Results Of Time History Analysis Research Articles

Performance-based seismic design of full and partial self-centering steel braced frames using modified lateral force distribution

Design base shear and lateral force distribution are two important aspects of the seismic design process. Different design methods have been proposed to obtain the design base shear of different types of structures. However, the study related to the lateral force distribution is limited, especially for self-centering (SC) structures. The primary objective of this research is to develop a performance-based seismic design (PBSD) procedure for steel braced frames considering the modified lateral force distribution. Six-story steel braced steel frames are designed to explore the influence of the energy dissipation factor on the lateral force distribution of the frames. Results of nonlinear time-history analyses indicated that the lateral force acting on the roof increases with the decrease in energy dissipation factor. Following the parametric analyses of single-degree-of-freedom (SDOF) systems, the energy factor design spectra are established and the corresponding PBSD procedure for the steel braced frames is proposed. Finally, the four six-story braced steel frames are redesigned using the PBSD procedure with the modified lateral force distribution to illustrate the efficiency of the proposed design method. Analyses results indicate that the steel braced frames designed by the proposed design can satisfy the expected inter-story drift limit with a reasonably uniform distribution of peak inter-story drifts. Furthermore, the ‘partial’ SC system appears to be a desirable option that effectively controls the residual displacement and peak acceleration responses at the same time, and could achieve a manageable cost control compared to the full SC systems.

A low computational cost seismic analyses framework for 3D tunnel-form building structures

Numerical modeling of tunnel-form buildings entails the utilization of 3D finite element models consisting of large numbers of shell and fiber elements. Accordingly, the non-linear analysis of such models under seismic excitations is both time-consuming and computationally expensive, especially in the case of large scale structures. To address this challenge, this study investigates the efficiency of the Endurance Time Method (ETM) to evaluate the structural responses, location of damage initiation, and the overall performance of tunnel-form structures under the Design Basis Earthquake (DBE) hazard level. Comparison with results derived from pushover and time-history analyses indicated the acceptable accuracy of ETM with significantly less computational efforts. The computation time required for the ETM was less than 25% of pushover (until total failure of the system) and time history analyses on the five- and ten-story tunnel-form buildings. The maximum differences between the results of ETM and time history analysis used to estimate the story drifts and shear forces were 4–6% and 1–4.5%, respectively. Considering the reliability of ETM and its appropriate accuracy, this method can be considered as a suitable alternative to the conventional methods to provide a low computational cost seismic analyses framework for non-linear tunnel-form buildings and similar structural systems.

A simple method for approximate estimation of the displacement response of base-isolated structures

The present study is a new attempt to provide a simple method for approximate estimation of the displacement response of base-isolated structures. For this purpose, first the accuracy of the existing methods which have been presented for base-isolated buildings is examined by comparison their estimations with the results of time history analyses of the 2, 4, 6, 8, 10 and 12 stories base-isolated models under 40 near-field and 40 far-field ground motion records. Then, a new method is proposed which is based on equivalent linear SDOF system with considering modified values of optimum damping ratio and effective period factor for base-isolated structures. The optimum damping ratio for base-isolated structures is obtained 31% and 23% for far- and near-field ground motion records, respectively. An effective period factor of 2.02 and 1.99 is extracted as a suitable value for obtaining the period of equivalent linear structure subjected to far- and near-field ground motion records, respectively. Results show that the proposed method not only has practically more simple procedure than the others, but also can provide reasonable estimates of the maximum nonlinear roof displacement of base-isolated structures subjected to an individual ground motion record. It’s worth mentioning that most of the equivalent linearization methods presented in the past use displacement ductility ratio as a key parameter for estimating the nonlinear displacement of base-isolated structures while the proposed method doesn’t need to such a parameter which cannot be easily predicted without performing nonlinear time history analysis. Therefore, as another attempt in this study, suitable approximate values of ductility ratio and other important coefficients are recommended for a number of existing approximate methods to improve their estimations for base-isolated structures.

A method for seismic design of RC frame buildings using fundamental mode and plastic rotation capacity

A seismic design method is proposed for RC frame buildings, with focus on two of the seven virtues of earthquake resistant buildings, namely deformation capacity and desirable collapse mechanism. Fundamental lateral translation mode of the building and plastic rotation capacity of beams are included as input to estimate lateral force demand. Guidelines are provided to proportion beam and column cross-sections through: (a) closed-form expressions of flexural rigidities to maximize participation of the fundamental mode, and (b) relative achievable plastic rotation capacity using current design and detailing practice. This method is seen to surpass two prominent displacement-based design methods reported in literature. Results of nonlinear static pushover and nonlinear time history analyses of buildings of three different heights designed by this and the said two methods are used to make a case for the proposed method; the proposed method is able to control plastic rotation demand in beams and provide at least 20% more lateral deformation capacity than the said methods.

Seismic analysis of extended piles in sand considering effect of scouring and effect of water as added mass

Scouring is one of the major causes of the failure of bridges and offshore wind turbines. Piles under extreme scouring and earthquake loads are vulnerable to failure and need to be investigated thoroughly. In this study, the seismic responses of an extended concrete pile with no scour and four scouring depths conditions are investigated under an input motion based on a Kobe earthquake record. The effect of water during an earthquake has a significant impact on the structure as hydrodynamic pressure; however, it is generally ignored in the seismic analysis of piles. The present study uses the concept of added mass to consider the effects of water. The combined soilstructure-interaction and shear beam model is used to simulate pile and soil using the beam-on-nonlinear-Winkler foundation (BNWF) method. Soil is divided into multiple layers with increasing shear modulus and the mass of each soil layer is lumped into two nodes of the layer. Modal analysis and nonlinear time history analysis are performed on different models. Results of the modal analysis show the decrease in fundamental frequencies of the pile with increasing scouring depths. The time history analysis results are reported in the form of Arias and Housner intensities, envelopes of acceleration, velocity, displacement of pile top, and envelopes of shear force and bending moment of the pile. Results show significant changes with scouring depth and the effect of water.

Dynamic and pushover analysis of multi-storey reinforced concrete building using different load distribution pattern

One of nature's most dangerous phenomena are earthquakes which cause significant harm to both people's lives and property. In this study, four alternative approaches are used to demonstrate the distribution of lateral loads and compare its impacts on the results of a non-linear static pushover analysis of a ten-storey reinforced concrete (RC) building and study its response to the impact of an earthquake. In order to determine how the structure would respond to earthquake effects, pushover analysis which is an alternative way of time history analysis was adopted and the predicted results are compared with those of nonlinear time history analysis. Given that the building is situated in an area that is actively experiencing earthquakes and has rocky soil, the distributed lateral force is assumed to be equivalent to the design base shear. The study indicated an almost good suitable fit between two categories of pushover loading methods regarding security of the building, maximum base shear, and maximum displacement. The paper also presents a comparison between the results of nonlinear time history analysis at a particular roof displacement with that of pushover analysis.

Evaluation of dynamic analysis of diagrid tall steel building subjected to wind with control approach of operation and acceleration criteria

SummaryConsidering the widespread use of diagrid systems in tall buildings, research on the behavior of this structural system against wind and control of comfort criteria based on acceleration seems necessary. According to various regulations, it can be found that there is not much guidance for designing this structural system, and its requirements have not yet been included in the design regulations. Therefore, one of the most important objectives of the present study is to study and apply the diagrid structural system as one of the new structural systems in high‐rise buildings. For this purpose, the effect of various parameters, including the acceleration of stories and the base shear under the dynamic wind load, was evaluated which was less studied so far. It is expected that the performance of diagrid will be evaluated by more accurately understanding the diagrid in tall structures and examining the operation and comfort criteria based on acceleration against wind load using dynamic analysis of time history in three methods: Cholesky, ergodic, and autoregressive (AR). The results were compared with the formulas of ASCE7 regulations and AIJ‐GBV‐2004 and ISO 10137: 2007 and National Research Council of Canada (NBCC) comfort criteria. Studies show that the acceleration of diagrid system stories, based on the ASCE7 dynamic‐wind response prediction equations, has exceeded the allowable limit of ASCE 7 regulations (20 gal or 20 milli‐g). This is due to the overestimation of the ASCE 7 equations relative to the results of wind time history analysis. Hence, in the 50‐story structure, the maximum roof acceleration resulting from ASCE 7 equation is 1.83 times the analytical results, in 70‐story structure 2.07 times, and in the 100‐story structure 1.87 times the results of dynamic‐wind time history analysis.

Comparison of Nonlinear Dynamic Analysis of Time History and Endurance Time Method in Tall Structures with Frame-Wall System

In this study, the seismic response of tall concrete structures with a special dual frame-wall concrete system is investigated using the endurance time method, and the results are compared with nonlinear time history analysis results. For this purpose, first, appropriate analytical models including buildings with concrete framed-wall system and 20, 30, and 40 stories are modeled non-linearly in PERFORM 3D software, and then, main nonlinear time history analyses are carried out for seven ground motions (accelerogram) further from the fault based on the FEMA P695 code and the endurance time accelerogram of (in) series. The results of the analysis are compared using indices (shear, relative displacement, and acceleration). The results indicate that the endurance time method is accurate in two indices of shear and acceleration, but the accuracy of the relative displacement index of the floor decreases as the number of stories of the structure increases.

Elastic Investigation of Piers Numbers Effects in Transverse Direction on the Stiffness of Continuous and Simply Supported Bridges

Piers were important structural members between superstructure and foundation of bridge which were designed to transfer and resist the vertical loads and horizontal loads (dead load of structure, traffic loads, water pressure, ice, and wind) to the foundation. The objective of this study was to evaluate the effect of number of piers in transverse direction (horizontal distance or spacing) between piers on the static and dynamic responses for two types of bridges structure which they include continuous and simply supported bridges. Static and dynamic analysis methods were adopted to analyse the bridges structures by using CSI-bridge software. The results of static analysis for two types of bridges models (continuous and simply supported) showed that the increasing of piers numbers had significant effects on the increasing of bridge structure stiffness by decreasing the downward vertical deflection due to service loads, increasing of the resistance of prestressed loads to service loads, and increasing of compression stresses. The results of time history analysis for two types of bridges models showed that the values of natural frequency were increased when the piers numbers were increased. Traffic load frequency (vibration frequency) values were decreased with increasing of bridge piers number because of the bridge structure had more stability. It can be concluded that the values of natural frequency were more than values of traffic load frequency, indicating that the bridges structures had enough stiffness and stability.

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.

Risk-targeted design of seismically isolated buildings

Seismic isolation has been used to improve the seismic performance of buildings for the last decades. However, it has been shown that the code-based displacement capacity of isolation members does not provide adequate collapse probabilities for different Risk Categories of ASCE 7–16. Seismic design codes stipulate to determine maximum isolator displacement at the Risk-Targeted Maximum Considered Earthquake (MCER) level. Although the code-based isolator displacement capacity is checked with the mean displacement of the MCER level nonlinear time history analysis results, extreme event effects and incorporating uncertainties into fragility curves create additional displacement demands. Therefore, there is a need to amplify the code-based displacement capacity of isolators to obtain sufficient collapse performance. A previous study, assumed elastic behavior and rigid mass model for the superstructure, proposes an iterative probabilistic approach. In the present study, seven different seismically isolated buildings are modeled, and the nonlinear behavior of isolators and superstructures is considered in the incremental dynamic analyses achieved for all models to obtain fragility curves. Although the seismic isolation considerably decreases the spectral accelerations of the superstructure, it is shown that the R-μ-T relations for diaphragm level accelerations, transferred accelerations from base to superstructure, become more sensitive to nonlinear behavior at the superstructure mode periods. The fragility curves of five case study buildings are obtained using the FEMA P695 procedure, and it is figured out that the collapse probability of isolation units is highly correlated with the required isolator displacement to code-based maximum isolator displacement (D/DM) ratio. Then, two simple equations with high correlation coefficients are acquired to estimate the required displacement capacity for prescribed risk-target levels. Further, the obtained equations are verified for two different isolated building models, and their results are compared with the Incremental Dynamic Analysis (IDA) results. The verification results show that the given equations can be used in the preliminary design phase of seismically isolated mid-rise reinforced concrete buildings.

Role of model identification and analysis method in the seismic assessment of historical masonry towers

The seismic assessment of historical monuments typically involves many issues, the most crucial of which are: retrieving geometrical and mechanical data; implementing a suitable numerical model; choosing the more appropriate type of analysis and properly interpreting the results. The more well-managed these issues are the better the reliability of the seismic assessment. The Civic Tower of L'Aquila (Italy) is considered in this paper as a typical case in which lack of data and discrepancy of sources make the seismic assessment hard to be suitably performed. The key role of the identification of the numerical model was firstly evidenced. Different numerical models were implemented to identify the best-fitting geometrical and mechanical properties of the structure. The considered case-study also led to the discussion of some issues concerning the identification of the numerical model of historical towers. Code-compliant linear and non-linear dynamic analyses were then carried out to compare the seismic performance of the tower assessed through the different methods based on regulations. A concrete damage plasticity model for masonry and a set of spectrum-consistent earthquakes were adopted. The stress maps, the horizontal displacement peaks (useful to avoid the pounding phenomenon with the adjacent Margherita palace), and the evolution of damage were investigated. The study highlighted that the widely adopted response-spectrum modal analysis (RSMA) may underestimate -even remarkably-the displacement demand compared to the more rigorous time-history linear analysis (THLA), this being an inconsistency of the code-compliant linear analyses. On the other hand, the results of the non-linear time-history analysis (NLTHA) confirmed the displacement demand predicted by the THLA, while the damage evolution under tensile stress was found in agreement with the crack pattern detected on the structure. The influence of the vertical component of the earthquake was also investigated, founding that the vertical component of the ground motion does not significantly affect the results.

Loading protocols for seismic performance evaluation of buckling-restrained braces in RC frames

The selection of a proper loading protocol for seismic performance evaluation of structural dampers poses quite a challenge at the very first step of quasi-static cyclic tests. Existing loading protocols for buckling-restrained braces (BRBs) are mainly developed on the basis of the seismic responses of BRB-steel frame systems. However, the behavior of a BRB in reinforced concrete frames (RCFs) differs markedly from that in the steel frames. In this study, a set of loading protocols were constructed to provide a reference for seismic performance assessment of BRBs in RCFs. The methodology for the development of loading protocols was illustrated and a series of BRB-RCFs varying in building heights and story shear resisted by BRBs were designed. An extensive analytical investigation of seismic responses of BRBs under far-field, near-fault, and long-duration earthquakes was systematically evaluated and quantified. Based on the rainflow counting method, the control parameters in terms of numbers of cycles, amplitudes, and residual core strains were determined to construct loading sequences. The effectiveness of the proposed loading protocols was discussed and verified through comparisons with existing loading sequences and time-history analysis results of a validated BRB-RCF model. It is indicated that the proposed loading protocols can conservatively reflect the actual seismic damage demands with a reasonable agreement.

Progressive collapse resistance of reinforced concrete frame structures subjected to column removal scenario

The progressive collapse of a building occurs due to an unusual loading condition or when there is a sudden loss of the primary structural element. This could lead to a partial collapse or entire collapse of the building due to the failure of one or more structural elements. This study investigates the progressive collapse demand of the RC building by linear static analysis and vertical displacement responses using non-linear dynamic analysis. ETABS software is used for corner column removal, edge column removal, penultimate column removal and internal column removal scenarios. The RC buildings considered in this investigation are G + 6, G + 8 and G + 10 storey. The results showed that progressive collapse demand value of the bottom end of adjacent columns were found to be safe for all damage cases. However, the top end of the adjacent columns was found to be unsafe. The most critical damage case was found to be the penultimate column removal case that possessed highest PCD values. Finally, the vertical displacement of time history results is represented graphically and later the results of time history analysis is compared with linear static analysis results.

Seismic design and evaluation of controlled outriggered rocking wall (CORW) using equivalent energy design procedure (EEDP)

Next-generation earthquake engineering design seeks to develop robust structural systems to limit structural and non-structural damage after strong earthquake shaking. In this study, an innovative high-performance tall building, named Controlled outriggered rocking wall (CORW) system is proposed. The CORW system improves the conventional reinforced concrete (RC) core wall by adding a damped outrigger system and a damped controlled rocking base. The CORW is designed using the novel Equivalent energy design procedure (EEDP) to meet different performance objectives for different shaking intensities. Detailed EEDP for the CORW system is presented in this paper. Three prototype buildings (with 25, 35, and 45 stories) located in Vancouver Canada were designed using the proposed procedure. Robust finite element models of the CORW systems were developed in OpenSees and subjected to a series of nonlinear time history analysis. The results of the nonlinear time history analysis indicate that the CORW system designed using EEDP performs well under different levels of earthquake intensities, where it can be used reliably as an effective seismic force resisting system for high-rise application.

Performance-based seismic design methodology for tall buildings with outrigger and ladder systems

The plethora of efforts needed for conducting the Performance-Based Seismic Design (PBSD) to investigate the tall buildings' performance requires sustainability in developing this approach and facilitating its implementation to obtain an effective final design. To tackle this issue, novel performance-based design procedures are proposed in this study. The novel PBSD procedures commence with integrating the optimization and simplification techniques into the preliminary design stage to rapidly obtain an optimal initial amount of materials required to establish outrigger and ladder systems models. Then, conducting a nonlinear time history analysis with the aid of ETABS and Peforme-3D software to scrutinize the extent to which the initial design approaches the final design and the superiority of each system under frequent or rare earthquake events. The global and component responses are examined for each system under three performance levels: immediate occupancy IO, life safety LS, and collapse prevention CP. The optimization and simplification techniques in the preliminary design stage have reduced the overwhelming efforts required to generate and prepare an optimized preliminary design for both systems. Furthermore, the results of the nonlinear time history analysis found that the ladder system reduced the structure's global response by 27.2% for the lateral displacement and 30.4% for the base shear than the outrigger system. However, the evaluation of the components acceptance criteria confirmed that the ladder system increased the plastic damage distribution in the external structural system's components and reduced it in the internal one. Generally, the preliminary design approached the final design by varied ratios, 60–80% in the ladder system and 60–95% in the outrigger system for different components. Therefore, it is recommended to apply this approach to the outrigger system and improve the ladder system's efficiency. Finally, this study is limited to an optimized preliminary design that approaches the final design and reduces the iteration number of the PBSD procedures. Thus further researches are required to improve and extend the applicably of these procedures.

Seismic vulnerability assessment of free-standing massive masonry columns by the 3D Discrete Element Method

Free-standing masonry column is a recurrent typology of built cultural heritage. Usually raised without seismic design, columns are subject to intense rocking and overturning under strong ground motions. In this paper, a strategy to assess their seismic vulnerability using the 3-D Discrete Element Method is proposed. It includes calibrating the elastic parameters (joint stiffness parameters) using ambient vibration test data and developing fragility curves from a large set of time history analysis results. This procedure was verified for four selected water towers, a representative archaeological monument in Pompeii, Italy. Based on preliminary structural analyses, the water towers were modeled as rigid monolithic blocks. The outcomes of modal analysis were used to calibrate the input elastic parameters, starting from a range of values given by analytical calculations derived by literature. Artificial earthquakes with peak ground acceleration levels ranging from 0.167 g to 0.803 g were implemented as ground motions. Collapses and non-collapses configurations were subdivided through a threshold concerning the maximum allowable rotation of the blocks. Fragility curves were statistically developed; these revealed the overturning probabilities of the investigated structures and also enabled to conduct a parametric study of the Rayleigh damping constants.

Assessment of Viscous Damper Placement as Passive Energy Dissipation on High-rise Building, a Numerical Study

The methods to reduce the impact of earthquake loads is using a damper, namely passive control, active control, and semi-active. One of the advantages of passive control is because of its simplicity in design, installation, and especially in its maintenance. Viscous Fluid Damper (VfD) is one type of dampers which has been widely applied. A numerical analysis of the seismic response of VfD on the ten-story reinforced concrete (RC) frame that receives lateral loads as earthquake loads already compared to conventional RC frame. Displacement and acceleration are an earthquake response parameter which is a direct impact on the structure. The results of numerical time history analysis with the excitation of the Kobe-X earthquake record show that the presence of structural passive control has succeeded in reducing displacement on the top floor and also on the floors below. On the roof floor, for the X longitudinal direction, there was a reduction up to 7.721 percent. Whereas on the 6th, 4th and 2nd floors the smaller reduction occurred by around 6.624% to 2.932%. Likewise, with the acceleration parameters, generally able to reduce acceleration. For frames with placement of VfD in 1st – 2nd floor can reduce acceleration to 10.293 percent whose data is taken at the top of the building. For further, need an experimental laboratory program to validate the numerical analysis to ensure the prediction behaviour of the structural design.

Comparative study on the effect of outrigger on seismic response of tall buildings with braced and Wall Core. II: Determining seismic design parameters

SummaryIn this paper, the effect of outrigger on key seismic design parameters of tall buildings with braced or shear wall core is studied. Based on the results of modal response spectrum (MRS) and nonlinear time history (NLTH) analyses of 44 models with braced or wall core, discussed in the companion paper and pushover analysis and presented in this part, the effect of outrigger and its placement level on parameters of seismic design including response modification factor R, overstrength factor Ω0, deflection amplification factor Cd, and damping ratio of structures are studied. New local measures for Cd and Ω0 are proposed based on the NLTH results. It is observed that outrigger results in greater increase in strength and stiffness of structures with wall core than buildings with braced core, but decreases the ductility of both systems. The obtained R is less than the values given by the code in most cases. For the main structures without outrigger, using the code prescribed value for Cd underestimates the real displacement of the structures with wall core and overestimates it for those with braced core. Outrigger results in increasing Cd of structures with braced core and reduction of Cd in those with wall core. The considerable variation of two proposed measures for Cd shows the inappropriateness of application of a single value of Cd for all stories. Outrigger increased the damping ratio of buildings with shear wall core considerably.

Seismic Performance Evaluation of Steel Buildings with Oil Dampers Using Capacity Spectrum Method

This study proposes a capacity spectrum Method (CSM)-based procedure to estimate the maximum seismic performance of steel buildings passively controlled with bilinear oil dampers. In the proposed CSM, the maximum seismic response of a building was estimated, in the acceleration-displacement response spectrum, as the intersection between the capacity curve and the damping-adjusted demand curves, using the equivalent linearization method. The building equivalent damping ratio was determined by the sum of the inherent damping, and the square root of sum of squares (SRSS) of the hysteretic damping and the viscous damping of the supplemental oil devices. The calculation steps of the proposed CSM are explained in detail based on the equivalent single degree of freedom (ESDOF) system, and its accuracy was examined by comparison with time history analysis (THA) results. Two model steel buildings of 4 and 10 stories, uniformly equipped with oil dampers along the height, were subjected to six selected earthquake ground motions scaled to be compatible with Level-2 earthquakes, as defined in the Japanese Building Standard Law. The seismic performance of the buildings was estimated by the proposed CSM procedure and compared with the results of nonlinear THA in terms of the maximum story displacements and the shear forces. It was observed that the proposed CSM scheme provided a satisfactory accuracy to assess the maximum nonlinear response of steel buildings passively controlled with oil dampers.