Floor Acceleration Research Articles

Optimal design of Magnetorheological damper for seismic response reduction of Base-Isolated structures considering Soil-Structure interaction

The base isolation technique is a well-known strategy used for the seismic protection of structures. It introduces a highly deformable device between the structure and the foundation to isolate the main structure from the ground motions. Despite the continuous enhancement of base isolation techniques, the limitations of this strategy are well known to researchers and engineers. Base isolation systems undergo large displacement under strong earthquake motions, which usually results in structural damage and users’ discomfort. In this study, a magnetorheological (MR) damper is used to improve the performance of a benchmark base-isolated building by placing the MR damper between the base of the structure and the foundation. The soil-structure interaction (SSI) effect is included in the study by considering three types of soil, denoted dense, medium, and soft. The soil is introduced through two degrees of freedom, namely, sway and rocking, based on the cone simplified linear model. The MR damper parameters are optimized using a particle swarm optimization (PSO) algorithm under a set of benchmark earthquake records. The optimal local parameters related to each benchmark earthquake and soil type are averaged to obtain general optimum parameters. The acquired general optimum parameters are then used to study the dynamical response of the benchmark base-isolated building controlled by the MR damper, considering an array of 100 natural earthquake ground motions. The obtained results show an enhancement of the dynamic response under all soils conditions compared to non-optimized and base isolation only scenarios. Several engineering demand parameters (EDP) and response indicators were investigated in this study, including top floor and base displacements, inter-storey drift, storey drift, top floor acceleration, and base shear. The results obtained show the effectiveness of the optimization in improving the response of base-isolated structures. It also demonstrates the dependence of damper parameters on soil typologies.

Seismic Performance of MRSF Structures Damped with Steel Slit Shear Panels

The steel slit shear panel (SP), made by cutting slits in a monolithic steel plate, dissipates seismic energy through the flexural behavior of individual plate links separated by slits. To investigate the influence of different link designs and steel properties, three scaled SP specimens are designed and tested under quasi-static cyclic loading. Test results show that plate links with a larger width-thickness ratio buckle earlier and larger, and associated pinching in hysteresis is more serious. Significant strain hardening of low yield steel promotes plump shear hysteresis even with obvious out-of-plane buckling deformation. To investigate the effect of different SPs on the overall structural performance, moment resisting steel frame (MRSF) structures installed with SPs are built. Time-history analysis results show that the installation of SPs reduces the story displacement responses but increases the floor accelerations. The low yield SP works best in reducing both the maximum and residual inter-story drift.

Estimation of Floor Response Spectra for Self-Centering Structural Systems with Flag-Shaped Hysteretic Behavior

Spectral floor-acceleration demands are necessary for the seismic design of acceleration-sensitive non-structural components (NSCs). Existing studies to estimate floor response spectra (FRS) using empirical equations are based on elasto-plastic and stiffness degrading hysteretic behavior of the primary structure. In the present study, the FRS for self-centering (SC) structural systems with flag-shaped hysteretic behavior under far-fault ground motions are investigated. The FRS and dynamic amplification factor (DAF) are obtained from nonlinear response history analysis (NLRHA) of SC structural systems with flag-shaped hysteretic behavior. An equation to estimate the FRS is proposed and verified by carrying out NLRHA using a different set of far-fault ground motions. The equation to estimate FRS is shown to predict floor acceleration demands with very good accuracy.

EVALUATION OF EQUIVALENT LINEARIZATION ANALYSIS METHODS FOR SEISMICALLY ISOLATED BUILDING USING LEAD-RUBBER BEARING

The paper presents an evaluation of the linearization analysis method for isolated multistory building structures using lead-rubber bearings (LRB) considering vertical stiffness and critical buckling load. The effective linear parameters of LRB are estimated by the single-mode spectral analysis method, calculating for a typical target spectrum by TCVN 9386:2012. A set of time history analyses is conducted on both the equivalent linear model and the bilinear model. A comparison of seismic responses between the two models is performed where the bilinear model included considering the effects of vertical stiffness and critical buckling load. The results show the conservative design in force and displacement of the isolated structure by the linearization analysis method, but nonconservative design in floor accelerations. Significantly higher displacement estimates by the linearization method may lead to over-designed bearing displacement capacity.

Comparative response assessment of different steel plate shear walls (SPSWs) under near-field ground motion

Steel plate shear walls (SPSWs) are widely used due to their high energy absorption and exceptional lateral resistance. However, the behavior of different infill panels varies, and their response under near-field seismic excitations is uncertain. To better understand how different infill panels contribute to seismic structural responses, this paper presents findings from research conducted on different types of SPSWs. The study of different types of SPSWs with infill panel aspect ratios of 1:1 and 2:1 under cyclic loading was the first part of this parametric investigation, and the second part was the study of the structural response of 3D frames with an aspect ratio of 3:4:2 and a 1:1 infill panel under near-field ground motions. The results of energy dissipation, acceleration, and drift were compared. This study's goal was to better understand how various infill panels react to near-field ground motions and cyclic loading. With a 2:1 aspect ratio, unstiffened SPSWs reduced plastic dissipation by 78% while increasing lateral stiffness by 10%. With minimal peak transient and residual interstory drifts and effectively damped floor acceleration, unstiffened SPSW demonstrated the best viscous effect and dissipated 69.42% of the total system.

Earthquake Early Warning for Estimating Floor Shaking Levels of Tall Buildings

ABSTRACTThis article investigates methods to improve earthquake early warning (EEW) predictions of shaking levels for residents of tall buildings. In the current U.S. Geological Survey ShakeAlert EEW system, regions far from an epicenter will not receive alerts due to low predicted ground-shaking intensities. However, residents of tall buildings in those areas may still experience significant shaking due to the acceleration amplification caused by tall buildings’ dynamic behavior, as recently experienced by residents of the 52-story building in downtown Los Angeles (DTLA) during the 2019 M 7.1 Ridgecrest earthquake. Using more than 400 recorded response data acquired from 77 instrumented buildings in California, here we compare the Federal Emergency Management Agency (FEMA) P-58 and American Society of Civil Engineers (ASCE) 7-16 simplified equations for peak floor acceleration (PFA), finding that the ASCE estimation is close to the median of data recorded in large and long-distance events, whereas the current FEMA estimation is not suitable. In the second part of this article, four instrumented tall buildings in DTLA are extensively studied, and the performance of the simplified and response spectrum (RS) methods giving both an estimation of the free-field horizontal peak ground acceleration (PGA) and pseudospectral acceleration is evaluated. The results show that the RS method is as accurate as the response history analysis as long as the ground-motion RS is accurate, whereas the ASCE 7-16 prediction is conservative. However, when ground-motion RS or PGA is estimated for DTLA using a ground-motion model (GMM), the performance of the RS method significantly degrades due to underestimation by the GMM at long periods. The results of this study imply that a nonergodic GMM, which may give more accurate prediction in Los Angeles, could improve the results for PFA when the building’s behavior is dominated by a few long-period fundamental modes, as is the case for the 52-story building in DTLA.

Integration of peak seismic floor velocities and accelerations into the performance-based design of steel structures

This paper elaborates towards the homogenization of the seismic behavior between the structural and non-structural components of steel structures in the framework of performance-based seismic design. In particular, for the seismic performance level that the steel structure is set to satisfy, simple expressions that relate peak seismic floor velocity and acceleration with their corresponding peak ground counterparts are provided. These expressions are functions of the number of stories and are derived by non-linear regression analyses of the peak seismic floor velocity and acceleration data derived by non-linear time-history analyses of various steel structures subjected either to far-fault, or to near-fault seismic motions. Numerical examples are presented in order to demonstrate the efficiency and range of applicability of the proposed expressions. It is concluded that by means of the proposed expressions, the seismic performance of a class of acceleration- or velocity-susceptible non-structural components in steel structures is expected to be in line with that of the structural components.

Performance based design of a new hybrid passive energy dissipation device for vibration control of reinforced concrete frames subjected to broad-ranging earthquake ground excitations

This paper evaluates a new hybrid passive device that comprises two different passive energy dissipation categories, namely, viscoelastic damper and friction damper. The device operates as a synergic unit using a novel locking mechanism. The hybrid passive device offsets the drawbacks of individual devices and facilitates improving the performance of structures for multiple excitation vibration levels due to wind and seismic events. Both devices are connected in a parallel combination using locking mechanisms and exhibit distinct hysteretic characteristics under small and significant deformation. The viscoelastic damper alone will be activated for extreme winds and minor to moderate earthquakes, and both devices will work simultaneously for strong earthquakes. Considering the advantages of such a new device, this article presents a new seismic design method structures equipped with a hybrid passive dampers device using the energy-balance concept. The method accounts for the structural member's inelastic behaviour, phase transformation of the stiffness and the energy dissipation capacity of the energy devices to achieve the intended performance level under the design earthquake hazard. The proposed method has been applied on a six- and a twelve-storied reinforced concrete building. The seismic performance is verified through nonlinear time history analysis using a set of near and far-field ground motions. Numerical results are investigated in terms of the inter-story drifts, the residual drifts, the floor acceleration and the yield mechanisms. The structure's performance corresponds well with the target performance levels.

Teaching-Learning Based Optimization of Nonlinear Isolation Systems under Far Fault Earthquakes

Seismic isolation systems exposed to far-fault earthquakes can reduce floor accelerations and story drift ratios to acceptable levels. However, they exhibit different structural performances in each earthquake due to different excitation frequency contents. By optimizing the isolation system parameters, their performance may be maintained at the best level under different far-fault earthquakes. In this study, the optimization of the parameters of the nonlinear isolation system of a 5-story benchmark building is performed by Teaching-Learning Based Optimization (TLBO) algorithm to minimize peak floor accelerations under historical far-fault earthquakes with and without exceeding a specified base displacement limit. According to the results of the analyses, it can be said that TLBO algorithm is a robust algorithm with low standard deviations for determining optimum nonlinear isolation system parameters.

Multi-objective optimal design of SC-BRB for structures subjected to different near-fault earthquake pulses

The purpose of this research is to propose a two-objective optimum design technique of self-centering buckling restrained brace (SC-BRB) characteristics based on a multi-objective cuckoo search (MOCS) optimization algorithm for structures subjected to pulse-like near-field seismic excitations. The optimal geometric specifications of shape memory alloy (SMA) and buckling-restrained brace (BRB), including the length of SMA bars, BRB core, area of SMA, and BRB core, are optimized with the aim of simultaneous reduction of the maximum displacement and acceleration of the floors of structures. The numerical analyses are then performed on 3-story and 6-story frames that have been modeled in OpenSees software under pulse-like near-fault earthquakes. The optimized SC-BRB using MOCS considerably reduces the seismic responses of both structures. The SC-BRB also performs better than BRB in severe earthquakes with a shorter fault distance and a larger moment magnitude. Furthermore, SC-BRB has a steady energy dissipation capacity due to more energy absorption in SMA. In terms of maximum top floor displacement and acceleration, the 3-story SC-BRB frame (SC-BRBF) results are reduced the values of 28.3% and 20.1% than the BRB frame (BRBF) for all considered near-fault earthquake pulses, respectively. Moreover, the optimized SC-BRB is considerably capable of reducing the hazards originating from residual drifts and deformations.

New insights into the relationship between seismic intensity measures and nonlinear structural response

This paper focuses on the probabilistic analysis of Intensity Measures (IMs) and Engineering Demand Parameters (EDPs) in the context of earthquake-induced ground motions. Several statistical properties, which are desirable in IMs when they are used to predict EDPs, have been analysed. Specifically, efficiency, sufficiency and steadfastness have been quantified for a set of IMs with respect to two EDPs: the maximum inter-storey drift ratio, MIDR, and the maximum floor acceleration, MFA. Steadfastness is a new statistical property proposed in this article, which is related to the ability of IMs to forecast EDPs for large building suites. In other words, this means that efficiency does not significantly vary when different types of buildings are simultanously considered in the statistical analyses. This property allows reducing the number of calculations when performing seismic risk estimations at urban level since, for instance, a large variety of fragility curves, representing specific building typologies, can be grouped together within a more generic one. The main sources of uncertainty involved in the calculation of the seismic risk have been considered in the analysis. To do so, the nonlinear dynamic responses of probabilistic multi-degree-of-freedom building models, subjected to a large data set of ground motion records, have been calculated. These models have been generated to simulate the dynamic behavior of reinforced concrete buildings whose number of stories vary from 3 to 13. 18 spectrum-, energy- and direct-accelerogram-based IMs have been considered herein. Then, from clouds of IM-EDP points, efficiency, sufficiency and steadfastness have been quantified. For MIDR, results show that IMs based on spectral velocity are more efficient and steadfast than the ones based on spectral acceleration; spectral velocity averaged in a range of periods, AvSv, has shown to be the most efficient IM with an adequate level of steadfastness. For MFA, spectral acceleration-based-IMs are more efficient than velocity-based ones. A comparison is also presented on the use of linear vs quadratic regression models, and their implications on the derivation of fragility functions. Concerning sufficiency, most of the 18 IMs analysed do not have this property. Nonetheless, multi-regression models have been employed to address this lack of sufficiency allowing to obtain a so-called ‘ideal’ IM.

Self-centering Devices with Paralleled Friction Spring Groups: Development, Experiment and System Behavior

ABSTRACT A new self-centering device employing paralleled friction spring groups is developed. The fundamental working principle and fabrication procedure are first presented, followed by quasi-static tests. The devices have flag-shaped hysteretic behavior with excellent self-centering capability and moderate energy dissipation. Stable performance under multiple rounds of loading is also exhibited, suggesting that the devices are fully reusable after undergoing several major earthquakes. A system level analysis is carried out, confirming that the devices are effective in eliminating the residual deformation. A hybrid system is finally proposed enabling a well-balanced control of the peak deformation, residual deformation, and peak floor acceleration.

Seismic evaluation of industrial steel moment resisting frames with shape memory alloys using performance-spectra-based method

Seismic evaluation of industrial facilities may need to employ multi-performance indicators in various yielding stages towards a more delicate design methodology. This paper develops a performance-spectra-based method to evaluate the seismic performance of industrial steel moment resisting frames (MRFs) with shape memory alloys (SMAs) considering multi-performance indicators and hysteresis transition. The work commences with demonstration of the hysteresis transition of a steel MRF with SMAs from the ‘self-centring stage’ to the ‘post self-centring stage’. Then, based on the single-degree-of-freedom (SDOF) analogy and the hysteretic model considering hysteresis transition, a performance-spectra-based space for quantifying the seismic behaviour of an industrial steel MRF with SMAs including multi-performance indicators is established. In this study, the performance-spectra-based space employs three essential indicators (i.e. energy modification factor, peak acceleration ratio and residual displacement ratio). To demonstrate the application and validate the adequacy of the proposed method for seismic evaluation of industrial steel MRFs with SMAs, two prototype steel MRFs equipped with SMA connections for industrial use are designed and assessed under representative earthquake ground motion ensembles. By comparing essential structural seismic response quantities (i.e. maximum roof displacement, peak interstorey drift ratio, peak floor acceleration and interstorey residual drift ratio) predicted by the proposed method with those calculated by nonlinear-response-history analyses (NL-RHAs), the effectiveness of the proposition is confirmed.

Earthquake ground motion estimation for buildings using absolute floor acceleration response data

AbstractEarthquake ground motion information is useful for structural vibration control during earthquakes and post‐earthquake inspections of structural damage. However, in practice, the ground input data on a specific location may not be available because of instrumentation limitations or technical issues. Numerous studies have been devoted to identifying the unknown excitation. Most of these studies required responses in relative coordinates or from systems with direct feedthrough. On the other hand, acceleration monitoring systems in applications measure global responses of buildings, which corresponds to the absolute acceleration of a point on a structure, that is, the measured floor acceleration is the sum of the unknown earthquake‐induced ground motion and acceleration relative to the ground when an earthquake happens. In this study, a ground motion estimation approach was developed based on an extended Kalman filter (EKF) with an embedded Bayesian noise parameter updating technique, which requires a relatively small number of sensors for absolute acceleration measurements. Numerical simulations, a shaking table test, and a real‐world application were performed to demonstrate the applicability and accuracy of the proposed method.

Seismic Performance of the Inerter and Negative Stiffness–Based Dampers for Vibration Control of Structures

Passive energy dissipation devices or supplemental damping devices have been successfully implemented into structures for controlling the excessive vibrations under wind and seismic excitation. Recent developments in the form of negative stiffness dampers (NSDs) and inerter-based vibration absorbers (IVAs) as potential energy dissipation devices are of considerable interest to researchers. The present study evaluates the performance of the combined NSD and IVA as a possible alternative to the traditional energy dissipation devices such as viscous dampers (VDs) and viscoelastic dampers (VEDs). The mathematical formulation and optimal design of the combined NSD and IVA mechanism are presented. A 20-storey benchmark building is modeled as a multi-degree-of-freedom (MDOF) shear building. The dynamic equations for the MDOF building are written in the state-space form, and a simple optimization approach based on effective modal damping is prescribed. Comparative performance between traditionally applied and novel IVA and NSD is investigated. The design considerations to analyze structures employing combined NSDs and IVAs are developed. It is demonstrated that NSDs and IVA-based passive energy dissipation devices are the most efficient devices in reducing inter-storey drifts and floor accelerations compared with VDs and VEDs using the same damping coefficient.

Seismic assessment of steel frame subjected to simulated directivity earthquakes: The unilaterality of fault normal component at various rupture distances

Directionality is prominent in the fault normal component of ground motion. It has a different effect on stations in the rupture direction on the tectonic fault than it does on stations in the opposite direction. Such pulse-like features observed in forward and backward directivity stations affect both low-rise and high-rise structures, depending on their fundamental period and the pulse period of ground motion. However, systematic availability of both forward and backward directivity ground motion for unilateral and bilateral earthquakes of the same magnitude at a similar rupture distance for a given fault is rare. So, multiple directivity scenarios are simulated using fracture mechanics-based principles. Using OpenSees, steel moment-resisting frames of 1, 5, and 9 stories, well designed according to building codes, are modeled, and their non-linear response is evaluated. Stations at constant rupture distances are used to compute fragility for each scenario separately. Variation inter-storey drift and peak floor acceleration, along with the hysteretic behavior of panel-zone springs, have also been studied for each of the directivity scenarios. Finally, the results obtained are compared to what is expected by HAZUS.

Seismic assessment approaches for mass‐dominant sliding contents: The case of storage racks

AbstractThe typical view of seismic performance of structure‐content systems is one of segregation: Contents are assumed to be of low mass relative to the supporting structure, leading to a separate treatment of the two, whereby one first analyzes the structure and then subjects any contents to the resulting peak floor acceleration responses. Racking structures are of the exact opposite persuasion, having massive “palletized” contents that can slide on top of light cold‐formed steel frames, with Content‐Structure‐Sliding Interaction (CSSI) governing global and local response. In support of assessment and design, three approaches are investigated to capture CSSI: (i) introducing friction sliders per pallet and running nonlinear response‐history analysis, (ii) increasing the model viscous damping and using elastic response‐history analysis, and (iii) reducing the horizontal seismic loads in tandem with modal response spectrum analysis. Each comes with its own challenges and modelling/analysis needs, offering different levels of accuracy (or no appreciable capability at all) when assessing the actual sliding displacement of contents. Three case studies are employed to calibrate empirical relationships for damping amplification and seismic load reduction, largely removing the bias of simpler alternatives (ii) and (iii), respectively, to level the ground for future code applications.

An enhanced sequential ground motion selection for risk assessment using a Bayesian updating approach

In the context of probabilistic prediction of seismic demand parameters, this research presents a novel method for sequential ground motion selection based on the Bayesian updating approach. Unlike traditional methods of ground motion selection, which rely on spectral shape or intensity measures, the ground motions in this approach are chosen directly through a probabilistic evaluation of the structural responses. In this method, the probability density function produced by a limited number of nonlinear analyses closely resembles the curve produced by a large number of nonlinear analyses. To accomplish this, a large number of linear analysis results obtained in a short period of time are utilized to form the initial belief in a Bayesian model. Subsequently, in a two-way interaction, the results of fresh nonlinear analysis can be introduced sequentially to improve the predictive power of the model while concurrently selecting new ground motions. For this purpose, three case study buildings, including an 8-, 12-, and 20-story structure, were studied at three hazard levels. The efficiency of the proposed approach was investigated for sequential ground motion selection, and its effect on the probability density function of the peak floor acceleration and maximum story drift were studied. The proposed method is highly independent of the hazard level or the height of the building, and it may be used in almost every situation with significant improvement. This method of selecting ground motions is fairly efficient in terms of computing. The results show a significant improvement, especially at higher demand values that are more important in risk assessment. Significant improvements in peak floor acceleration results are also observed, which are notoriously difficult to evaluate.

Effects of demand parameters in the performance-based multi-objective optimum design of steel moment frame buildings

This paper investigates the effects of various demand parameters involved in the optimal performance-based design of steel moment frame buildings. These parameters include the peak values of story drift ratio, floor velocity, floor acceleration, and residual drift ratio. The performance-based design is performed to minimize two conflicting objectives, including the initial construction cost and the expected annual repair cost of the building under the design constraints. FEMA P-58 is utilized as a comprehensive and popular methodology for seismic risk assessment and building loss estimation in this paper. It evaluates the building performance in a probabilistic manner in terms of several metrics such as repair cost, repair time, carbon emissions, embodied energy, injuries, and fatalities. The Monte Carlo analysis is used to consider various uncertainties affecting seismic performance. A set of three steel moment-resisting frame (MRF) buildings representative of low-to mid-rise frames are selected as a case study in this research. The results of this study indicate that the story drift ratio has a major role in the performance-based optimal design of steel MRFs. Accordingly, despite increasing the two parameters of floor acceleration and floor velocity due to increasing the initial cost of structural moment frames, the annual repair cost of the building decreases when the story drift ratio decreases.

Seismic response of dual concentrically braced steel frames with various bracing configurations

Steel dual structures incorporating special concentrically braced frames (SCBFs) and special moment frames (SMFs) are, in general, effective for their high lateral stiffness and ductility. As such, ductile dual CBFs are often treated as a remedy for mitigating permanent drift demands along with the potential drift concentration in isolated stories. However, despite the benefits and interest, the effect of the bracing configuration on the seismic response of the dual concentrically braced systems, designed following the current capacity design approach stipulated in AISC Seismic Provisions, in particular, has not been thoroughly examined to date. The present study aims to shed light on the impact of the bracing scheme on the seismic behavior of dual SCBFs coupled with SMFs as backup frames. For this purpose, 4- and 10-story dual SCBFs with the commonly used chevron, split X, V, and cross X type bracing configurations are designed according to ASCE 7 and AISC 341. Then, the frames are subjected to a suite of ground motions. The nonlinear time-history analysis results are contrasted and discussed in terms of the peak and permanent drift demands, horizontal floor acceleration demands along with brace ductility demands.