Results Of Time History Analysis Research Articles

INVESTIGATING OF SEISMIC PARAMETERS OF RC FRAMES REHABILITATED BY ECCENTRICALLY BRACING WITH VERTICAL LINK

Most of existing concrete structures do not have suitable seismic performance due to various reasons, therefore they need seismic rehabilitation. One of the seismic rehabilitation method in structural level is using of steel bracing. New investigation of steel bracing can be referred to eccentrically bracing with single vertical link. This method of rehabilitation provides many advantages such as increasing in ductility, stiffness, lateral resistance, architectural compatibility, low weight, and the fewest changes in primary structural system. In this paper, two existing 3- and 9-story RC frames are assessed on the basis of FEMA356. Eccentrically bracing with single vertical link is used for seismic rehabilitation of these frames. The results of nonlinear time history analysis based on the maximum inter-story drift, maximum roof displacement and plastic rotation in critical elements of original and rehabilitated frames for two performance levels of Life safety (LS) and Collapse Prevention (CP) are presented. The results indicate that single vertical link can lead structures to the desired performance level with minimum cost and braced span number.

Response prediction of nonlinear hysteretic systems by deep neural networks

Nonlinear hysteretic systems are common in many engineering problems. The maximum response estimation of a nonlinear hysteretic system under stochastic excitations is an important task for designing and maintaining such systems. Although a nonlinear time history analysis is the most rigorous method to accurately estimate the responses in many situations, high computational costs and modelingtime hamper adoption of the approach in a routine engineering practice. Thus, various simplified regression equations are often introduced to replace a nonlinear time history analysis in engineering practices, but the accuracy of the estimated responses is limited. This paper proposes a deep neural network trained by the results of the nonlinear time history analyses as an alternative of such simplified regression equations. To this end, a convolutional neural network (CNN) which is usually applied to abstract features from visual imagery is introduced to analyze the information of the hysteretic behavior of the system, then, merged with neural networks representing the stochastic random excitation to predict the responses of a nonlinear hysteretic system. For verification, the proposed deep neural network is applied to the earthquake engineering area to predict the structural responses under earthquake excitations. The results confirm that the proposed deep neural network provides a superior performance compared to the simplified regression equations which are developed based on a limited dataset. Moreover, to give an insight of the proposed deep neural network, the extracted features from the deep neural network are investigated with various numerical examples. The method is expected to enable engineers to effectively predict the response of the hysteretic system without performing nonlinear time history analyses, and provide a new prospect in the relevant engineering fields. The supporting source code and data are available for download at https://github.com/TyongKim/ERD2.

Characteristics of FBD and DDBD techniques for SMRF buildings designed for seismic zone-V in India

This paper presents the results of the nonlinear time history analysis of six different reinforced concrete moment frames. The frames were designed using two major seismic design methods as Force Based Design (FBD) and Direct Displacement Based Design (DDBD) in which former is a conventional method while later one is a performance based approach of design. However, in this study, 4, 8 and 12 stories RC framed building with two × three bays were designed according to Indian standard and the Federal Emergency Management Agency (FEMA) considering two design approaches (FBD and DDBD) studied. Analysis and design for this study performed using SAP2000 v15.1 tool. Both design approaches performed using nonlinear time history analysis for five different real recorded ground motions of north-eastern region, India. The parameters such as inter-storey drift, displacement, material strain, and ductility demand obtained from the analyses of the frames designed using both design methods are noted and compared. This study examined, discussed and shown about the applicability of existing methods adopted considering the FBD and DDBD in a cost-effective manner to achieve desired level of performance.

Performance of EBFs equipped with an innovative shape memory alloy damper

Given their unique characteristics, Shape Memory Alloys (SMAs) have significant potential for use in different areas of engineering. The phase shift characteristics of these alloys allow them to memorize a certain shape, and if deformed, revert back to that shape through a thermal process. Given the vast potentials of SMAs, they can be utilized to address the limitation of conventional eccentrically braced frames (EBFs) with vertical links in order to achieve better residual and maximum interstory drifts. This paper presents a vibration control system equipped with SMAs to achieve improved operational domain. The Compared to conventional EBFs, the proposed system named recentering damping device (RDD) is easy to fabricate and implement and allows for the redesign of fuse members. A numerical analysis is performed for a 9-story steel frame building using nonlinear analysis program OpenSees to evaluate the system performance. Results of time history analysis demonstrate better self-centering behavior and lower residual interstory drifts of the proposed system as compared to EBF.

Structural evaluation of steel self-centering moment-resisting frames under far-field and near-field earthquakes

The design formulations, structural performance, and energy-based results of steel buildings of self-centering moment-resisting frames (SC-MRFs) with post-tensioned (PT) connections of three different energy dissipation devices including bolted top and seat angles, beam bottom flange friction devices, and bolted web friction devices are investigated under far-field and near-field earthquakes. The results of nonlinear time-history analysis using DRAIN-2DX program show that the simplified analysis used to estimate the inertial forces distributed through collector beams is not sufficiently precise since it leads to the underestimation of design beam axial forces and connection moments in exterior bays. The overestimation of design displacement demands of SC-MRFs with bottom flange friction devices results in bigger sections of beams and columns than are necessary. In addition, this SC-MRF system is not as effective as other two systems to dissipate the input energy. It is recommended to symmetrically use beam flange friction devices to achieve higher level of energy dissipation and lower fabrication costs. On the other hand, SC-MRFs with web friction devices is identified as the most efficient system to minimize the recoverable energy and SC-MRFs with top and seat angles provides medium level of energy dissipation. Different from far-field earthquakes, the input energies of SC-MRF structures under near-field ground motions are distinguishable by hikes of maximum values at the early stages of earthquakes. Furthermore, absolute energy components are substantially higher than relative energy components under near-field earthquakes. These phenomena should be considered in the formulations of energy factor and energy-based design procedures of SC-MRF systems.

The Seismic Investigation of Off-Diagonal Steel Braced RC Frames

Abstract Steel bracing may be used to enhance the seismic strength of RC frames. Concentric steel bracing generally reduces ductility, which is a key component of seismic design. To overcome the problem, ductile steel brace-RC frame systems are therefore usually introduced in the form of eccentric braces. In the present study, the Off-Diagonal Bracing System (ODBS), which works as a concentric type of bracing, is investigated. In this paper the response of ODBS elements to cyclic loading is first explored and compared with those of other types of bracing such as X-bracing and inverted-V bracing systems. The time history analysis responses and cyclic hysteresis responses of a number of lowrise to mid-rise RC frames retrofitted with different types of bracing systems are then evaluated and compared. It is shown that under seismic excitation, a much reduced base shear is experienced by frames retrofitted with ODBS compared to other bracing systems. The results of time history and cyclic hysteresis response analyses also indicate a far greater energy dissipation capacity and ductility for the ODBS compared to other bracing systems. It is also concluded that ODBS performs best in lowrise frames. The out-of-plane buckling response of the ODBS is also investigated, and it is shown that a double-plated central connection can control such an adverse response.

Nonlinear seismic response of a base isolated single pylon cable-stayed bridge

The vulnerability of single pylon cable-stayed bridges under strong ground motions are of great concern to both researchers and engineers. This paper investigates the seismic performance of reinforced concrete (RC) single pylon cable-stayed bridge equipped with friction sliding bearings under bi-directional earthquake excitations. Three-dimensional numerical models, consisting of the dynamic p-y elements, the fiber elements and the multi-layer shell elements, were developed in this study based on the prototype of a single pylon cable-stayed bridge in Guizhou Province, China, to investigate its seismic response. Three different types of friction sliding bearings were implemented between the base of the RC pier and the pile foundation to mitigate the dynamic response of the bridge superstructure under strong ground motions. The results of the nonlinear time history analyses indicate that friction sliding bearings can effectively reduce the base shear and the bending moment of the RC pier, as well as the seismic responses of substructure, at the cost of increasing the absolute displacement of the deck. To reduce the deck displacement, viscous dampers were implemented in parallel to the friction sliding bearings. Parametric studies and optimization analyses were performed on the critical parameters of the viscous dampers with the target of minimizing the absolute displacement of the deck and reducing the base shear of the RC pier to an acceptable level. The numerical results show that implementation of friction sliding bearings together with the viscous dampers is an effective seismic control approach to mitigate the seismic damage of the single pylon cable-stayed bridge.

A new simplified approach for assessing nonlinear seismic response of plan-asymmetric structures considering soil-structure interaction

An innovative approximate method is presented to consider the plan asymmetry, nonlinear structural behaviour and soil-structure interaction (SSI) effects simultaneously. The proposed method so-called Flexible base 2DMPA (F2MPA) is an extension of 2 degrees of freedom modal pushover analysis (2DMPA) approach to consider foundation flexibility in seismic response analysis of plan asymmetric structures which itself were developed based on Uncoupled Modal Response History Analysis method for inelastic fixed-base asymmetric structures. In F2MPA for each mode shape using 2DMPA procedure, the elastic and inelastic properties of 2DOF modal systems corresponding to the fixed-base structure are initially derived. Then in each time step, displacements and inelastic restoring forces of the superstructure are computed from modal equations of the flexibly-supported structure. In each time step, the nonlinear secant stiffness matrix corresponding to the n-th MDOF modal equations of soil-structure system is updated using the corresponding modal 2DOF system of fixed-base structure. To update the transformed modal stiffness matrix of the SSI system, this matrix is partitioned and it is assumed that the non-linear variation of the superstructure can be estimated from the variation of modal stiffness matrix of the fixed-base structure. Accuracy of the proposed method was verified on an 8-story asymmetric-plan building under different seismic excitations. The results obtained from F2MPA method were compared with those obtained by nonlinear response history analysis of the asymmetric soil-structure system as a reference response. It was shown that the proposed approach could predict the results of the nonlinear time history analysis with a good accuracy. The main advantage of F2MPA is that this method is much less time-consuming and useful for the practical aims such as massive analysis of a nonlinear structure under different records with multiple intensity levels.

Identification of dominant modes of single-layer reticulated shells under seismic excitations

Single-layer reticulated shells are a type of spatial lattice structures with closely spaced natural frequencies and significant higher mode effects. An accurate calculation of the seismic responses of these structures necessitates the modal combination of dozens or even hundreds of modes when using mode superposition method. To precisely identify the modes that are significantly contributory to the seismic responses, the time-averaged modal strain energy ratio, denoted as TAMSER for brevity, is derived and then selected as the only evaluation metric, taking into account both the modal characteristics of a given structure and the spectral characteristics of an input ground motion. In terms of this metric, a methodology for identifying dominant modes is developed for structure systems under unidirectional or multi-directional seismic excitation. Based on this methodology, the dominant modes of two typical single-layer reticulated shells are identified and investigated, and then the responses resulting from the modal combination of these identified dominant modes are compared to that of the first 30 and 250 modes, as well as the time history analysis results. It is finally concluded that this proposed methodology is capable of accurately identifying the dominant modes of single-layer reticulated shells subjected to seismic excitations, and the incorporation of the identified dominant modes only into the modal combination is appropriate and sufficient to consider the higher modes effects and to get a satisfactory engineering accuracy in the mode superposition analysis.

A preliminary study on seismic behavior of the graphite reflector in molten salt reactor

Graphite reflector submerged in molten salt in TMSR has important influences on the evaluation of structural stability of the reflector and the safety of the reactor because of the collision and fluid–solid interaction. In this paper, an approach is proposed for simulating the non-linear responses of the reflector under seismic excitation. And then, with this approach, a finite element model of the reflector in TMSR is built to investigate its dynamic characteristics and the seismic time history responses. From the results of the seismic time history analysis, the displacements of the control rod channel, which are required for the evaluation the safety of the reactor, are obtained as the boundary condition for the control rod drop test. At the same time, the impact forces between the components are provided that is required for the stress evaluation of graphite components.

Creation of innovative earthquake resistant steel buildings by dividing the structure into inner and outer parts having interaction by hysteretic dampers

In conventional seismic design codes of building structures severe structural damage due to large earthquakes is accepted, provided that the structure does not collapse, and life safety of the residents is guaranteed. The extent of this allowed damage can be so high that after earthquake, demolition and reconstruction of the building becomes inevitable, particularly in near-fault areas. The idea followed in this study is to relocate the damage to some predetermined fuse elements, other than the main structural members, so that the building could be quickly and easily repaired, even after major earthquakes. For this purpose, dividing the building’s structure into two inner and outer parts with different dynamic characteristics, and creating dynamic interaction between them by using yielding plate or hysteretic dampers was investigated. At first, a 5-storey building model was developed and divided into two interactive parts using hysteretic dampers at roof level, and it was tried, by developing a code in MATLAB environment for solving the nonlinear equations of motion, to find the optimal values of the initial stiffness and the yielding strength of dampers, assuming their behavior to be elastic-perfectly plastic. The results of nonlinear time history analyses (NLTHA), by using a set of selected earthquakes accelerograms, showed that by using appropriate initial stiffness and yield strength, the inert-story drifts of both inner and outer structures can be significantly reduced. Then, 5-, 8- and 11-story steel braced buildings were designed by ETABS, and were divided into inner and outer substructures, and then were modeled in PERFORM-3D software using hysteretic dampers with optimal stiffness and strength, obtained from the MATLAB program, and the seismic responses were compared. Final results of the NLTHA show that the inter-story drift values of outer and inner substructures decreases in average by 20 % and 65 %, respectively, comparing to the original structure. This drift reduction leads to decrease of the damage indices of outer and inner substructures by almost 20 % and 80 %, respectively, comparing to the original structure, making it possible in most cases to have an easily repairable structure.

Compatibility Forces in Floor Diaphragms of Steel Braced Multi-Story Buildings

Floors have a key role in the seismic behaviour of structures, especially in multi-story buildings. The in-plane behaviour of a floor system influences the seismic response of the structure significantly and affects the distribution of lateral forces between seismic resisting systems and over the height of the structure. In buildings where the seismic resisting systems are in the same location in plan on each floor over the height of the building, inertial and displacement compatibility shear forces are the principal shear forces generated at the interface between the floor system and the seismic-resisting system. These two are called interface diaphragm forces. These interface forces must be transferred into the appropriate lateral load resisting system and the interface must be well designed and detailed. Determination of the magnitude of the interface loads on concrete diaphragms are not well understood and still a matter of debate. There is no consensus of a design procedure for determining the diaphragm actions and distribution into the seismic resisting systems. In this paper, interface forces generated in floor diaphragms by asymmetrical actions of the braced framing system on each side of the building in the direction of analysis have been investigated. A numerical study using Numerical Integration Time History Analysis (NITH), has been undertaken to evaluate the interface forces of concrete floor diaphragms in a 12-story braced steel building. The results of nonlinear time history analyses using ground motion records from three different earthquakes are presented.

Design method for steel restrainer bars on railway bridges subjected to spatially varying earthquakes

The unseating and pounding damage at expansion joints during earthquakes emphasizes the need to restrain the relative opening and closing displacements between adjacent bridge spans. Traditional cable restrainers perform well in preventing unseating damage but may increase the pounding damage between adjacent decks sometimes. Considering the limitation of the cable restrainer, a steel restrainer bar installed between a pier and a deck is proposed to restrain both the relative opening and closing displacement to prevent unseating and pounding damage to railway bridges. In this paper, theoretical formulas for the mechanical properties of the restrainer bar, experiments and numerical analyses are conducted. A design procedure for the steel restrainer bar, which considers spatially varying earthquakes and the hysteretic behaviour of the restrainer, is developed. The procedure accounts for the dynamic out-of-phase motion characteristics between adjacent bridge spans, and the pounding is assumed to be completely avoided by the restrainer bar. A series of case studies with different pier heights and different earthquake characteristics are conducted to evaluate the effectiveness of the design method; the time history analysis results are consistent with the case study results. The restraint effect of the rails is also considered in this paper. The results of the numerical analyses reveal that both the pier height and the rails significantly influence the effectiveness of the steel restrainer bar. As the pier height increases, the effectiveness of the restrainer in reducing the relative displacement also increases, whereas the effectiveness in mitigating shear decreases with a pier height greater than 15 m. The rails reduce the effectiveness of the steel restrainer bar when subjected to spatially varying ground motions.

Sustainability of civil infrastructure using shape memory technology

The last few decades have clearly demonstrated the vulnerability of our civil infrastructure systems to problems like aging, natural, and man-made hazards including earthquakes, hurricanes, blasts, etc. The conventional materials such as steel and concrete have proven to be limited in terms of their ability to withstand the extreme demands imposed on them by modern societies. The limitations in currently used construction materials combined with the consistently growing population worldwide present new challenges and demands for researchers in the field of structural engineering. Hence, there is an urgent need for new materials that are capable of extending the service life of structures with minimal or no need for maintenance or repairs against natural and man-made hazards. Shape memory alloy (SMA) is a class of “Smart Materials” that have recently emerged as potential construction material with unique thermomechanical properties, namely shape memory effect and superelasticity. Two applications of SMAs in civil structures are discussed in this paper. The first application involves the use of SMA in performing seismic rehabilitation of RC bridge columns that lack flexural ductility. In this application, SMA is used in the form of thermally prestressed spirals that can apply large active confinement pressure to the columns at their plastic hinge regions to improve their flexural ductility. The experimental results of large scale shake table tests performed on two RC columns, one of which is retrofitted with SMA are discussed. The results demonstrate the great ability of SMA spirals in mitigating the damage even under strong levels of ground shaking. The second application focuses on utilizing superelastic SMA fibers as reinforcement for polymeric composite bars. The newly developed composite material is named SMA–Fiber Reinforced Polymer (SMA–FRP), and is studied as seismic reinforcing bars for moment resisting concrete frames. The results of nonlinear time history analysis prove that using SMA–FRP bars at the plastic hinge regions of the frames helps significantly in limiting the residual drifts and enhancing the energy dissipation of the frames.

Numerical Investigation of Nano Particles Dispersion and Deposition in Fully Developed Laminar Pipe Flows

The aim of this paper is to study the deposition and dispersion of nano particles in fully developed laminar pipe flows numerically. To simulate particle transport and to locate the position of particles, the Eulerian - Lagrangian method is used under the conditions of one-way coupling. Due to studied range of particle diameters from 5 nm to 100 nm, the main effective force for particle deposition is the Brownian diffusion force. After studying the mesh independency and validating results, time history analysis of particle transport is also performed by injecting the particles from the inlet surface and tracking them at each moment. Furthermore, the effective parameters, i.e. particle diameter, pipe length and diameter, temperature and particle density are studied comprehensively. The results of time history analysis of particle transport show that nano particles with less diameters are more deposited in less time. Furthermore, maximum number of escaped particles from the pipe occurred at 0.035 s after injecting the particles for all studied particle diameters due to the studied flow rate and length of the pipe. The output of this study can provide a guideline for evaluating nano particle transport and deposition in fully developed laminar pipe flows.

The effect of mainshock-aftershock on the residual displacement of buildings equipped with cylindrical frictional damper

Recently, Friction dampers become popular due to the desirable performance in the energy dissipation of lateral loads. A lot of research which has been conducted on these dampers results in developing friction dampers with low sensitivity to the number of cycles and temperature increases. Friction dampers impose high residual drifts to the buildings because of low post-yield stiffness of the damper which results from increasing lateral displacement and period of buildings. This issue can be more critical under strong aftershocks which results in increasing of structural damages. In this paper, in addition to the assessment of aftershock on steel buildings equipped with friction dampers, methods for controlling residual drifts and decreasing the costs of retrofitting are investigated. Utilizing rigid connections as a lateral dual system and activating lateral stiffness of gravity columns by adding elastic braces are as an example of effective methods investigated in this research. The results of nonlinear time history analyses on the low to medium rise steel frames equipped with friction dampers illustrate a rise in residual drifts as the result of aftershocks. In addition, the results show that different slip loads of friction damper can affect the residual drifts. Furthermore, elastic stories in comparison to rigid connections can reduce residual drifts of buildings in an effective fashion, when most slip loads of friction dampers are considered.

Evaluation of a Simplified Method to Estimate the Peak Inter-Story Drift Ratio of Steel Frames with Hysteretic Dampers

In this paper, a simplified method is proposed to estimate the peak inter-story drift ratios of steel frames with hysteretic dampers. The simplified method involved the following: (1) the inelastic spectral displacement is estimated using a single-degree-of-freedom (SDOF) system with multi-springs, which is equivalent to a steel frame with dampers and in which multi-springs represent the hysteretic behavior of dampers; (2) the first inelastic mode vector is estimated using a pattern of story drifts obtained from nonlinear static pushover analysis; and (3) the effects of modes higher than the first mode are estimated by using the jth modal period, jth mode vector, and jth modal damping ratio obtained from eigenvalue analysis. The accuracy of the simplified method is estimated using the results of nonlinear time history analysis (NTHA) on a series of three-story, six-story, and twelve-story steel moment resisting frames with steel hysteretic dampers. Based on the results of a comparison of the peak inter-story drift ratios estimated by the simplified method and that computed via NTHA using an elaborate analytical model, the accuracy of the simplified method is sufficient for evaluating seismic demands.

Identifying stiffness irregularity in buildings using fundamental lateral mode shape

Soft or extreme soft storeys in multi-storied buildings cause localized damage (and even collapse) during strong earthquake shaking. The presence of such soft or extremely soft storey is identified through provisions of vertical stiffness irregularity in seismic design codes. Identification of the irregularity in a building requires estimation of lateral translational stiffness of each storey. Estimation of lateral translational stiffness can be an arduous task. A simple procedure is presented to estimate storey stiffness using only properties of fundamental lateral translational mode of oscillation (namely natural period and associated mode shape), which are readily available to designers at the end of analysis stage. In addition, simplified analytical expressions are provided towards identifying stiffness irregularity. Results of linear elastic time-history analyses indicate that the proposed procedure captures the irregularity in storey stiffness in both low- and mid-rise buildings.

Applicability of equivalent linearization methods to irregular isolated bridges

Equivalent linearization methods are widely applied to assess the seismic demands of buildings and bridges. Several regulation codes include methodologies to conduct linear analyses with equivalent structures. Normally, bridges are highly irregular structures due to the different pier heights. This study evaluates the applicability of a group of equivalent linearization methods proposed in the literature to predict the seismic response of isolated bridges with height irregularities. The isolation system is composed by lead rubber bearings. A parametric study is carried out in bridges with three types of height irregularities and combinations of relative height between piers. The bridges were subjected to a family of seismic records from a subduction zone to evaluate the equivalent elastic methods based on the results of nonlinear time-history analysis. Especial emphasis is devoted to assess the influence of the bridge irregularity on the ability of the methods to predict the nonlinear bridge demands. The results show the applicability of the equivalent linearization methods as function of the ductility demands and the height irregularity of the bridges. This study also discusses the parameter combinations that the different approaches lead to underestimate or overestimate the bridges demands.

The Effects of Using Different Seismic Bearing on the Behavior and Seismic Response of High-Rise Building

The effects of using different seismic bearings were investigated to reduce the seismic response of buildings by assuming the vulnerability of 20-story regular RC building in this paper. The method of this study was that the studied building was studied in three different models in terms of its connection to the foundation. In the first model, the structures were placed on the rigid bearing and in the second and third models; lead-rubber bearings and friction pendulum bearings were placed at the counter between the structure and foundation, respectively. Then, the dynamic analysis was used to assess the behaviour and seismic response of the mentioned models. The results of the study showed that the structures in the first model functioned like cantilever column that would become uniaxial and biaxial bending under the effects of earthquake around the vertical axis of structure. Due to the tensile (tension) weakness in concrete, seismic loads caused major cracks in the tension part of the structures according to the place of the neutral axis that could lead to the collapse of structure. In addition, the use of mentioned seismic bearings under the earthquake caused the structure like a semi-rigid box slid on this equipment that reduced the structure's stiffness and increased the period of the structure in comparison with the first model. Using the studied seismic bearings caused the displacement of the roof of the first and twentieth stories of the structure become approximately equal and prevented the creation of the bending moment in the first model. The results of non-linear time history analysis showed that using the studied seismic bearings caused the response of the structure reduced significantly when the structure was placed on rigid bearings. It could be very valuable regarding the limitation of the capacity of the structure's members.