representing medium to stiff soil. Eighteen real bedrock earthquakes are extracted from PEER database and incremental nonlinear dynamic analyses are conducted for different apparent wave propagation velocities (namely, 100, 420, 1000 m/s, and ∞). Fragility curves are hence generated for three pre-selected performance levels (namely, Operational, Life Safety, and Complete Collapse) considering some modeling and capacity uncertainties. The fragility curves are finally compared to examine the joint effect and severity of non-synchronized support motions and soil-structure interaction on the seismic performance and vulnerability of investigated bridges. Results demonstrate that the wave passage effect and SSI significantly affect the bridge response and the probability of exceeding various pre-selected performance levels, especially for scenarios with low apparent velocity and schemes with soft soil.

Optimal seismic retrofit of fractional viscoelastic dampers for minimum life-cycle cost of retrofitted steel frames

Previous studies have shown that fractional derivative models present a plausible level of accuracy when it comes to capturing the behavior of viscoelastic dampers due to being capable of expressing the effects of excitation frequency and ambient temperature in viscoelastic damper parameters. Considering the possibility of earthquake occurrence during a structure lifetime, minimizing the life-cycle cost of a building is a practical way of optimizing the seismic design or retrofit plan and performance of the structure. In this paper, the total life-cycle cost of a structure with fractional viscoelastic dampers is chosen as the objective of an optimization problem while considering the effects of the changes in the ambient temperature and uncertainty related to the ground motion excitation which are the decisive parameters in the life-cycle cost analysis. The results for a 5-story steel frame show lower total life-cycle cost, initial cost, and story drift for the optimized structure compared to one with uniform distribution of dampers. Also, more use of dampers in optimized structure with life-cycle cost minimization objective, compared to a structure designed to minimize the cost of the dampers, has led to a significant reduction in the expected life-cycle damage costs.

Quantifying the Influence of the Horizontal Slab Systems on the Building Resisting Performance against Earthquake

Floor systems play an integral part with the vertical components in resisting the lateral as well as the gravity loading. The purpose from this study is to identify the effect of the horizontal slab systems on the building resisting performance against ground motion excitations.

Collapse Simulation and Response Assessment of a Large Cooling Tower Subjected to Strong Earthquake Ground Motions

Large cooling towers in thermal power plants and nuclear power plants are likely to suffer from strong earthquakes during service periods. The resulting destructions of the cooling towers would endanger the power plants and threaten the security of the related areas. It is important to use effective means to evaluate the safety status of the cooling towers and guide further precautions as well as retrofitting efforts. This paper is therefore focused on an elaborate numerical investigation to the earthquake-induced collapses of a large cooling tower structure. A complete numerical work for simulation of material failure, component fracture, structural buckling and system collapse is presented by integrating the stochastic damage constitutive model of concrete, refined structural element models, and some other key techniques. Numerical results indicate that the damage behavior and collapse mode of the cooling tower are affected notably by the randomness specification of ground motions. The collapse mechanisms of the cooling tower are studied from the energy absorption and dissipation points of view. An effective energy-based criterion is introduced to identify the collapse of the cooling tower under ground motion excitations. While distinct collapse modes are observed, the collapse criterion can predict well the damage and failure of the cooling tower. The proposed methodology is vital to better understanding the disastrous mechanisms and potential failure paths in optimal design of the cooling towers to ensure safety.

Numerical modelling of cracking in gravity dam under static and seismic loadings with multiple pre-embedded discontinuity FEM

This paper presents a numerical study on the earthquake analysis of a concrete gravity dam. The numerical approach is based on the multiple pre-embedded discontinuity finite element method. The equations of motion with the ground motion excitation, static self-weight, hydrostatic reservoir load and the hydrodynamic reservoir loads as the boundary conditions are solved with explicit time stepping. Mass proportional damping is used instead of the usual stiffness proportional damping since the latter drastically decreases the critical time step of explicit time integration. The Koyna dam with the genuine accelerograms recorded during the 1967 earthquake is analyzed as the numerical example. The simulation results agree with the previously reported results by other studies, i.e. similar crack propagation in the monolith is predicted here.

Influence of Traffic Load and Far-Field Seismic Excitation in Cables of Cable-Stayed Bridge

Bridges are always expose to the cyclic live loads in the form of moving traffic and wind loads. This could jeopardise the service life of the bridges peculiarly in fatigue. With the rise of seismic activities nowadays, bridges may become more susceptible to fatigue damage. Thus, in this study, the static and dynamic behaviour of a cable-stayed bridge in terms of fatigue of steel elements are addressed by focusing on the stress variation in stay cables due to the effects of moving traffic loads and ground motion excitations. Fatigue Load Model 4 (FLM 4) has been adopted as a dynamic response on the bridge subjected to moving load while a two set of ground motions considering horizontal and vertical motions from far-field faults has been used as seismic excitation on the bridge. The results of this study revealed the notable influence of traffic loads on stress variations of cable-stayed bridges with significant effects due to ground motions scaled to the expected return period.

ANALYSIS OF LOCAL SITE EFFECTS ON SEISMIC RESPONSE OF FRAME STRUCTURES SUPPORTED BY FOUNDATIONS AT DIFFERENT ELEVATIONS

Frame structures supported by foundations with different elevations (such as a structure on the slope) undergo different ground motions because of the local site effect. Seismic response of the structure under non-uniform excitation of ground motion is different from that under uniform excitation on upper embedding end or lower embedding end. Sites with the same height difference and different soil conditions were set up in this paper to subject them to ground motions related to different positions and the seismic response of structures built on the sites under uniform and non-uniform strong seismic excitation were compared. The results show that the predominant period and amplitude of ground motions on upper embedding end and lower embedding end are different and affected by soil conditions. The extent and law of amplification of shear force, displacement and storey drift of structures under non-uniform excitation differ from those under uniform excitation when soil changes from soft to medium-hard soil. In addition, the position and degree of plastic hinges change under non-uniform excitation compared to uniform excitation. The uniform excitation using the ground motion on the upper embedding end is not always safe for the frame structure supported by foundations at different elevations.

Ground motion selection and modification evaluation for highway bridges subjected to Bi-directional horizontal excitation

Reinforced concrete (RC) highway bridges are primary lifeline infrastructures where accurate seismic structural analysis is essential to ensure the safety of bridges. The nonlinear seismic behavior of bridge structures generally exhibits distinct differences in the longitudinal and transverse directions, and thus studies on bi-directional ground motion excitation are needed. Performance-based earthquake engineering (PBEE) methodology explicitly takes into account uncertainties in hazard, structural response, damage, and loss estimation such that enables a comprehensive understanding of the structural performance in a probabilistic manner. For a given large ground motion scenario, this study takes advantage of the PBEE methodology (the first two stages of hazard and structural response) to develop a benchmark probability distribution of the seismic demands (PDSD) for a given bridge structure considering different assumed intercept angles of the input ground motions with the bridge longitudinal axis. The accuracy and reliability of the PDSD estimates from various ground motion selection and modification procedures for the purpose of future bridge designs are evaluated against this reference benchmark PDSD. Such an evaluation is conducted for several engineering demand parameters of three representative RC bridges in California.

A Simplified Model for Investigation on the Effects of Seismic Actions on Masonry Arch Bridges

Masonry bridges are vulnerable structural systems to the ground motion excitation and their survival in case of such incidents has to be studied in detail. In this work, a simplified model for dynamic analysis of masonry bridges based on rocking motion of rigid blocks is proposed. Using this model, nonlinear time integration analysis on these bridges can be done with ease and in a short time. The proposed model was used in evaluation of seismic performances of a monumental masonry bridge subjected to both horizontal and vertical seismic actions. The study shows the importance of vertical component of ground motion in determination of internal forces and shear sliding deformation at bottom of the bridge’s pier. The proposed model has also shown its ability in defining the effectiveness of a seismic retrofit approach for the same bridge system in a comparative study. According to this investigation, seismic performances of the bridge can be significantly improved in case of adding ductility to its deck assembly.

Current Status and Future of Regional Seismic Network Monitoring in the Central and Eastern United States

AbstractThe central and eastern United States (CEUS) is an area of generally low-to-moderate seismic hazard with a number of large cities with high seismic risk, a history of occasional damaging earthquakes, and seismic activity induced by wastewater disposal. Seismic monitoring in the CEUS, which began at the beginning 1900s, has undergone many changes through time. Over the past two decades, broadband digital seismic stations connected by internet communications have become widespread. Modern data processing systems to automatically locate earthquakes and assign event magnitudes in near-real time have become the norm, and, since the inception of the Advanced National Seismic System in 2000, more than 10,000 earthquakes have been located and cataloged. Continuously recorded digital seismic data at 100 samples per second are allowing new avenues of research into earthquake source parameters, ground-motion excitation, and seismic wave propagation. Unfortunately, over the past two decades the number of regional seismic network (RSN) centers has diminished due to consolidations and terminations, as funding has tightened. Nevertheless, the public in different parts of the CEUS still looks to local experts for information when earthquakes take place or when they have questions about earthquakes and seismic hazard. The current RSNs must evolve to encompass the need for local seismic information centers and to serve the needs of present and future research into the causes and effects of CEUS earthquakes.

Nonlinear dynamic behaviour of severely corroded reinforced concrete columns: shaking table study

A set of benchmark, medium scale, shaking table tests on corroded reinforced concrete (RC) columns is conducted with the aim of investigating the effects of corrosion damage on the nonlinear dynamic behaviour of RC bridge piers. The experimental programme consists of an uncorroded control specimen and two corroded RC column specimens, with identical structural details. An accelerated corrosion procedure is used to corrode the RC columns. The uncorroded and corroded specimens are subjected to far-field long duration ground motion excitations. The two corroded columns had 51% and 65% average mass loss ratios. The testing sequence includes slight, extensive, and complete damage levels, followed by an aftershock to examine the cascade effect on the nonlinear dynamic response of the proposed RC columns. The experimental results show that corrosion changes the failure mode of the RC columns, and has a significant negative impact on the residual strength (about 50% mass loss results in about 80% strength reduction) and drift capacity of RC columns.

Significance of Ground Motion Scaling Parameters on Amplitude of Scale Factors and Seismic Response of Short- and Long-Period Structures

In this study, an ensemble of ground motions is selected and scaled in order to perform code-compliant bidirectional Nonlinear Response History Analysis for the design purpose of both short- and long-period structures. The followed scaling method provides both the requirements of the Turkish Earthquake Code regarding the scaling of ground motions and compatibility of response spectra of selected ground motion pairs with the target spectrum. The effects of four parameters, involved in the followed scaling method, on both the amplitude of scale factors and seismic response of structures are investigated. These parameters are the number of ground motion records, period range, number of periods used in the related period range, and distribution of weight factors at the selected periods. In the analyses, ground motion excitations were applied to both fixed-base and seismically isolated structure models representative of short- and long-period structures, respectively. Results revealed that both the amplitudes of scale factors and seismic response of short-period structures are more prone to variation of investigated parameters compared to those of long-period structures.

Earthquakes Effect on the Behavior of Multi-Story RC Building Using Non-Linear Dynamic Analysis

One of the buildings that were designed depending on the old Syrian Code is selected and modeled using SAP2000 by taking its non-linear properties to be checked under dynamical loading. Three earthquake records are taken and applied to the model as Time History loading cases. The dynamical displacement of the top roof of the building and the hysterical diagrams of the relation between base shear and roof displacement is compared and discussed. Asymmetry of shear walls and cores is responsible of the differences in responses of building elements, and insufficient nonlinear modeling of shear walls prevents from finding the real capacity of the system, although comparing pushover curves with hysteric loops from the applied ground motion excitations shows that the building is capable, depending on its old design, to withstand various types of extreme ground motions and earthquakes.

Development of a Refined Analysis Method for Earthquake-Induced Pounding between Adjacent RC Frame Structures

Pounding of two adjacent structures is one of the factors that cause damage and hinder sustainable use of reinforced concrete (RC) frame structures under strong ground motion excitations. This study developed a pounding analysis method with a refined beam-column element in order to solve the pounding problem between two RC frame structures. The analysis method combines the fiber beam-column element model with the element sections discretized into concrete and longitudinal rebar fibers, the Hertz-damp contact element model to describe the pounding between beam-column elements, and the method to integrate the pounding force into the system dynamic equilibrium equation. The pounding can be considered either at the level between the story slab to slab or at the level between story slab to mid-column. The application of the proposed method in pounding analyses to provide a rational seismic separation gap between two adjacent RC frame structures is finally conducted to increase their safety and sustainability under strong earthquakes.

Numerical study on site response considering ground motion spatial variation

Identical support excitation (ISE) of ground motion has been utilized to study the site response, whereas, to a large-scale site, multiple support excitation (MSE) considering the ground motion spatial variation may be more applicable. In this paper, the spatially varying earthquake ground motions (SVEGMs) were simulated first. Then, a large-scale four-layered free-field model was established using ABAQUS. Taking ISE and MSE into account, the significant durations of acceleration responses and shear wave velocities at many observation points were obtained. Comparing the results under MSE with those under ISE, it was shown that: i) the significant durations of acceleration responses under MSE were shorter than those under ISE; ii) even though the input peak ground acceleration (PGA) for MSE is lower, the shear wave velocity under MSE is not necessarily higher. Therefore, the spatial variation of ground motion may have important influence on the seismic response of large-scale site.

Comparing seismic performances of pilotis and bare RC frame structures by shaking table tests

This study investigates the role of masonry infill walls in the responses and failure modes of RC frames subjected to earthquake, by applying shaking table tests on two three-story, two-bay, 1/4-scale RC frame structures. Two types of RC frame structures, an infilled RC frame without infill walls in the first story (pilotis frame) and a bare RC frame, are constructed according to the seismic design codes of China. The pilotis and bare frames are simultaneously deployed on a shaking table when I-ELC180 ground motion and Wolong ground motion excitations are used. The two specimens are subjected to a sequence of dynamic tests with increasing intensity. This paper mainly presents and compares the phenomena observed from the shaking table tests of the pilotis and bare frames, including the dynamic properties, acceleration and displacement responses, evolution of damage and final failure mechanisms. The tests indicate that, compared with the bare frame, the infill walls in the pilotis frame can significantly affect the failure mode and damage distribution. Plastic hinges first appear in the 1st story columns for the pilotis frame under seismic loading, while plastic hinges first appear at the beam ends in the 1st story for the bare frame; The damage to the pilotis frame is increasingly concentrated in the 1st story as the earthquake intensity increases, while the damage distribution along the height of the bare frame remains almost the same as the earthquake intensity increases. The maximum interstory drift ratio of the pilotis frame is up to 50% larger than that of the bare frame under the same earthquake intensity.