Static Pushover Research Articles

Study on the Development of Seismic Fragility of Steel SMRF

Abstract: Vulnerability assessment of the structure is the most important and wide area of research which requires more input from the engineers and seismologist. The seismic vulnerability assessment of the structure can be evaluated by developing Fragility curves. Fragility curves shows the conditional probability of the structure exceeding the particular performance limit of the given damage state during strong ground motions. Fragility curves can be developed for different parameters like spectral displacement (Sd), spectral acceleration (Sa) Peak ground acceleration (PGA) , Inter storey drift ratio (IDR) etc. This paper describes about the different methods used in deriving the Fragility curves like conventional methods, Nonlinear Dynamic analysis methods and Nonlinear Static analysis methods. Also the fragility analysis of 5 Storied Steel Moment Resisting Frame (SMRF) has been carried out based on the parameters suggested by HAZUS M.H 2.1. Nonlinear static pushover analysis of the frame has been carried out in ETABS2016. Fragility curves are developed based on the pushover analysis results. The damage states defined as per HAZUS are Slight damage (SD), Moderate damage (MD) Extensive damage (ED) and Complete damage (CD). After carrying out the fragility analysis for the steel SMRF, it has been found out that, as the spectral displacement increases probability of failure for the slight damage of the structure is very high and the probability of failure for the complete damage is very low. Hence the probability of failure of the structure reduces from slight damage to complete damage. Keywords: Fragility curves, vulnerability assessment, Nonlinear static pushover analysis, HAZUS M.H 2.1.

Seismic endurance of box-girder bridge with different piers

According to recent data, many bridges tend to fail due to seismic inadequacy in design. Circular piers are more effective in reducing scour depth and increasing the durability of bridges than an elliptical bridge pier. The question is “whether circular pier performs better under seismic action than elliptical pier or not”. So a comparative study was conducted to analyze the behavior of circular and elliptical piers under earthquake forces, and complete nonlinear static pushover analysis was carried out for two different bridge models separately. Pushover curve data from Indian Standard Code was used to formulate the problem and performance outcomes of the bridges were analyzed. It was found that single circular pier showed no failure and better performance in both directions of the bridge, with more displacement capacity than double elliptical piers when subjected to incremental lateral loads. The design capacity (D/C) ratio in the longitudinal direction was 1.8 times more for single circular pier bridge and in the transverse direction, the ratio was 1.4 times more for double elliptical pier bridge for the considered earthquake situation. Results also showed 47% less drift value for single circular pier bridge compared to double elliptical pier bridge.

Simplified bar-system model for tubular structures by considering local joint flexibility

Local joint flexibility (LJF) may produce significant effect on performance of tubular structures. According to the local deformation characteristics at tubular joints, a simplified bar-system model is presented by using tie-strut members to simulate the local flexibility at joints. Based on proposed equations for calculating the LJFs of different types of tubular joints in literature, analytical equations for determining the dimensions and material properties of the presented tie-strut members are deduced. Additionally, nonlinear static pushover test was conducted for a small-scale tubular structure specimen and linear vibration test was carried out for another specimen. Load-displacement curves at some specified positions in the static test and displacement-time curves at some specified positions in the vibration test are obtained. Three different finite element models are then presented, which include using 3-D shell elements to simulate the entire tubular structures (3-D model), using beam elements to simulate the entire structures with rigid connection assumed at tubular joints (rigid frame model), and using beam elements to simulate the tubular members and using the presented tie-strut model to simulate the tubular joints (presented bar-system model). Finite element results obtained from the three different methods are compared with the experimental results, and the comparison indicates that both 3-D model and the presented bar-system model can produce accurate predictions compared to the experimental results while the rigid frame model overestimates the stiffness and load-carrying capacity and underestimates the vibrating amplitude of the specimens. As the presented bar-system model has much higher efficiency compared to 3-D model, it is more challenging for analyzing both linear and nonlinear behavior of tubular structures in practical design.

EVALUASI KINERJA STRUKTUR BANGUNAN DENGAN METODE PUSHOVER ANALYSIS PADA GEDUNG RUMAH SAKIT STROKE NASIONAL BUKITTINGGI

Earthquake phenomena are natural phenomena that greatly affect buildings, especially high-rise buildings. Planning of earthquake-resistant building structures is very important in Indonesia, considering that most of the area is located in earthquake areas with moderate to high intensity. Buildings in earthquake-prone areas must be designed to be able to withstand earthquakes. The latest planning trend is performance based seismic design, which utilizes computer-based non- linear analysis techniques to analyze the inelastic behavior of structures from various ground motion intensities (earthquakes), so that their performance can be known in critical conditions. Further action can be taken if it does not meet the required requirements. The purpose of this study was to determine the pushover analysis procedure to evaluate the performance of the building structure and to determine the pattern of the collapse of the building structure after being analyzed by pushover. This study uses the Static Pushover Analysis method with reference to the displacement coefficient method (FEMA 356, 2000). The lateral load used is the result of an equivalent static analysis which is carried out monotonically in one direction. Research shows that there are several conclusions. First, the displacement of the maximum pushover result (δmax) in the XZ direction, which is in step-7, is greater than the displacement target (δt), with a figure of 61541.4681 mm > 58.212 mm. Second, the displacement of the maximum pushover result (δmax) in the YZ direction, namely in step-5, is greater than the displacement target (δt), with a figure of 10450.54 mm > 58.212 mm. Third, the evaluation in the XZ direction is still safe even though max > t, because all beam elements in the portal appear plastic hinges with levels A-B which are still elastic, all marked in pink. The distribution of plastic hinges that occur at all steps shows that no structural member exceeds the performance limit so that it can be said that the performance of the structural component is in a safe condition. Fourth, the structural members in the YZ direction are also safe because max > t and the plastic hinge distribution scheme shows the structural members marked in pink with levels A-B. Keywords: pushover, non-linear, plastic hinge

Hysteresis Envelope Model of Double Extended End-Plate Bolted Beam-to-Column Joint

In this study, a hysteresis envelope mathematical model for the double extended end-plate bolted beam-to-column joint is proposed. The aim of a proposed joint model is to provide a more realistic behaviour of steel frames under seismic loading by using nonlinear static pushover analysis. The hysteresis envelope model defines the ratios between the monotonic properties of the joints and the properties of the joints during cyclic deformation. The proposed models are based on the hysteresis curves of the joints obtained by numerical simulations. The numerical model takes into account the geometric nonlinearity of the connecting elements, preloading of bolts, contacts between plates and bolts, and nonlinear properties of steel. Nonlinear static pushover analyses of steel frames are performed where the behaviour of the joints is described using the proposed hysteresis envelope models. The results are compared with the nonlinear static pushover analyses of steel frames with a trilinear monotonic joint model. Based on the results, the values of maximum peak ground acceleration for moment-resisting frames with the monotonic model of joints and hysteresis envelope model are estimated.

Novel post‐tensioned rocking piles for enhancing the seismic resilience of bridges

AbstractThe rocking pile foundation system is a relatively new design concept that can be implemented in bridges to improve their seismic performance. This type of foundation prevents plastic damage at the bridge piers and the foundation system, which are difficult to repair and can lead to collapse. However, lack of adequate energy dissipation in this type of foundation can result in large deck displacements and subsequent catastrophic failures of the bridge. The present study proposes a novel foundation system that integrates post‐tensioned piles with the rocking foundation to simultaneously prevent plastic hinging at the piers and reduce the deck displacements during severe earthquakes. The effectiveness of the proposed foundation system is investigated and compared against the rocking pile and conventional fixed‐base foundation systems using identical bridge configurations. Three‐dimensional finite element models of these bridges were developed to capture possible nonlinear behavior of the bridge as well as soil‐structure interaction effects. Six strong earthquakes with both horizontal components were selected and scaled to the appropriate seismic hazard level with a return period of 2475 years. Static pushover and nonlinear time‐history analyses were then performed to compare the dynamic response of the bridges, including deck displacements, pier and pile inertial forces, and other nonlinear behavior experienced by the structure. The results reveal that by integrating the post‐tensioned piles with the rocking foundation, the deck displacements were reduced to an acceptable limit without subjecting the bridge to any damage. In contrast, the bridge with the fixed base foundation experienced extensive damage at the piers, and the bridge with the rocking foundation experienced substantial deck displacements that ultimately led to unseating, resulting in the collapse of both bridges. It was therefore concluded that the proposed rocking foundation system with post‐tensioned piles is the superior alternative and can be implemented in practice as an attractive solution due to the seismic protection it offers.

Wind-resistant performance and failure modes for a semi-submersible offshore platform during jacking closure

Currently, the construction process of large offshore platforms worldwide involves building the topside and hull separately and then having them integrated. The jacking closure scheme by overlapped support tower bearing the topside is a new integration method, which has a great advantage over other traditional methods in vertical bearing capacity. But its weak bearing capacity to lateral forces like wind load limits the jacking height. Moreover, few engineering examples and academic researches also cause insufficient understanding of it. Therefore, this paper, based on the first deep-water semi-submersible platform with the10,000-ton oil storage worldwide, i.e. “Deep Sea No. 1” energy station, aims to evaluate the wind-resistant performance during the jacking closure process. Firstly, the background and the jacking closure scheme of this project are introduced in detail. Secondly, the finite element models (FEMs) of the jacking system are established according to the overlapped characteristics of jacking towers and the corresponding failure criteria. Then, the wind-induced vibration response is simulated, and the static pushover is conducted to evaluate the ultimate bearing capacity and its influencing factors. The failure criteria are checked using ANSYS parameter design language. It is found that “the occurrence of tensile stress on the contact surface” is the first and main failure mode of the structure. Additionally, the results also reveal that the strand cables can not only improve the ultimate bearing capacity of the structure but also change its weak direction. However, the improvement is gradually weakened with the increase of jacking height. In contrast, the contribution of the bracing pipes to the structural bearing capacity is always significant. These findings can be used to develop a more reasonable closure plan.

Performance evaluation of a RC frame structure from element level to structure level

Ductility is an important design parameter which controls the flow of plasticity in a structure to improve the ability to sustain severe load with nonlinear deformation. In the present study, the flow of ductility from element level to structure level has been assessed. For the study purpose, a RC frame structure has been considered and the displacement ductility capacity of the frame has been evaluated from the best possible combinations of curvature and rotation ductility capacity of beams and columns. To generate the capacity curve of the structure, nonlinear static pushover analysis has been performed. It has been observed that the variation of section ductility of the column is more significant with diameter and spacing of the transverse reinforcement bar than the same for beam section. A reduction of structural ductility has been observed as compared to the element ductility which needs more detailing requirement for the fulfillment.

Seismic performance assessment of steel moment-resisting frames with self-centering viscous-hysteretic devices

This study evaluates the seismic performance of the steel frame with self-centering viscous-hysteretic devices (SC-VHD frame) and investigates the effect of design parameters of the SC-VHD, preload, loading stiffness, unloading stiffness and viscous damping ratio, on seismic performance through conducting nonlinear static pushover analysis, fragility analysis and risk assessment on 4-, 8-, and 12- story SC-VHD frames with different design parameters. Results indicate that the maximum normalized base shear and the ultimate roof drift ratios of the SC-VHD frames increase by more than 13.3% and 42.9%, respectively, compared with the conventional frames. The SC-VHD frames have acceptable low probabilities of collapse and probabilities exceeding considered residual drift under the maximum considered earthquake (MCE) and during a 50-year period. Accordingly, the most sensitive parameter regarding the probabilities of collapse is loading stiffness, followed sequentially by unloading stiffness, viscous damping ratio and preload. While for the probability exceeding considered residual drift under the MCE, the parameters of ring springs, preload, loading stiffness and unloading stiffness, have a significant effect on the 4- and 8-story SC-VHD frames, and the viscous damping ratio of the viscous damper has the most significant effect on the 12-story SC-VHD frame. For the probability exceeding considered residual drift during a 50-year period, the most sensitive parameter is the viscous damping ratio.

Experimental study and numerical simulation of seismic behaviour of corroded reinforced concrete frames

One of the major durability issues of Reinforced Concrete (RC) structures is the steel reinforcement corrosion. Its effect needs to be evaluated on the lateral load capacity of the structures for understanding their seismic performance. In the present paper, the precise FE modelling and simulation of corroded RC frames subjected to lateral loads and its validation with full scale shake table tests on corroded RC frames is demonstrated. The rebar corrosion is induced in RC frames by accelerated impressed current technique and shake table tests are performed by simulating real time earthquakes with increasing excitation till failure of the frames is achieved. The tests showed that there was higher degradation of the frequency of the corroded structure with respect to pristine structure with increase in earthquake excitation levels. Numerical simulation of the tests is carried out using 2D micro-modelling, incorporating the effect of bond strength reduction, reduction in rebar diameter and decrease in mechanical properties of corroded steel in the model. Nonlinear static pushover analysis is performed to evaluate load displacement characteristics of the corroded and uncorroded frames and the results are compared with dynamic pushover obtained from shake table tests. It was observed that compared to un-corroded frame there is 13% reduction in the failure load due to earthquake excitation for the 7.5% corroded frame and numerical results for 10% corroded frame showed 22% reduction in the lateral load carrying capacity.

Evaluation of R-Factor for 5 Storey RC Frame (IMRF) Building

According to IS 1893 part 1 (2016), the philosophy of earthquake resistant structures allows for some damages and inelastic lateral displacement in the structure for energy dissipation during an earthquake. The non-linear behaviour of elements in the structure plays a crucial role in earthquake resistance. There are three detailed classes for distinct seismic zones in different national codes. In India, the draught IS 13920 advocated the usage of IMRF (intermediate moment resisting frame) in zones II and III. The 5 story IMRF is designed and detailed as per IS 1893 (part 1) 2016, IS 13920 (2016), IS 1893 draft, IS 13920 draft, IS 456 (2000). In addition, nonlinear static pushover analysis was performed on IMRF and SMRF RC frame buildings in accordance with FEMA 356. (Displacement Coefficient method) During the analysis, two distinct load patterns (i) parabolic as per IS 1893 (part 1) 2016 (ii) fundamental mode shape are utilised, and the influence of p-delta is also taken into account when evaluating the response reduction factor. The analysed R-factor for studied frame building for fundamental mode shape loading was found to be near to the initial estimated R-factor during the design.

Comparative Assessment of the Seismic Behavior of Reduced‐Core Length and Conventional Buckling‐Restrained Bracing Systems

AbstractThe buckling‐restrained braced frame (BRBF) is an appropriate lateral system for resisting seismic excitations. This system has high lateral stiffness and stable hysteretic loops mainly due to restraining global buckling of the brace core under compression. Even though BRB is an improved version of the conventional brace, it is not yet an ideal system since there are deficiencies regarding the performance of this system. Notable decrease of lateral stiffness after yielding of the BRB cores and significant residual drifts are instances of the deficiencies in BRBFs, which have been recognized as problematic and costly in recent earthquakes.Reducing the core length of the BRB can effectively increase the lateral stiffness as well as decreasing the drifts and residual drifts of the structure without any increase in the cross‐sectional area of the core. In this research nonlinear static pushover analyses have been conducted on 4 buildings including 4 and 9 story structures with conventional and reduced core length BRBs with diagonal and inverted chevron brace configuration. Both pinned and rigid end‐connections were considered for the beams in the braced bays. A 4‐story dual SMRF‐BRBF was included for better assessment of the contribution of frame action to the structural behavior. The results indicate that the reduced length BRBs have much better seismic behavior. Furthermore, they are replaceable, repairable and more cost‐efficient compared to conventional BRBs.

Perilaku Seismik Struktur Rangka Komposit RCS dengan Variasi Rigidity Zone Factor Pertemuan Balok-Kolom

This research evaluated the effect of connection’s rigidity zone factors of RCS frame’s connections on the seismic behavior of regular RCS frames of a five-story office building located at seismic design category (SDC) of D. The variations on rigidity zone factors were 0, 0.25, 0.50, 0.75, and 1.0, respectively for Model MS, MSR025, MSR05 MSR075, and MR with the same elements’ dimension. A 3-D finite element modeling was conducted to do a linear elastic analysis for structural design and nonlinear static pushover analysis for evaluating the structural seismic performance. The results show that all models have met the strength and serviceability design limits. The seismic performances in terms of base shear, elastic stiffness, and ductility of all RCS models increased with an increase in the values of the rigidity zone factor. The structural performance level according to FEMA 440 was life safety (LS) for the Model MSR05, MSR075, and MR, while for the Model MS and MSR025 was collapse prevention (CP). The seismic energy dissipation for all RCS frames was an intermediate category indicated by the numbers of developed plastic hinges less than 20% of the total potential plastic hinges

Use of timber panels to reduce the seismic vulnerability of concrete frame structures

This paper discusses a seismic retrofit method that is intended for reinforced concrete (RC) frame structures and that sees the replacement of the existing masonry infills with timber structural panels. The intervention is aimed at enhancing the overall behaviour of the buildings without modifying the original concrete structural system. In the presented research over 200 finite-element models comprising bare, masonry-infilled and retrofitted single-storey single-bay frames were subjected to nonlinear static analyses. The response of the timber-infilled frames was then optimised, thus enabling the definition of a few simple design rules to guide the implementation of the retrofit system. Subsequently, the efficacy of the proposed solution was further investigated by applying the timber-based retrofit to an entire case-study building modelled via finite elements and subjected to nonlinear static pushover analysis. The comparison of the results obtained for the building in the original and retrofitted configurations indicates that the proposed system can favour the development of ductile behaviours and improve the seismic response of RC frame structures significantly.

RC walls in Australia: displacement-based seismic design in accordance with AS 1170.4 and AS 3600

ABSTRACT Displacement-based methods, such as a non-linear static pushover analysis (e.g. the capacity spectrum method), have many advantages compared to traditional force-based design methods. However, implementing a non-linear analysis and design method in accordance with the Australian Standard for concrete structures (AS 3600) introduces many difficult technical issues into the design, of which the standard provides little guidance. The aim of this study is to provide a framework and general guidance for designers who wish to perform non-linear displacement-based analysis methods for RC wall buildings. The paper will present how these methods can be used in accordance with the Australian Standard for earthquake actions (AS 1170.4) to assess seismic compliance and then provide recommendations for the requirements stipulated by AS 3600, which includes an experimentally validated tension stiffening model, nonlinear stress-strain material curves, mean material properties and material strain limits. The paper is concluded with a case study example of how a displacement-based seismic assessment can be performed using a typical case study building.

COMPARATIVE SEISMIC PERFORMANCE ASSESSMENT OF UHPC JACKETING AND SCED BRACING TECHNIQUES

Observed damage to existing pre-seismic code buildings in previous earthquakes has raised interest among the engineering community for improving the performance of these structures using different seismic retrofit measures. This paper throws light on contemporary techniques for the seismic retrofit of RC buildings, namely ultra-high-performance concrete (UHPC) jacketing and self-centering energy dissipative (SCED) braces. Detailed fiber-based numerical modeling of a benchmark structure is undertaken to evaluate the effectiveness of the selected retrofit measures. The case study structure is a two-story pre-seismic code residential building designed for gravity and wind loads, exhibiting poor seismic performance. Along with the two retrofit strategies investigated, several parameters are also considered. Inelastic static pushover and incremental dynamic analyses are conducted to select the retrofit measures and assess their effects on seismic performance. Using a collection of far-field earthquake records and a set of performance criteria, fragility functions are constructed to assess the vulnerability of the benchmark structure with and without the retrofit solutions. The study shows that the adopted index that links cost to the seismic performance obtained from the fragility functions can provide a rational ranking of the selected retrofit approaches relative to the existing building and support selecting the most effective and economical alternative.

Estimation of inter-story drifts at onset of damage statesfor RC high-rise buildings

In this paper, a detailed probabilistic seismic damage analysis of RC high-rise buildings was performed and as a result, the damage states (DSs) and appropriate performance levels (PLs) were defined in a quantitative manner. DSs were quantified using inter-story drift, where the drifts were determined at the onset of each DS. The analysis was performed on three RC high-rise buildings: 20-story, 30-story and 40-story with core wall structural system. Probabilistic seismic damage analysis was performed for 60 earthquake records, recorded on rock and stiff soil, and scaled to two intensity levels associated with probability of exceedance, 10 % in 50 years - 475-year return period (10%/50) and probability of exceedance, 2 % in 50 years - 2475-year return period (2%/50). In addition to these analyses for estimation the damage index, nonlinear static pushover analyses (NSPAs) were performed using different modal combination patterns. Large deviations among the pushover curves for individual considered modal combination patterns were observed. In order to adequately select the parameters for calculating damage index, an analysis of drifts and shear forces for individual modal combination patterns was performed. The functional dependencies between inter-story drifts and damage index were derived using the regression analysis. Based on the derived dependencies, the values of inter-story drifts at the onset of considered DSs for high-rise buildings were proposed.

Stability Analysis of Prefabricated Steel Structure Building Based on Pushover Method

Prefabricated steel structure building is a kind of structural system which is widely used in recent years. It is very necessary to study the stability of it by Pushover method. Through the investigation of a building under construction in Hebei Province, China, the finite element analysis software SAP2000 is used to establish numerical simulation models of four kinds of staggered truss steel structure, so as to calculate its static pushover performance. It includes base shear-peak displacement curves, performance points and plastic hinge development. The main conclusions are as follows: the terminal shear and displacement of the hybrid staggered truss structure are obviously better than that of the Vierendeel staggered truss structure in the process of pushover. The base shear of the structure can be greatly increased and the overall deformation of the frame can be reduced when the side spans is set up. Until the end of the pushover, there is no plastic hinge on the column, which meets the seismic design requirements of “strong column and weak beam”, indicating that the prefabricated steel structure staggered truss system has good seismic performance.

Seismic Fragility Curves for Performance of Semi-rigid Connections of Steel Frames

A steel frame with a semi-rigid connection is one of the most widely used structural systems in modern construction. These systems are cheap to make, require less time to construct and offer the highest quality and reliable construction quality without the need for highly skilled workers. However, these systems show greater natural periods compared to their perfectly rigid frame counterparts. This causes the building to attract low loads during earthquakes. In this research study, the seismic performance of steel frames with semi-rigid joints is evaluated. Three connections with capacities of 50, 70 and 100% of the beam’s plastic moment are studied and examined. The seismic performance of these frames is determined by a non-linear static pushover analysis and an incremental dynamic analysis leading finally to the fragility curves which are developed. The results show that a decrease in the connection capacity increases the probability of reaching or exceeding a particular damage limit state in the frames is found. Doi: 10.28991/cej-2021-03091714 Full Text: PDF

RESEARCH INTO THE DEVELOPMENT OF A RESPONSE MODIFICATION FACTOR FOR DIAGRID STRUCTURES

The diagrid structure system has recently gained popularity as an effective structural system for high rise buildings. The effectiveness of the system relies on the truss action developed by the perimeter diagrid system to resist lateral loads. In active seismic zones, ductile performance of the lateral resisting system is highly desired to dissipate energy developed in the structure from the ground shaking. In this study, the seismic performance of the diagrid system is investigated. Non-linear static pushover analysis followed by non-linear dynamic analysis were conducted to study the inelastic behaviour of diagrid systems. Through methods of analysis, the response modification factor of the modelled system was calculated. The results indicate that the reserve strength of a diagrid system is comparable to and often greater than that of many other lateral structure systems, such as steel special truss moment resisting frames and other structural systems as prescribed by the ASCE 7-10.