Suite Of Ground Motions Research Articles

Effects of sample size of ground motions on seismic fragility analysis of offshore jacket platforms using Genetic Algorithm

Intense seismic excitations might be considered as one of the most important loading cases imposed on offshore jacket platforms. To perform seismic assessments for these structures, appropriate suites of ground motions can be selected by different methodologies. Since time history analyses of fixed offshore platforms are truly time consuming, number of selected records can considerably change the computational expenses. This paper quantitatively investigates the effects of sample size of records on seismic performance of these structures. Thus, several record suites with different sample sizes are selected from a target population with 40 records so that the differences between the logarithmic mean and standard deviation of their response spectra become minimum using Genetic Algorithm. The results show that the fragility curves based on the suites with larger sample sizes are closer to the target fragility. Nevertheless, according to the results of the investigated case study, some suites with smaller sample sizes (e.g. 20 or 25) could lead to relatively acceptable accuracy at almost half the computational expenses. Also, the correlation coefficients between the collapse fragilities of the selected record sets and the target one are very close to unity, indicating that these collapse fragilities are linearly correlated with the target one.

Numerical assessment of vertical ground motion effects on highway bridges

Field evidence of recent earthquakes shows serious bridge damages due to the direct compression or tension in the columns and some flexural and shear failures caused by the variation in axial force of the columns. These damages could not be produced solely by the horizontal seismic excitations; the vertical component of the earthquake is involved. This paper presents a numerical study highlighting the presence of vertical seismic excitation. Nonlinear time history analyses are conducted on detailed three-dimensional models of multi-span simply supported and multi-span continuous bridges using a suite of representative ground motions. The results showed the significant influence of vertical excitation on the bridge responses. Therefore, it is imperative to include more efficient criteria to upgrade the design codes and extend practical techniques that consider and cope with the structural effects of vertical ground motion along with the horizontal excitations.

Seismic performance evaluation of steel frames with pre-pressed spring self-centering braces

Pre-pressed spring self-centering energy dissipation (PS-SCED) braces with flag-shaped hysteretic responses are expected to undergo large deformation while re-centering to their origin, providing a self-centering capability for the structures. Three 4-, 8-, and 16-story braced frames with PS-SCED braces were analyzed to assess their seismic performances in comparison to conventional steel braced frames (CSBFs) and buckling restrained braced frames (BRBFs). Detailed analytical models of the PS-SCED braces were implemented and seven suites of ground motions with three different hazard levels were used to assess the structural performances with respect to interstory drifts, residual deformations, and peak floor accelerations during earthquakes. It was shown that the CSBFs underwent larger average responses than the BRBFs and the PS-SCED braced frames. The interstory drifts and peak floor acceleration responses of the frames with the BRBs were comparable and often better than those of the frames with the PS-SCED braces due to their high energy dissipation and simpler hysteretic cycles; the difference in the performance of these two braces tended to decrease with the increase in the seismic hazard level and the building height. The PS-SCED braces reduced the residual deformations more effectively than the BRBs and CSBs.

Quantifying near fault pulses using generalized Morse wavelets

Recently, a family of analytic wavelets known as generalized Morse wavelets (GMW) has been established in the literature. This versatile wavelet family uses two parameters (γ and β) to adjust its frequency or time domain properties. By varying these two parameters, this wavelet family includes many commonly used analytic wavelets. In this paper, GMW are examined, along with the continuous wavelet transform (CWT), to determine their suitability for extraction of ground motion pulses from a suite of acceleration records. GMW are compared to a popular pulse model that has recently been adapted for wavelet analysis in the literature. It is shown, based on two different metrics, that GMW performs comparably with the other model for this suite of ground motions. Since CWT is repeated with different wavelet parameters for each ground motion, the parameter selection and pulse extraction process are examined. It is shown that using the maximum wavelet coefficient alone does not allow for the extraction of pulses that best match the time series or the spectral response. Alternative criteria are proposed in order to optimize pulse selection using performance-based metrics.

Modeling criteria of older non-ductile concrete frame–wall buildings

The purpose of seismic provisions included in modern building codes is to obtain a satisfactory structural performance of buildings during earthquakes. However, in the United States and elsewhere, there are large inventories of buildings designed and constructed several decades ago, under outdated building codes. Some of these buildings are classified as non-ductile buildings. Currently, under the ATC-78 project, a methodology is being developed to identify seismically hazardous frame–wall buildings through a simple procedure that does not require full nonlinear analyses by the responsible engineer. This methodology requires the determination of the controlling plastic collapse mechanism, the base shear strength, and the ratio between the story drift ratio and the roof drift ratio, called parameter \(\alpha\), at collapse level. The procedure is calibrated with fully inelastic nonlinear analyses of archetype buildings. In this paper we first introduce an efficient scheme for modeling frame–wall buildings using the software OpenSees. Later, the plastic collapse mechanism, the base shear strength, and values of \(\alpha\) are estimated from nonlinear static and dynamic analyses considering a large suite of ground-motion records that represent increasing hazard levels. The analytical experiment included several frame–wall combinations in 4 and 8-story buildings, intended to represent a broad range of conditions that can be found in actual buildings, where the simplified methodology to evaluate the risk of collapse can be applicable. Analysis results indicate that even walls of modest length may positively modify the collapse mechanism of nonductile bare frames preventing soft story failures.

Experimental Studies on Seismic Response of Skew Bridges with Seat-Type Abutments. I: Shake Table Experiments

Girder unseating in skewed bridges with seat-type abutments has been frequently observed in past earthquakes. This has been attributed to excessive in-plane rotation of the superstructure, and a handful of numerical studies have been conducted to investigate the cause of this rotation. The most common explanation has been pounding between the superstructure and abutment back wall, but this phenomenon has not been verified by experimental testing. Accordingly, this paper describes an experimental investigation into the behavior of skewed bridges, with special attention being given to the interaction between the superstructure and abutment. Shake table experiments are described on a family of four single-span simply supported bridge models with skew angles of 0°, 30°, 45°, and 60° and five different expansion gaps. These models were subjected to a suite of ground motions that varied by type (near field and far field), intensity, and input direction. In total, 876 experiments were conducted. Data collected included superstructure displacements and accelerations, and impact forces at the abutments. The results confirm that a skew bridge experiences large in-plane rotation when the obtuse corner of the span impacts the adjacent back wall with or without slippage along the face of the wall. As a consequence, these bridges can experience large in-plane displacements normal to the back wall at their acute corners, and larger support lengths are required to prevent unseating than for straight bridges. The comprehensive data set obtained in these experiments may be used in future studies to validate numerical models for skew bridges and improve the empirically based minimum support length requirements that are specified for skew bridges in many bridge design codes.

Evaluation of SCEC CyberShake Ground Motions for Engineering Practice

This paper evaluates CyberShake (version 15.12) ground motions for potential application to high-rise building design in the Los Angeles region by comparing them against recordings from past earthquakes as well as empirical models. We consider two selected sites in the Los Angeles region with different underlying soil conditions and select comparable suites of ground motion records from CyberShake and the NGA-West2 database according to the ASCE 7-16 requirements. Major observations include (1) selected ground motions from CyberShake and NGA-West2 share similar features, in terms of response spectra and polarization; (2) when selecting records from Cyber-Shake, it is easy to select motions with sources that match the hazard deaggregation; (3) CyberShake durations on soil are consistent with the empirical models considered, whereas durations on rock are slightly shorter; (4) occasional excessive polarization in ground motion is produced by San Andreas fault ruptures, though those records are usually excluded after the ground motion selection. Results from this study suggest that CyberShake ground motions are a suitable and promising source of ground motions for engineering evaluations.

Assessment of the Seismic Site Amplification in the City of Ivanec (NW Part of Croatia) Using the Microtremor HVSR Method and Equivalent-Linear Site Response Analysis

The city of Ivanec is located between valley of the Bednja River and Mt. Ivanščica and this area can be prone to significant seismic site amplification due to local site characteristics. This study presents the first assessment of seismic site amplification for the city of Ivanec by the microtremor horizontal-to-vertical-spectral-ratio (HVSR) method and the equivalent-linear (EQL) site response analysis. Based on microtremor measurements and HVSR analysis, fundamental soil frequency and HVSR peak amplitude maps indicate potentially seismic danger zones. The 1-D EQL site response analysis was performed using multiple suites of earthquake ground motions scaled to the 95- and 475-year return periods of peak ground accelerations. Site amplification maps at the predominant peak frequency and ground surface indicate two microzones, one with high amplification in the central part of the city due to soft soil characteristics, and the other with small amplification in the transitional zone from alluvial basin towards the foothills of Mt. Ivanščica. HVSR peak amplitudes and site response peak amplifications showed similar spatial distributions with similar predominant peak frequencies but with different amplitude levels. Site amplification maps provided significant information about potential resonance effects for structures of certain heights that can be correlated with the local ground shaking characteristics.

A compliant tuned liquid damper for controlling seismic vibration of short period structures

Abstract Tuning of sloshing frequency of liquid to short period structures possess challenges in Tuned Liquid Dampers (TLD). However, seismic vulnerability of short period structures cannot be overemphasized. In such instances, frequency tuning is facilitated by inserting a spring between the structure and the TLD. An implementation of this is presented herein by mounting the TLD on an array of compliant elastomeric pads, referred herein as compliant TLD. The effectiveness of compliant TLD is demonstrated through simulation that involve numerical solution of the derived equation of motion. The nonlinear sloshing, wave breaking and resulting dissipation are described by Sun's model. The infeasibility of tuning of conventional TLD to short period structures and consequent inefficiency are shown to be largely eliminated by the proposed implementation. Significant reductions in the peak and root mean square (r.m.s) floor accelerations and storey displacements are observed while (the structures are) subjected to a suite of ground motions pertaining to varying hazard level. The parameters for the proposed system are chosen based on the parametric variations of the controlled responses. The performances of the compliant TLD are also verified with experimental investigations using shake table test on scaled model of a building-TLD system. The experimental results corroborate with the trends observed from simulation. The tuning invoked by the compliant mechanism is observed to largely retain the sloshing and dissipation through wave breaking. Further, the design of the compliant TLD is presented and enumerated with an example of realistic building frame.

Seismic performance improvement of tension-only-braced frames with Energy-Dissipative Rocking Columns

Steel braced frames consisting of strong beams, weak columns, and tension-only braces (BTFs) have been extensively used in low-rise steel buildings, since it is prefabricated rapidly and is cost-efficient. However, BTFs are vulnerable to soft story failure due to the negligible post-yielding stiffness of tension-only braces. A novel device of Energy-Dissipative Rocking Column (EDRC) is proposed to improve the seismic performance of BTFs. An EDRC consists of dual hinge supported steel column branches and replaceable steel strip dampers between them. Firstly, the effectiveness of EDRCs were verified through nonlinear time history analyses on benchmark multi-story BTFs using a suite of far-field ground motion records. Secondly, systematic parametric studies are conducted to quantify the effect of key influencing factors such as stiffness ratio of EDRC to BTF and bending stiffness of column branches on the seismic performance improvement of BTF-EDRC systems. The results show that the maximum inter-story drift and inter-story drift concentration under earthquakes as well as the residual drift after earthquakes can be significantly mitigated by coupling with EDRC. It is found that the maximum inter-story drift ratio can be reduced by 50% when the stiffness ratio of EDRC to BTF reached 1.0. The most effective stiffness ratio of EDRC to BTF for mitigating the inter-story drift concentration and residual drift ratio falls in a range of no more than 0.2 and 0.4, respectively.

Design of buildings through Linear Time-History Analysis optimising ground motion selection: A case study for RC-MRFs

A novel framework for Eurocode 8-compliant design using Linear Time-History Analysis (LTHA) is discussed. LTHA overcomes the approximations typical of other linear seismic analyses but it still lacks a suitable approach for seismic input selection compatible with Eurocode 8-prescriptions. The critical aspect is to find a balanced compromise to control seismic input variability, suiting design purposes, but still being able to capture specific response features such as pulse-like effects. A LTHA design procedure has been included in ASCE/SEI 7-16 (2017) and described in FEMA P-1050 (2015), suggesting spectral-matching of three ground motions as input, so that LTHA can be used as alternative to Response Spectrum Analysis (RSA). Herein, a 12-storey regular Reinforced-Concrete Moment-Resisting-Frame building is employed as case-study. Different ground motion selection strategies are compared. Firstly, three suites of spectrum-compatible ground motions are used referring to requirements of Eurocode 8 for nonlinear time-history analysis and compared with LTHA-ground motion selection procedure included in FEMA P-1050.A new index for LTHA ground motion selection (Ieq) is proposed to control response variability in relation to the dynamic properties of the structure aimed at obtaining a suitable input for LTHA design. A target value for Ieq is proposed on the basis of structural response for far-field and near-field suites and an ad hoc suite of pulse-like ground motions selected considering pulse periods lower than the fundamental period of the structure. The difference in terms of design results between LTHA and RSA is employed as benchmark to evaluate the suitability of Ieq for LTHA design.

The effect of amplitude scaling limits on conditional spectrum‐based ground motion selection

SummaryAmplitude scaling is commonly used to select ground motions matching a target response spectrum. In this paper, the effect of scaling limits on ground motion selection, based on the conditional spectrum framework, is investigated. Target spectra are computed for four probabilistic seismic hazard cases in Western United States, and 16 ground motion suites are selected using different scaling limits (ie, 2, 5, 10, and 15). Comparison of spectral acceleration distributions of the selected ground motion suites demonstrates that the use of a scaling limit of 2 yields a relatively poor representation of the target spectra, because of the small limit leading to an insufficient number of available ground motions. It is also shown that increasing scaling limit results in selected ground motions with generally increased distributions of Arias intensity and significant duration Ds5‐75, implying that scaling limit consideration can significantly influence the cumulative and duration characteristics of selected ground motions. The ground motion suites selected are then used as input for slope displacement and structural dynamic analyses. Comparative results demonstrate that the consideration of scaling limits in ground motion selection has a notable influence on the distribution of the engineering demand parameters calculated (ie, slope displacement and interstory drift ratio). Finally, based on extensive analyses, a scaling limit range of 3 to 5 is recommended for general use when selecting ground motion records from the NGA‐West2 database.

Variability in Seismic Collapse Probabilities of Solid- and Coupled-Wall Buildings

Reinforced concrete walls are a common lateral load–resisting system for buildings. Common wall configurations are solid planar or solid flanged (H, I, T, L, or C-shaped cross sections) and coupled planar or flanged walls. A coupled-wall system comprises two solid walls linked at most floors by coupling beams that typically have length-to-depth ratios ranging from 2 to 4. In low- to midrise buildings, solid walls are expected and designed to form a single plastic hinge at the base of the wall; in mid- to high-rise buildings, a second hinge is expected in the upper half of the buildings. In coupled walls, plastic hinges are expected at the ends of the coupling beams and at the base of the wall. Because coupled walls offer a more distributed plastic mechanism, they are considered to provide superior earthquake performance to solid walls. This study employs nonlinear incremental dynamic analysis with the suite of ground motions from crustal earthquakes and a set of six idealized wall buildings of varying height to test the hypothesis that coupled walls provide reduced earthquake collapse risk over solid walls; to establish the sensitivity of collapse to modeling decisions and model parameters; and to identify design decisions that can reduce earthquake collapse risk.

Validation of stochastic ground motion model modification by comparison to seismic demand of recorded ground motions

An important consideration for the adoption of stochastic ground motion models in performance-based earthquake engineering applications is that the probability distribution of target intensity measures from the developed suites of time-histories is compatible with the prescribed hazard at the site and structure of interest. The authors have recently developed a computationally efficient framework to modify existing stochastic ground motion models to facilitate such a compatibility. This paper extends this effort through a validation study by comparing the seismic demand of recorded ground motions to the demand of stochastic ground motion models established through the proposed modification. Suites of recorded and stochastic ground motions, whose spectral acceleration statistics match the mean and variance of target spectra within a period range of interest, are utilized as input to perform response history analysis of inelastic single-degree-of-freedom (SDoF) case-study systems. SDoF systems with peak-oriented hysteretic behavior, strain hardening, and (potentially) degrading characteristics, experiencing different degree of inelastic response, are considered. Response is evaluated using the peak inelastic displacement and the hysteretic energy given by the work of the SDoF restoring force as engineering demand parameters (EDPs). The resultant EDP distributions are compared to assess the effect of (and validate) the proposed modification. It is shown that the proposed modification of stochastic ground motion models can provide results that are similar to these from recorded ground motion suites, improving any (in some cases large) discrepancies that exist for the initial, unmodified stochastic ground motion model.

On the Seismic Fragility Assessment of Concrete Gravity Dams in Eastern Canada

In recent years, probabilistic methods, such as fragility analysis, have emerged as reliable tools for the seismic assessment of dam-type structures. These methods require the selection of a representative suite of ground motion records, resulting in the need for a ground motion selection method that includes all the relevant ground motion parameters in the fragility analysis of this type of structure. This article presents the development of up-to-date fragility curves for the sliding limit states of gravity dams in Eastern Canada using a record selection method based on the generalized conditional intensity measure approach. These fragility functions are then combined with the recently developed regional hazard data to evaluate the annual risk, which is measured in terms of the unconditional probability of limit state exceedance. The proposed methodology is applied to a case study dam in northeastern Canada, whose fragility is assessed through comparison with previous studies and current safety guidelines. It is observed that the more accurate procedure proposed herein produces less conservative fragility estimates for the case study dam.

Methodology for selection of the most damaging ground motions for nuclear power plant structures

A methodology for selecting the most unfavorable ground motions (MUGMs) for practical dynamic analyses of nuclear power plant (NPP) systems is proposed. A series of correlation analyses are carried out to select relevant intensity measures (IMs) that best characterize structural demand parameters and consequently the damage potential of ground motions on NPP structures. Predictive demand models are developed as a function of the selected IMs using a large suite of as-recorded ground motions. A damage index is defined and the MUGMs were identified according to the estimated damage values. The reliability of the methodology is verified by comparing the identified MUGMs with the “actual MUGMs” that resulted in the peak damage values among all dynamic simulations. As an alternative to traditional spectrum-based record selection methods, the proposed methodology will ensure a very low risk of failure of NPP structures under the design level earthquake and can be utilized in seismic margin assessment.

Temperature effect on seismic performance of CBFs equipped with SMA braces

Shape memory alloys (SMAs) exhibit superelasticity given the ambient temperature is above the austenite finish temperature threshold, the magnitude of which significantly depends on the metal ingredients though. For the monocrystalline CuAlBe SMAs, their superelasticity was found being maintained even when the ambient temperature is down to −40C. Thus this makes such SMAs particularly favorable for outdoor seismic applications, such as the framed structures located in cold regions with substantial temperature oscillation. Due to the thermo-mechanical coupling mechanism, the hysteretic properties of SMAs vary with temperature change, primarily including altered material strength and different damping. Thus, this study adopted the monocrystalline CuAlBe SMAs as the kernel component of the SMA braces. To quantify the seismic response characteristics at various temperatures, a wide temperature range from −40 to 40C are considered. The middle temperature, 0C, is artificially selected to be the reference temperature in the performance comparisons, as well the corresponding material properties are used in the seismic design procedure. Both single-degree-of-freedom systems and a six-story braced frame were numerically analyzed by subjecting them to a suite of earthquake ground motions corresponding to the design basis hazard level. To the frame structures, the analytical results show that temperature variation generates minor influence on deformation and energy demands, whereas low temperatures help to reduce acceleration demands. Further, attributed to the excellent superelasticity of the monocrystalline CuAlBe SMAs, the frames successfully maintain recentering capability without leaving residual deformation upon considered earthquakes, even when the temperature is down to −40C.

Hybrid base-isolation of a nonlinear building using a passive resettable stiffness damper

The resetting semi-active stiffness damper (RSASD) has been shown to be effective at protecting civil structures subject to near-field earthquake ground motions. Recently, a modified version of the RSASD was proposed in which the semi-active components of the damper were replaced by a novel mechanism for achieving resetting of the damper force. The resulting resetting passive stiffness damper (RPSD) represents a more reliable and robust alternative to its semi-active counterpart. The objective of the research presented herein is to investigate the RPSD for hybrid base-isolation of a nonlinear building subject to a suite of near-field earthquake ground motions. The performance of the RPSD is benchmarked through a comparison with other passive, semi-active, and hybrid seismic protective systems. It is shown that the RPSD is effective in reducing the peak base drifts while maintaining the building response within the elastic range. Furthermore, it is shown that the performance of the RPSD generally compared well with the other protective systems in terms of reducing the peak base drifts, but was not as effective at reducing the peak base absolute accelerations or floor drifts.

A ground motion selection and modification method through stratified sampling

In this article a ground motion selection and modification (GMSM) method is presented, suitable for the probabilistic seismic assessment of the damage state and collapse potential of building structures. The objective is to predict the probability distribution of the engineering demand parameters in a future earthquake event, provided that the spectral acceleration at the fundamental period of the structure is given. The GMSM method uses a vector-valued intensity measure that incorporates the Normalized Spectral Area parameter. Through stratified sampling on the Normalized Spectral Area, optimised ground motion suites are formed. Its advantage over other GMSM methods is that it implicitly matches the multivariate distribution of the response spectrum in the region of the structure elongated period, and it adopts an unbiased estimator of the response central tendency. The GMSM method is applied in the probabilistic response assessment of a first-mode dominated multi-degree-of-freedom system that represents a ten-storey building. Substantial reduction in the computational work is achieved, compared to another method.

Computational seismic evaluation of a curved prestressed concrete I-girder bridge equipped with shape memory alloy

This article proposes an analytical modeling approach for seismic evaluation of curved prestressed concrete (PSC) bridges using alternative restraint materials. The approach was applied to a simply supported, curved PSC I-girder bridge subjected to a suite of ground motions. Along with analyzing the response of the bridge without Shape Memory Alloy (SMA), the bridge was retrofitted with SMA components and restraints to reduce its response and both results were compared. SMA components used included dowels in the bearing pad, restraints connecting the girders to the abutments, and piles with SMA characteristics. Results indicated that the SMA components reduced seismic response, and specifically the deformations of the SMA bridge were up to 80% lower than the non-SMA bridge.