Suite Of Ground Motions Research Articles

SVD enabled data augmentation for machine learning based surrogate modeling of non-linear structures

The computationally expensive estimation of engineering demand parameters (EDPs) via finite element (FE) models, while considering earthquake and material parameter uncertainty limits the use of the Performance Based Earthquake Engineering framework. Attempts have been made to substitute FE models with surrogate models, however, most of these models are a function of building parameters only. This necessitates re-training for earthquakes not previously seen by the surrogate. In this paper, the authors propose a machine learning based surrogate model framework, which considers both these uncertainties in order to predict for unseen earthquakes. Accordingly, earthquakes are characterized by their projections on an orthonormal basis, computed using SVD of a representative ground motion suite. This enables one to generate varieties of earthquakes by randomly sampling these weights and multiplying them with the basis, resulting in a large data set that is needed to train machine learning models. The weights along with the constitutive parameters serve as inputs to a machine learning model with EDPs as the desired output. Four competing machine learning models were tested and it was observed that a deep neural network (DNN) gave the most accurate prediction. The framework is validated by using it to successfully predict the peak response of one-story and three-story shear frame buildings represented using nonlinear spring–mass–damper systems, subjected to unseen far-field ground motions.

Numerical investigation on the seismic performance of a RC framed building equipped with a novel Prestressed LEad Damper with Straight Shaft

This study aims at assessing the use of a novel damper, which is called the Prestressed LEad Damper with Straight Shaft (or PS-LED), for the seismic rehabilitation of RC framed buildings. This device provides high energy dissipation by the friction activated between a lead core and a shaft and achieves a high specific output force by preloading the lead during the assembly. In order to show the effectiveness of the PS-LED device for the retrofit of existing buildings, a RC structure designed according to past codes that ignored seismic actions is retrofitted with the PS-LED system considering two different damage targets: (i) in the first case, the structure is retrofitted in order to behave elastically under the design earthquake; (ii) in the second case, a partially dissipative behavior of the structure is conceived, with activation of plastic hinges, and limited and reparable damage. In order to assess the suitability of the design procedure, non-linear static analyses are performed on the upgraded building, showing a satisfactory agreement between the seismic performance and the design target. Non-linear dynamic analyses are further carried out considering a suite of bidirectional artificial ground motions with response spectra matching on average the target spectrum according to the Italian Building Code for the life-safety limit state. Finally, a comparison is performed between the performances of the building retrofitted with the PS-LED device and the building retrofitted with a conventional steel hysteretic damper (SHD), demonstrating that the PS-LED, thanks to its superior damping capacity, limits the increase in internal forces that usually affects frames equipped with SHDs, reducing the need of local strengthening of the columns and foundations and consequently the total cost of the seismic rehabilitation.

Effects of pulse-like ground motions and wavelet asymmetry on responses of cantilever retaining wall

Fault-rupture processes produce pulse-like ground motions in near-fault regions. These motions are characterized as large-amplitude long-period pulsing that can cause severe structural damage. This study investigates their effect on cantilever retaining wall responses through finite-difference-based numerical simulations. We collect a suite of large-amplitude ground motions which are classified into pulse-like, non-pulse-like, and ambiguous using a pulse indicator. It turns out that relative wall movements by pulse- and non-pulse-like motions are not particularly different. Instead, relative wall movement is highly affected by ground motion Arias intensity, regardless of motion type. Additional simulations using original and reversed ground motion assess the effect of wavelet asymmetry (the ratio of peak amplitudes or energy values in wavelet positive and negative directions) on the wall response. Ground motion asymmetric to the wall movement direction generates larger wall displacement than ground motion asymmetric opposite to the wall movement direction. Relative wall displacements differ by up to approximately 46% depending on the direction of ground motion against the wall movement.

The Seismic Performance and Global Collapse Resistance Capacity of Infilled Reinforced Concrete Frames Considering the Axial–Shear–Bending Interaction of Columns

This paper presents a mechanism and method for simulating the axial–shear–bending interaction of a reinforced concrete (RC) column. The three-dimensional model of a multi-story infilled RC frame was modeled using the OpenSees software. Static pushover and nonlinear dynamic analyses under fortification and rare earthquakes were conducted using the model. Finally, based on the incremental dynamic analyses of 22 suites of ground-motion records, the global collapse resistance capacity of the infilled RC frame was evaluated using the evaluation method of a normal distribution. The analytical results show that the axial–shear–bending interaction is a key factor that affects the seismic response of infilled RC frames. Under the fortification earthquake condition, no obvious damage to physical structures was evident; the influence was relatively minor. However, under the condition of a rare earthquake, severe damage to physical structures was evident, resulting in the underestimation of the lateral inter-story drift ratio, while the degradation rates of the load capacity and global collapse resistance capacities for the infilled concrete frames were highly overestimated when the axial–shear–bending interaction was not considered.

Seismic analysis of a base-isolated reinforced concrete frame using high damping rubber bearings considering hardening characteristics and bidirectional coupling effect

High damping rubber bearings (HDRBs) are commonly used in base isolation systems, which can protect the structures from heavy earthquake damage by elongating natural vibration periods of the structures and improving the energy dissipation capacity of the systems. For the HDRBs, the stiffness hardening effect is exhibited at large shear strain, which is favorable to control the displacement of the bearings. Meanwhile, the horizontal hysteretic behavior in the two orthogonal directions is coupled and related to the path history. Here, the influence of the stiffness hardening and bidirectional coupling effects of the bearings on the seismic responses of the isolation system is studied. The full-scale quasistatic cyclic tests of two HDRB specimens with different dimensions were carried out under unidirectional and bidirectional loads. In the numerical analysis, the DHI and the classical Bouc-Wen model are used to simulate the hysteretic behavior of the HDRBs, in which the DHI model can capture the increased damping and stiffness, and bidirectional coupling properties. The parameters of both bearing models are determined according to the test results. Time-history analyses of a six-story reinforced concrete (RC) frame building with an HDRB isolation system are carried out for a suite of earthquake ground motions. The influence on seismic response of superstructure and isolation layer is analyzed with or without considering the hardening effect and bidirectional coupling effect of HDRBs. The results demonstrate that with consideration of the stiffness hardening and coupling effects, the peak responses of the superstructure significantly increase under high-intensity earthquakes. The analytical results suggest that the stiffness hardening and the effect of bidirectional coupling of the HDRBs should be considered in the design for HDRB isolation systems.

Site specific seismic hazard analysis of monumental site Dharahara, Kathmandu, Nepal

This study was carried out to estimate the seismic requirement for analyzing and designing the monumental structure, Dharahara, in Kathmandu, Nepal. A probabilistic seismic hazard analysis of the study region was performed to develop the elastic response spectrum for the bedrock site. The obtained spectrum was compared with the existing seismic codes, NBC 105:2020 and IS 1893:2016, to estimate the target response spectrum. One-dimensional equivalent linear (EQL) and non-linear (NL) ground response analyses were performed using DEEPSOIL v7, incorporating a suite of ground motions. The three borehole logs exhibited very soft soil in the region. Due to the presence of soft soil strata in the region under study, strength-corrected modulus reduction and damping curves were adopted in the current study to overcome the effect of larger shear strain. The average response spectra from EQL analyses predicted higher values than NL analyses. Finally, a comparison of the proposed design response spectrum with the code spectra was performed. Comparisons revealed that the mean EQL and NL outputs fall in between the design spectra of similar soil classes in NBC 105:2020 and IS 1893:2016 seismic codes.

Performance-based plastic design of multi-story hybrid braced frames with buckling-restrained braces and shape memory alloy braces

By employing two or more types of braces, hybrid braced frames (HBFs) combine the merits of the adopted braces and offsets their drawbacks. However, the corresponding seismic design method is still under development. To this end, this paper develops a performance-based seismic design (PBSD) method, based on the performance-based plastic design (PBPD) method. In current work, the buckling-restrained braces (BRBs) and shape memory alloy braces (SMABs) are introduced, because they are known for high energy-dissipating capacity and excellent self-centering capability, respectively. The participation extent of BRBs and SMABs is controlled by the stiffness contribution factor. The equations governing the design results are derived and the step-by-step procedure is detailed. A six-story benchmark frame is selected to demonstrate the design method. The designed HBF is subjected to a suite of earthquake ground motions, and the mean demands of seismic responses are compared against performance targets. It indicates that the HBF designed by the design method can well meet the design targets. A parametric analysis is conducted to reveal the effect of changing the stiffness contribution factor on the seismic demands of the HBF. This study validates the proposed design method for HBF, and it may also shed light to the PBSD method of the structures equipped with the other types of displacement-dependent damping braces.

FOSM-based updated consecutive modal pushover procedure (FUCMP) for seismic uncertainty quantification of jacket offshore platforms

In this paper, the updated consecutive modal pushover (UCMP) procedure is implemented in the framework of the first-order-second-moment (FOSM) approach, called herein the FUCMP procedure, to propose a simplified and fast method in order to quantify the effects of epistemic (modeling) uncertainties in jacket type offshore platforms (JTOP). The UCMP procedure can take into consideration the progressive changes in the modal characteristics of structures together with the higher mode effects in predicting the seismic demands of JTOPs. The proposed FUCMP procedure is applied to a case study JTOP structure for the ductility level seismic performance evaluation considering the uncertainties in the material mechanical properties, mass and weight, and geotechnical properties. The sensitivity analysis on the seismic drift demands along the platform height to the variability in different random variables is also conducted. To this end, the results of the UCMP procedure for the modified platform models created by different uncertain variables are compared with those of the mean deterministic model. Moreover, the results of the FUCMP procedure are compared with those from the nonlinear response history (NRH) analysis conducted using a suite of far-field earthquake ground motions. These records are scaled to the design abnormal level earthquake (ALE) response spectrum. The results indicate that the FUCMP procedure is successful in predicting the standard deviation of the seismic demands considering various sources of uncertainty. Conceptually, the proposed procedure can be applied for the probabilistic seismic demand evaluation of the other structural systems.

Shaking Table Tests of a Novel Flat Slab-Flanged Wall (FSFW) Coupled System with Embedded Concrete-Filled-Steel-Tubes in Wall Piers

The flat slab-flanged wall (FSFW) coupled system has gained popularity in recent years; however, its seismic performance remains an issue, as beams and columns in it are commonly eliminated. To tackle this problem, embedding concrete-filled steel tubes (CFSTs) in wall piers has been proposed to strengthen the system; the viability of this approach has been verified at the member level. Along this line, this study embarks on a shaking table testing of a 1/8-scale five-story FSFW structure equipped with CFSTs in walls, with an aim to understand the overall seismic behavior of such an enhanced system. As with the practice in many countries, the plan layout of the test structure consisted of four rows of wall piers, thus presenting a ‘fish-bone’ floor configuration that relied only upon the walls to resist gravity and lateral loads. The structure was subjected to a suite of input ground motions along with white-noise excitations. By so doing, its damage progression, pattern and dynamic characteristics were clearly identified. Furthermore, a non-linear time history analysis was conducted using PERFORM-3D, and the goodness-of-fit of the computed responses to the experimental records was examined. Findings indicated that the application of CFSTs was instrumental in resisting the simulated earthquake loads acting on the FSFW system, hence the global response limits required by codes of practice were met, even in the case of extremely strong earthquakes. Nevertheless, the junction between the shear walls and floor slabs was found to be the weakest links in the whole system. Designers are thus cautioned to implement proper detailing in those regions to prevent local distress, though it did not appear to acutely impair the system’s collapse-resisting capacity.

Fragility Analysis of Helical Piles Supporting Bridge in Different Ground Conditions

This paper examines the seismic performance of a bridge–helical pile foundation based on the seismic fragility analysis, considering the element fragility of a coupled bridge–soil–foundation system. Nonlinear time history analyses were conducted using a finite-element modeling scheme that was validated using the results of large-scale shaking table tests of soil–piles–structure systems involving both liquefiable and nonliquefiable soils. Incremental dynamic analysis was conducted to generate the fragility curves for a two-span bridge supported on helical piles in nonliquefiable and liquefiable sites considering a suite of ground motions. The damage limit states were defined to describe the capacity of the bridge components. In total, 440 nonlinear time-history analyses were performed to evaluate the seismic demand of the helical piles and the bridge-reinforced concrete pier components, and the results were used to establish their fragility curves. The results revealed that the helical piles were the most fragile component in the nonliquefiable and liquefiable tests. The liquefiable soil could decrease the seismic demand on the column lateral deformation and increase the demand dispersion. On the other hand, the reinforced concrete pier exhibited a large drift response in the nonliquefiable soil, causing it to be more vulnerable to seismic hazards than in the case of liquefiable soil.

Seismic fragility assessment of building-type structures in oil refineries

A seismic fragility assessment methodology is presented for equipment-supporting reinforced concrete and steel buildings that are typically encountered in oil refineries. Using a suite of hazard-consistent ground motions and reduced-order models, incremental dynamic analysis is performed to obtain the seismic demand of the structural systems examined. Appropriate drift- and floor acceleration-sensitive failure modes are considered to define the limit state capacities of the supporting structure and the nested non-structural process equipment. Special care is exercised on the demand and capacity representation of structural and non-structural components, offering a transparent roadmap for undertaking analytical fragility assessment for equipment-supporting buildings typical to an oil refinery. The findings and the proposed methodology can be exploited by designers and facility managers for mitigating the risk of failure prior to the occurrence of an earthquake event, for designing the pertinent structures and their non-structural components by means of a risk-aware performance-based methodology, or as feed data in early warning systems.

Optimizing Viscous Damper Placement with Element Exchange Method and Energy-Based Distribution Methods for Building Structures

Viscous dampers enhance the seismic performance of buildings by increasing the capacity for damping. It remains a challenge to decide on the optimum placement for the added dampers and it has been a continuing subject of research. In this paper, Element Exchange Method and energy-based distribution methods are proposed. The proposed methods are compared against currently available methods through case studies. The case studies are based on a two-dimensional ten-story shear building adopting different nonlinearity of viscous dampers and a three-dimensional steel moment-resisting-frame (MRF) building with varying plan asymmetry. The methods then automatically develop damper placement schemes, and subsequently, the buildings are evaluated for their maximum peak inter-story drift against a suite of ground motions. The result shows that Element Exchange methods and energy-based distribution methods can successfully reduce maximum peak inter-story drift over all case studies.

Influence of ground motion characteristics (velocity pulse and duration) on the pile responses in liquefiable soil deposits

In this paper, three-dimensional finite-element analyses are conducted to study the effects of ground-motion duration and the presence of velocity pulse on the pile responses in liquefiable soils. The soil-pile system consisting of liquefiable granular deposits and rigid piles is modeled in OpenSeesPL. Four suites of ground motions, namely one pulse-like, one non-pulse-like, one long-duration, and one short-duration suite, are appropriately selected from worldwide earthquake events. Dynamic analyses are conducted for the soil-pile system under the excitations of four selected ground-motion suites. It is indicated that the pulse-containing ground motions bring in greater pile-bearing bending moments and shear forces as compared to the non-pulse-like suite, with the increase ratios of the mean peak bending-moments and shear-forces up to 27% and 34%, respectively. Furthermore, as compared to the short-duration suite, using the long-duration ground motions yields similar pile responses (bending moments and shear forces) but a substantial increase of the pile's permanent lateral displacements. The mean lateral displacements at pile head are 0.62 m and 0.17 m under the excitations of the long- and short-duration suites, respectively. In addition, correlation analyses indicate that peak ground velocity and Arias intensity can be regarded as effective intensity measures to evaluate the pile response (e.g., bending moment) and lateral displacement for a pile-reinforced liquefiable ground system. The results could provide useful insights for applying the pile-pinning approach in areas where pulse-like or long-duration ground motions are of the main concern.

Assessments on seismic performance of self-centering hybrid damping systems under far-field and near-field ground motions

This research is conducted to perform the assessment on seismic performance of novel self-centering systems equipped with hybrid energy dissipaters, which are composed of hysteretic energy dissipaters in parallel with viscous energy dissipaters. The hybrid energy dissipating mechanism equips the self-centering system with significant energy-dissipating capacity. An equivalent self-centering damping ratio ξSC is firstly proposed to quantify the energy-dissipating capacity from the two types of dampers, respectively. Single-degree-of-freedom systems (SDOF) with different dynamic parameters are then developed. Nonlinear response history analyses on SDOF are performed using a suite of far-field and near-field ground motions to make the assessment. It is shown that the near-field ground motions will lead to much more ductility demands and acceleration responses for the considered systems compared with far-field ground motions. Increasing the damping for the system will decrease the ductility demand but will lead to the dramatic acceleration amplification. Moreover, adjusting the equivalent self-centering damping ratio between the two types of damping will change dynamic response of the system. Increasing the proportion of hysteretic damping will decrease the ductility demand and increase the acceleration response of the considered system, especially under far-field ground motions. Ductility demand spectra of the considered system for seismic design are established based on results from SDOF analyses. Finally, system-level analyses are performed on a novel knee bracing frame with self-centering hybrid damping capacity to validate the practicability of ductility demand spectra, and conduct the assessment.

Seismic zonation and default suites of ground-motion records for time-history analysis in the South Island of New Zealand

The rise of performance-based earthquake engineering, in combination with the complexity associated with selecting records for time-history analysis, demonstrates an expressed need for localized default suites of ground motion records for structural designers to use in the absence of site-specific studies. In the current research investigation, deaggregations of probabilistic seismic hazard models (National Seismic Hazard Model, Canterbury Seismic Hazard Model, and Kaikōura Seismic Hazard Model) and the location-specific seismological characteristics of expected ground motions were used to define eight seismic hazard zonations and accompanying suite profiles for the South Island of New Zealand to satisfy the requirements of the New Zealand structural design standard NZS1170.5 for response-history analyses. Specific records, including 21 from the recent Kaikōura, Darfield, and Christchurch earthquakes, were then selected from publicly-available databases and presented as default suites for use in time-history analyses in the absence of site-specific studies. This investigation encompasses seismic hazards corresponding to 500-year return periods, site classes C (shallow soils) and D (deep soils), and buildings with fundamental periods between 0.4 and 2.0 seconds.

Seismic robustness assessment of steel moment resisting frames employing material uncertainty incorporated incremental dynamic analysis

Seismic robustness can be a useful tool to evaluate the structural system capacity to withstand seismic damages. Robust structures can minimize their vulnerability to the progression of damages (extensive) and thereby prevent or avert the collapse risk in the event of seismic shocks. Neglecting the effect of topology, the structural robustness can be reduced to the function of seismic record and material properties. It can be taken as the ratio of mean annual frequency of exceeding a given limit state of interest considering record variability alone and the mean annual frequency of exceeding the given limit state of interest incorporating record as well as material uncertainty. The present study attempts to quantify the seismic robustness of 4-storey and 8-storey moment-resisting frames in terms of the above two mean annual frequency of exceeding the given limit state of interest. Based on the past research reports, a 10% Inter-storey Drift Ratio, IDR where transient drift is competent to permanent drift is considered as the collapse limit state of interest for the investigation. The study buildings are modeled in the OpenSees platform and Incremental Dynamic Analyses, IDA of the adopted structures for the selected suite of ground motion records coming under various hazard levels are carried out. IDA curves, which relate the normalized seismic Intensity Measure, IM and maximum IDR, IDA curve derived fragility curves, robustness parameter, and correction for response reduction factor are presented for the given intensity hazard level.

Assessment of spectrum-matched time histories for floor response spectra generation

The floor response spectra (FRS) are usually determined by time history analyses using spectrum-matched time histories. However, previous studies reported that spectrum-matched time histories may cause a biased estimate of FRS, which raises a challenge of FRS generation for nuclear power plant structures. The study focuses on resolving the challenge. We begin the study by discussing characteristics of amplitude-scaled and spectrum-matched ground motions for seismic design and analysis of nuclear facilities. Then, we explore the influence of spectral matching on the change of ground motion parameters. Using suites of amplitude-scaled and spectrum-matched ground motions, we determine FRS for examples of typical structures, and investigate how the change of ground motion parameters due to spectral matching affects the biased estimate of FRS. The investigation shows that, the change of damping scaling factor is remarkably correlated with the biased estimate of FRS. Thus, we propose the improved spectrum-matched time histories for floor response spectra generation. Taken a service building of nuclear power plants as an example, the study shows that, the improved spectrum-matched are adequate to achieve approximate unbiased estimate of FRS under different damping values.

Influence of ground motion type on nonlinear seismic behaviour and fragility of corrosion-damaged reinforced concrete bridge piers

Two identical reinforced concrete (RC) bridge piers including a rectangular and a circular section are considered. The influence of corrosion damage, non-stationary characteristics of ground motions, and cross-sectional shape on nonlinear dynamic behaviour, failure mechanism and failure probability of these piers is investigated. An advanced modelling technique, capable of modelling coupled influence of inelastic buckling and low-cycle fatigue degradation of reinforcement, is employed to simulate the nonlinear structural behaviour of the piers. The considered bridge piers with various mass loss ratios (as a measure of corrosion) are subjected to a series of static pushover analyses and incremental dynamic analyses under three different suites of ground motions such as, Far-Field (FF), Near-Field With Pulse (NFWP), and Near-Field with No Pulse (NFNP). Furthermore, an advanced matching algorithm is used to investigate the effect of non-stationary content of near-field earthquake records including the presence of large pulses in ground motion time series on the nonlinear dynamic behaviour of the corrosion-damaged RC bridge piers. Finally, fragility curves are developed for each corroded bridge pier with different corrosion ratios subjected to each ground motion suite. Analyses results show that the failure mechanism of the corrosion-damaged bridge piers significantly depends on the cross-sectional shape and ground motion type. It is concluded that while both of the piers with slight corrosion levels are much more vulnerable under NFWP ground motions than those under FF and NFNP ground motions; the probability of failure of the extremely corroded bridge piers is approximately the same regardless of ground motion type.

Broadband ground motion simulations for Northeast India

This study presents broadband (BB) ground motions for two historical earthquakes (Mw ≥ 8) in Northeast (NE) India. We apply a recently developed geology-based 3D velocity model of the Northeastern region (NER) to simulate low frequency (LF) ground motions (f < 1Hz). These synthetics are further combined with high frequency (HF) scattering wave-fields (f > 1Hz) using a hybrid approach, thus yielding BB ground motions (0.1–10Hz). First, we calibrate the 3D model and scattering parameters by comparing the simulated ground motions against recorded data available for four events (Mw 5.2, Mw 5.5, Mw 6.3, and Mw 7.2) in NE India. Our results indicate that the variability of the bias for spectral acceleration (SA) lies within limits [-1,1] for all four events. From the statistical analysis of the ground motion, we also observe that the hybrid approach gives a realistic estimation of site-specific BB ground motion and consistently reproduces the prominent features of the observed motions up to 10Hz at most of the stations. Next, we simulate a suite of BB ground motions to estimate ground motion intensities for two historical earthquakes (viz., 1897 Assam event, 1950 Assam event) in NE India. Our results indicate that the variations in the ground motion intensities at hard rock and soft soil sites are manifestations of the local geology and surface topography incorporated in the 3D regional velocity model. We also observe that the peak ground acceleration (PGA) for both events exceed 1 g in the epicentral region. The BB ground motions obtained from this approach will be useful to engineers in seismic design and site-specific hazard estimation.

Reliability of structural wall shear design for tall reinforced-concrete core wall buildings

The developments of shear design acceptance criterion for structural walls in tall reinforced-concrete core wall buildings were reviewed and reliability studies were conducted. Over the past 10 years, the seismic action portion of the shear design acceptance criterion has consistently been in the general format of γFu ≤ ϕFn,e, where γ is the demand factor, Fu is the mean shear demand resulting from a suite of ground motions, Fn,e is the nominal shear strength computed from expected material properties, and ϕ is the uncertainty in Fn,e. Between the governing code, ASCE7-16, and the tall building guidelines, PEER TBI (2010, 2017) and LATBSDC (2011, 2014, 2017, 2020), there has been a lack of consensus in the recommendations for γ and ϕ factors. Thus, reliability studies were conducted on the shear design acceptance criterion, with 20- and 30-story case study buildings designed and analyzed per LATBSDC (2014) guidelines. Using closed-form solutions and Monte Carlo simulations, for the range of f’c considered, the shear design acceptance criterion using γ = 1.5, ϕ = 1.0, and a conservative dispersion in shear demand of ρD = 0.50 resulted in at least 94.2% reliability, conditioned upon MCE hazard level ground shaking, suggesting that the current LATBSDC (2020) recommendations provide adequate safety. For a reduction in safety (e.g., use of γ < 1.5), further reliability studies including a larger population of tall buildings will be needed to calibrate γ and ϕ factors.