Inelastic Time-history Analyses Research Articles

Seismic Response of Vertical Hybrid Concrete/Steel Frames Considering Soil–Structure Interaction

The aim of this study is to investigate the seismic behavior of concrete/steel mixed structures. In engineering praxis, many buildings consist of two parts: one made of reinforced concrete and the other made of steel. There are several difficulties in the code-based seismic design of these structures due to the different dynamic responses of each discrete part. Seismic design codes, such as the IBC and Eurocode 8, do not provide instructions for structures consisting of two parts. In addition, they use a single-loading scenario, but there are many locations that are affected by more than one earthquake in a short period. Another drawback is that recent provisions do not consider soil–structure interaction effects. The specific issue addressed here is the seismic response of mixed structures, which is evaluated through inelastic time–history analysis. More specifically, the response indices involve height-wise distributions for peak interstory drift ratios, maximum floor horizontal displacements, maximum floor accelerations, and plastic hinge formations in the frame elements when they are subjected to seismic sequences of earthquakes, as well as in far fault ground motions for different soil types. The results reveal that sequential ground motions lead to increased displacement demands, and they affect the permanent displacements. This phenomenon appears in both cases of stiff and flexible soil, as well as for both regular and irregular frames. It is found that soil–structure interaction generally leads to lower values of IDR, and maximum horizontal displacement and acceleration in comparison with the case of rigid soil assumptions.

An improved iterative technique for inelastic time history analysis of single degree of freedom (SDOF) elasto-plastic system

An improved iterative technique for inelastic time history analysis of single degree of freedom (SDOF) elasto-plastic system

Energy-based design method for special concentrically braced frames considering low-cycle fatigue damage of braces

Energy-based design methods have been widely used to evaluate structural performance in last few decades. However, energy-based design methods of SCBF proposed by previous researches did not fully consider the low-cycle fatigue damage of braces. To study energy-based design method of SCBF considering the effect of low-cycle fatigue damage of braces, inelastic dynamic time history analysis of six SCBF models is performed and experimental data is evaluated in this paper. Energy input spectrum of SCBF is modified and energy demand of braces are proposed by dynamic time history analysis. Energy dissipation capacity of braces based on low-cycle fatigue damage of braces is set up through the results of tests on braces and braced frames. A practical energy-based design method of SCBF is proposed by obeying the principle that energy demands are smaller than energy dissipation capacity of braces. An integral design process of energy-based design method of SCBF is shown in this paper by a designed SCBF example. The configuration detailing of braces meeting the regulations of AISC 341 is relatively stricter compared with the security domain of proposed energy-based design method. The configuration detailing of braces is not only related to ductility of the structure, but also intensity of design ground motion according to the energy-based design method proposed by this paper.

Seismic response of RC frames equipped with buckling-restrained braces having different yielding lengths

<abstract> <p>Buckling-restrained braces (BRBs) have proven to be a valuable earthquake resisting system. They demonstrated substantial ability in providing structures with ductility and energy dissipation. However, they are prone to exhibit large residual deformations after earthquake loading because of their low post-yield stiffnesses. In this study, the seismic response of RC frames equipped with BRBs has been investigated. The focus of this research work is on evaluating the effect of the BRB yielding-core length on both the maximum and the residual lateral deformations of the braced RC frames. This is achieved by performing inelastic static pushover and dynamic time-history analyses on three- and nine-story X-braced RC frames having yielding-core length ratios of 25%, 50%, and 75% of the total brace length. The effects of the yielding-core length on both the maximum and the residual lateral deformations of the braced RC frames have been evaluated. Also, the safety of the short-yielding-core BRBs against fracture failures has been checked. An empirical equation has been derived for estimating the critical length of the BRB yielding cores. The results indicated that the high strain hardening capability of reduced length yielding-cores improves the post-yield stiffness and consequently reduces the maximum and residual drifts of the braced RC frames.</p> </abstract>

Wall-diaphragm interactions in seismic response of single-story building systems

The objective of this paper is to investigate the seismic time history response of single-story buildings with a wide range of wall and roof diaphragm periods and ductility. Current design specifications, e.g., ASCE 7-16, are transitioning from accounting for inelasticity in only the vertical lateral force resisting systems, i.e., the walls, to including inelasticity in the horizontal lateral force resisting system, i.e., the roof diaphragm. It is not clear if the wall and roof inelasticity can be considered independently, nor if they can utilize their own seismic response modification coefficients. More fundamentally, the basic manner in which stiffness, mass, and ductility of the wall and roof interact is not fully understood. Here, a single-story building is approximated with a reduced order mass-spring model and parameters are varied so that a wide range of wall periods, diaphragm periods, and wall and diaphragm inelasticity are explored. The model is imposed to vibration analyses, elastic time history analyses, and inelastic time history analyses. The results show clear regimes where the wall and diaphragms interact, and those where the response is largely independent. When inelasticity occurs the ductility demands can be significant for the building component, wall or diaphragm, that initiates the yielding, and greater than traditionally expected. The model results are compared to current and proposed provisions for predicting force and ductility demands and conclusions are drawn with respect to the accuracy of available methods.

Investigating different methods for application of earthquake records in seismic evaluation of irregular RC bridges considering incident angles

Seismic response of 9 regular and irregular four-span RC bridges with different configurations was studied using elastic and inelastic time history analyses. The influence of using different approaches for applying the ground motion records, considering the interaction of the orthogonal horizontal components of the ground motion records and their incident angles, was investigated. For this purpose, the records were applied to the structural models using three different methods and the results were compared. To investigate the influence of the directivity of the records on the mean seismic response, the records were applied at 19 different incident angles. The use of the 100–30 combination rule for prediction of the maximum response was also investigated. In addition, the results from linear and nonlinear analyses were compared and the maximum ductility demands and drift ratios were predicted. The results indicated that as the irregularity of the bridges increased, the seismic response became more coupled in the two principal directions and also became more sensitive to the incident angle of the records. It was also shown that when the mean seismic response was considered, the results were less sensitive to the incident angle compared to the case that the response from the individual records was considered.

Investigating the Use of Natural and Artificial Records for Prediction of Seismic Response of Regular and Irregular RC Bridges Considering Displacement Directions

The seismic responses of continuous multi-span reinforced concrete (RC) bridges were predicted using inelastic time history analyses (ITHA) and incremental dynamic analysis (IDA). Some important issues in ITHA were studied in this research, including: the effects of using artificial and natural records on predictions of the mean seismic demands, effects of displacement directions on predictions of the mean seismic response, the use of 2D analysis with combination rules for prediction of the response obtained using 3D analysis, and prediction of the maximum radial displacement demands compared to the displacements obtained along the principal axes of the bridges. In addition, IDA was conducted and predictions were obtained at different damage states. These issues were investigated for the case of regular and irregular bridges using three different sets of natural and artificial records. The results indicated that the use of natural and artificial records typically resulted in similar predictions for the cases studied. The effect of displacement direction was important in predicting the mean seismic response. It was shown that 2D analyses with the combination rules resulted in good predictions of the radial displacement demands obtained from 3D analyses. The use of artificial records in IDA resulted in good prediction of the median collapse capacity.

Investigating the effects of combinations of irregularities on seismic ductility demands and mean response for four-span RC bridges considering displacement direction

The effects of combinations of different types of irregularities have not been studied in details in the past and current seismic design codes do not address this issue appropriately. In this research, 76 regular and irregular bridges with irregularities in both superstructure and substructure were designed and evaluated to investigate the impact of combinations of irregularities on the seismic ductility demands. The irregularity parameters considered in this study include irregularities in span arrangement, different lengths of columns, different abutments support conditions and different stiffness of superstructure. The bridges were designed and checked according to AASHTO provisions. Inelastic time history analysis was conducted using OpenSees software and ductility demands in bridge columns for different bridge configurations were predicted. Predictions of ductility demands were based on the mean responses obtained using a number of ground motion records. Finally, the effect of considering displacement directions in predicting the mean bridge response (i.e., using different methods for predicting the mean response) for irregular and regular bridges was investigated. The results indicate that the combinations of irregularities can significantly increase the ductility demands in some cases compared to the case of regular bridges.

Equivalent viscous damping for linked column steel frame structures

Linked column steel frame system (LCF) is a new lateral-gravity resisting system aimed at reducing the time and costs of repairs after a seismic event. The performance objectives of this system are highly dependent on the deformations and the sequences at which fuses are formed in the structure. Therefore, in order to meet the performance objectives of this system, using a performance-based design method (e.g. the direct displacement-based design (DDBD) method), that directly controls the structural response would be useful. In the DDBD method, the equivalent viscous damping (EVD) is an important parameter which represents the energy dissipation of a structure due to its nonlinear (hysteretic) behavior. In the literature, there are some EVD expressions for conventional structural systems; however, no expression has been given for the LCF system, yet. The purpose of this study is to develop some reliable EVD expressions for the LCF systems by investigating the nonlinear behavior of 20 single-story frame samples of different configurations with variable link beam lengths. To this end, the EVD of the frames was calculated based on their hysteretic response loops under cyclic loading. Furthermore, the EVD values was calibrated using the inelastic time-history dynamic analysis of the frames under 10 spectrum-compatible earthquake records to propose some new EVD expressions for the LCF systems with short and long link beams. The results indicate that the EVD expressions available in the literature are very conservative for the LCF systems with short link beams.

Nonlinear Inelastic Earthquake Analysis of 2D Steel Frames

In this work, a new method for nonlinear time-history earthquake analysis of 2D steel frames by a fiber plastic hinge method is presented. The beam-column element based on the displacement-based finite element method is established and formulated in detail using a fiber plastic hinge approach and stability functions. Geometric nonlinearities are taken into accounting by stability functions and the geometric stiffness matrix. A nonlinear dynamic algorithm is established based on the combination of the Newmark integration method and the Newton-Raphson iterative algorithm for solving dynamic equations. The proposed program predicts the nonlinear inelastic responses of 2D steel frames subjected to earthquakes as efficiently and accurately as commercial software. This study also shows that the initial residual stresses of steel should be considered in nonlinear inelastic time-history earthquake analysis of 2D steel frames while SAP2000 does not consider the effects of residual stresses.

A New Empirical Correlation for Estimation of EBF Steel Frame Behavior Factor under Near-Fault Earthquakes Using the Genetic Algorithm

The most important feature of the behavior factor is that it allows the structural designer to be able to evaluate the structural seismic demand, using an elastic analysis, based on force-based principles quickly. In most seismic codes, this coefficient is merely dependent on the type of lateral resistance system and is introduced with a fixed number. However, there is a relationship between the behavior factor, ductility (performance level), structural geometric properties, and type of earthquake (near and far). In this paper, a new and accurate correlation is attempted to predict the behavior factor (q) of EBF steel frames, under near-fault earthquakes, using the genetic algorithm (GA). For this purpose, a databank consisting of 12960 data is created. To establish different geometrical properties of models, 3−, 6−, 9−, 12−, 15, and 20− story steel EBF frames were considered with 3 different types of link beam, 3 different types of column stiffness, and 3 different types of brace slenderness. Using nonlinear time history under 20 near-fault earthquake, all models were analyzed to reach 4 different performance levels. 6769 data were used as GA training data. Moreover, to validate the correlation, 2257 data were used as test data for calculating mean squared error (MSE) and correlation coefficient (R) between the predicted values of (q) and the real values. In addition, the MSE and R were calculated for correlation in the train and test data. Also, the comparison of the response of maximum inelastic displacement of 5 stories EBF from the proposed correlation and the mean inelastic time-history analysis confirms the accuracy of the estimate relationship.

Effects of column and superstructure irregularity on the seismic response of four-span RC bridges

Seismic behaviour of a total of 76 bridges, including 4 regular and 72 irregular continuous RC bridges with irregularities in both superstructure and substructure are investigated in this study. Bridges with different span arrangements, various column heights, different support conditions at abutments, different superstructure transverse stiffness were considered in this study. The influence of different seismic intensity levels on the seismic response was also investigated. The bridges were designed and detailed according to American Association of State Highway and Transportation Officials (AASHTO) regulations. Seismic response of bridges was predicted using different methods of analysis including elastic multi-mode and inelastic time history analyses. To investigate the sufficiency of code provisions for predicting the seismic behaviour of irregular bridges, a comparison was made between the results obtained using linear and non-linear analyses in the longitudinal and transverse directions of the bridges and a new relationship was proposed to predict when the results from elastic analyses are not appropriate. The results indicate that in some cases significant deviations can occur between the predictions obtained using linear and nonlinear analyses, when simultaneous irregularities in both superstructure and substructure were considered. Current code provisions do not consider this issue in design of irregular bridges.

Effects of Aftershocks on the Potential Damage of FRP-Retrofitted Reinforced Concrete Structures

This study investigates the effects of aftershocks on the potential damage of reinforced concrete (RC) structures retrofitted by fibre-reinforced polymer (FRP). An eight-storey RC frame that was poorly confined due to deficient transverse reinforcement was selected and then retrofitted by FRP wraps to provide external confinement. Inelastic time history and damage analyses of the frame under single earthquakes and mainshock–aftershock sequences were performed. FRP retrofit did not ensure a damage-free structure, and it minimised the damage though. The results indicated different contributions of aftershocks to the damage of the retrofitted frame, and this finding illustrates the limitations of current seismic codes. The damage caused by single earthquakes is proposed to be used as a criterion for dealing with the effects of aftershocks as follows. If the mainshocks cause minor or light damage, the effect of aftershocks is negligible, whereas the effect of aftershocks needs to be considered if the mainshocks cause moderate or severe damage. The outcomes of this study can be used in making decisions about whether the effects of aftershocks should be considered.

Comparison of near and far-fault ground motion effects on low and mid-rise masonry buildings

This paper presents a comparison of near and far-fault ground motion effects on the seismic response of masonry structures, using nonlinear methods. To compare the influence on low to moderate rise buildings, a detailed field investigation and archive study were executed in Albania. Based on the archive and detailed field investigation over a hundred real residential masonry buildings constructed between 1963-1978, seven masonry buildings of 2-5 stories were selected to reflect the commonly constructed template designs. Analytical models of each building were prepared following the experimental data on the quality of the masonry constitutive components of the selected buildings. Seismic deformation capacities of each building were obtained by nonlinear static analyses. Inelastic time history analysis was used to predict the seismic displacement demands of the studied buildings. The significance of the findings is further investigated by examining the nonlinear behavior of a group of realistic buildings subjected to near and far-fault ground recordings. Results indicate that there is a remarkable difference in the seismic response of masonry structures between near and far-fault records. It can therefore be concluded that near-fault earthquakes may have significantly more damage potential on masonry structures than far-fault records.

A temporal hybrid dynamic integration algorithm strategy for inelastic time history analysis of high‐rise reinforced concrete structures under strong earthquakes

SummaryA complete earthquake time history analysis (THA) requires a stable, accurate, and efficient dynamic integration algorithm. It is not rare to encounter numerical divergence when some implicit algorithms are used to deal with severe materially or geometrically nonlinearities. For explicit algorithms, computational efficiency is always a major concern. A temporal hybrid dynamic algorithm (THDA) strategy, which is specialized in the inelastic THAs of high‐rise reinforced concrete (RC) structures experiencing severe plasticity development, is developed herein. A preliminary evaluation is carried out on three low‐rise structural models, that is, two frame structures and one wall‐frame structure, for each group of collected implicit algorithms and explicit algorithms. From the evaluation, four alternatives are generated for the subsequent detailed assessment. A general framework for the THDA is proposed and implemented on a finite element analytical platform. The four alternatives are assessed based on their performance on a high‐rise frame core‐tube RC structure. The assessment indicates that the proposed THDA strategy can give rise to a more compatible dynamic integration algorithm for the complete THAs of high‐rise building structures when they are experiencing severe damage. The concerns about the computational stability, accuracy, and efficiency of the dynamic algorithms can be well balanced by the THDA.

Seismic drift response of seesaw-braced and buckling-restrained braced steel structures: A comparison study

A numerical comparison study of the interstorey and the residual interstorey seismic drifts of steel structures equipped with the seesaw system and buckling-restrained braces is carried out. This investigation involves inelastic time-history seismic analyses of 2-, 5- and 8-storey 3-D steel structures, with specific orientation of columns and configuration of braces, for the design basis earthquake. The effects of soil-structure interaction and seismic incident angle are also considered in these analyses. Comparison of the seismic drift responses experienced by the seesaw-braced and the buckling-restrained braced steel structures, reveals different peak interstorey drift values but in many cases similar drift concentration along the height of the structures. Furthermore, the seesaw-braced steel structures exhibit, in general, larger peak residual drifts than buckling-restrained braced steel structures.

Seismic effects on leaning slender structures and tall buildings

A problem of joint effects of a structural leaning and seismic excitations can be particularly important for tall buildings and slender towers. Present paper addresses problems of loss of seismic resistance of leaning tall structures on examples of two selected typical structures: a 160 m high industrial chimney and a slender 30-storey high building. The problem is discussed first for strong seismic rockburst excitations (induced seismicity – where structural leaning often occurs) and also for strong natural earthquakes. Dynamic, non-linear time-history seismic response analyses of the 160 m high, reinforced concrete industrial chimney and the 30-storey shear wall building are carried out. Detailed parametric analyses of non-linear interaction of the effect of initial leaning and simultaneous seismic action, in form of inelastic time-history analyses, are performed. Unique changes in seismic responses of the leaning structures depending on combinations of various initial foundation tilts and seismic intensities are observed and studied in detail. It is demonstrated that the limitations on structural leaning described in an actual industrial mining code of practice are enough conservative. Various mechanisms driving seismic response at base and in the upper part of a leaning structure are recognized and carefully analysed for various combinations of initial ground tilts and seismic intensities.

Comparison of CFRP and GFRP Wraps on Reducing Seismic Damage of Deficient Reinforced Concrete Structures

The effectiveness of glass/carbon fibre reinforced polymer (GFRP/CFRP) wraps at plastic hinges has been confirmed; however, their respective effectiveness on reducing seismic damage of deficient reinforced concrete structures has hardly been compared. The current study aims at this comparison, providing some useful information to realise the better FRP for confinement retrofitting of reinforced concrete structures poorly confined due to deficient transverse reinforcement. Four- and eight-storey poorly confined reinforced concrete frames were selected to represent low- and mid-rise building structures, respectively. These deficient frames were then strengthened by CFRP/GFRP wraps via external confinement. Inelastic time history and cumulative damage analyses of the original, CFRP- and GFRP-retrofitted frames were performed. The damages of CFRP- and GFRP-retrofitted frames were compared with each other and with the damage of the original frame. The comparison results show that CFRP and GFRP wraps at plastic hinges substantially reduce damage. More importantly, GFRP wraps are more effective than CFRP wraps by reducing a higher amount of the cumulative damage index. The conclusions can help in deciding on the type of FRP to be used for confinement retrofitting of RC structures poorly confined due to deficient transverse reinforcement.

Characterization of Near-Fault Effects on Potential Cumulative Damage of Reinforced Concrete Bridge Piers

The objective of this study is to characterize the potential damage of near-fault ground motions on reinforced concrete bridge piers, providing information for engineering practice. To achieve this objective, 200 real near-fault pulse-type records and a bridge pier were selected. The same number of real far-fault ground motions was also selected for comparison. Inelastic time history and damage analyses were performed for the pier subjected to the selected near-fault and far-fault ground motions. The obtained results showed that more than 91% near-fault motions caused the pier to collapse while the pier survived under almost all far-fault motions. The responses of the pier under near-fault motions were characterized by one or few large hysteretic cycles. The damage indices of the pier subjected to near-fault ground motions increased rapidly when the pier underwent pulses of near-fault motions, and the pier damage was mainly caused by pulses. Collapse of the pier subjected to near-fault motions occurred at the pulse time. The duration to collapse, which was defined as the duration from light damage to collapse, of the pier under near-fault motions was extremely short (approximately 2.3–2.5 s). These special and negative structural damage characteristics of near-fault ground motions should be considered in designing and mitigating seismic hazards for structures located in near-fault regions.

Residual Displacement Demand Evaluation from Spectral Displacement

In this study, residual displacement demands are investigated for SDOF systems with period range of 0.1-3.0 s for near-field and far-field ground motions. The effects of stiffness degradation and post yield stiffness ratio on residual displacements are investigated. The modified-Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading. An elastoplastic model is used to represent non-degrading structures. For inelastic time history analyses, Newmark’s step by step time integration method was adapted in an in-house computer program. Based on time history analyses, a new simple equation is proposed for residual displacement demand of a system as a function of structural period (T), ductility (µ), strain hardening ratio (α) and spectral displacement (Sd).