Pushover Analysis Method Research Articles

Comparative Seismic Risk Assessment of Basilica-type Churches

ABSTRACTThis paper discusses the seismic risk assessment of a Basilica-type church according to the provisions of the Italian Guidelines. A comparison between the results obtained with local and global approaches is reported, based on a knowledge process aimed to characterize the geometric and mechanical parameters required for a reliable structural analysis. To perform the global analyses the finite element technique was employed, with proper assumptions to account for the nonlinear behavior of masonry. Illustrating a case study, the paper critically discusses about the employability of pushover analysis methods for the seismic assessment of basilica-type churches.

Evaluation of ductility and response modification factor in moment-resisting steel frames with CFT columns

One of the methods to strengthen the structures against the seismic lateral loading is the employment of the composite columns. A concrete-filled tube (CFT) has the cumulative advantages of steel and concrete. Concrete-filled steel tube columns have been widely used in the moment-resisting frame (MRF) structures, located in both non-seismic zones and high-risk seismic zones. In this paper, the results of studies on two important seismic parameters of ductility and the response modification factor (RMF) of the MRFs with CFT columns are submitted. While the studies are carried out, the effects of span length-story height ratio, the strength of materials and seismic behavior of MRFs are considered. In this regard, the ductility, RMF and the strength of 36 models of the steel MRFs with CFTs are analyzed. The fiber plastic hinges numerical simulation and pushover analysis method are used in the calculations. Based on the obtained results, the RMFs suitable for the 5-, 10- and 15- story frames are proposed.

A generalized normal flow method with online step controls for pushover analysis of nonlinear softening structures

This paper introduces a robust path-following method for pushover analysis of softening structures due to geometric and/or material nonlinearities. The proposed method is a generalized form of the conventional normal flow method (NFM) that provides dimensional consistency in the correction iterations and improves the convergence properties of the algorithm. The generalized NFM is further enhanced by two online step control processes, namely, the Convex Step-size Control (CSC) process and the Hyper-Plane Step Control (HPSC) process. The CSC process introduces a set of inequality constraints on the step size to form a convex solution space and (i) enhance the convergence properties of the algorithm by preventing overshooting in highly nonlinear areas, and (ii) improve robustness of the path-tracing process by controlling the resolution of the traced equilibrium path (or spacing/distance between successive equilibrium points) making it unlikely to miss (highly nonlinear) sections of the path. The HPSC process introduces a hyper-plane boundary in the solution space to prevent backward path-tracing. The performance of the proposed generalized NFM with the CSC and HPSC processes is validated through examples with structures exhibiting snap-back and snap-through response due to geometric and material nonlinearities. The proposed method is also compared with existing path-following methods typically used for the pushover analysis of nonlinear structures.

Applicability of the N2, extended N2 and modal pushover analysis methods for the seismic evaluation of base-isolated building frames with lead rubber bearings (LRBs)

In the recent investigations, the pushover analysis has been mainly used for the seismic evaluation of fixed-base structures, whereas a limited number of research investigations have focused on the applicability of pushover analysis to base-isolated buildings. Therefore, this study attempts to extend the modal pushover analysis (MPA) and the extended N2 (EN2) method to medium-rise base-isolated building frames to account for the effect of higher modes in predicting the seismic demands of these structures. Since the displacement at the isolation level and subsequently the effective stiffness of the isolation system are not predetermined at first, an iterative process was used to fulfill the MPA method for base-isolated frames. The original N2 method, which was recently extended to base-isolated buildings, was also implemented with three different lateral load distributions, i.e. the inverted triangular, the first mode and the PSC load patterns. For this purpose, two steel moment-resisting building frames including low-rise (3-storey) and medium-rise (12-storey) ones were considered. The structures were isolated with lead rubber bearing (LRB) isolation systems. Three types of isolators with different stiffnesses including the hard (H), normal (N) and soft (S) isolators were selected. It was observed that N2 method with the PSC load distribution, in most cases, gives better estimates of the seismic demands for low-rise base-isolated frames. Also, the MPA and the EN2 methods can result in accurate estimates of the seismic demands for medium-rise base-isolated frames with the hard type isolators, but their accuracy deteriorates with the increase in the damping and decrease in the stiffness of isolators.

Spectrum-based pushover analysis for estimating seismic demand of tall buildings

A quick and accurate estimate of the seismic demand of tall buildings is one of the important issues in the seismic evaluation of buildings. Nonlinear response time history analysis method (NLRHA) is considered as the most accurate and rigorous method, but the computation requirement of NLRHA for tall buildings can be complex and very time-consuming. Whereas the conventional pushover procedure is not capable of reasonably estimating seismic demand of mid and high rise-buildings, due to the oversimplification of conventional pushover approaches and the complexity of seismic performance of buildings. In this paper, a quick, yet effective, method of analysis, named as a spectrum-based pushover analysis (SPA) method, is proposed to estimate the seismic demand of tall buildings. In the SPA procedure, the very complex and complicated dynamic coupling effect of modes of the nonlinear seismic performance of a building is simplified, and the consecutive pushover technique is adopted to consider the simplified coupling effect. Comparison of the results obtained from NLRHA, the modal pushover analysis method, the consecutive modal pushover analysis method and the proposed SPA method is made. It is seen that the SPA method predicted the seismic demand of buildings well, including the inter-storey drift ratio and hinge plastic rotation, which are very close to those of NLRHA. Owing to its accuracy, effectiveness, and spectrum-based calculation procedure, the proposed SPA procedure is considered to be a very promising tool for predicting the seismic demand of tall buildings.

PERFORMANCE-BASED DESIGN METHOD FOR IRREGULAR PLANE RC FRAME-SHEAR-WALL STRUCTURE WITH DISSIPATION DEVICES

Based on the introduction of the critical angle, design of irregular plane RC frame-shear-wall structure was achieved using the improved multimode pushover analysis method. First, the critical angle of seismic wave was determined using the wavelet transform in MATLAB. Second, based on the performance requirement from the designed structure, the multimode pushover analysis method and the capacity spectrum were used to calculate the supplement damping ratio, and the corresponding viscous damper was selected and placed properly. Finally, the seismic performance was evaluated using the torsional angle capacity method. The results indicate that the critical angle of seismic wave show significant influence on the performance of the irregular plane RC frame-shear-wall structure. Meanwhile, the structural design based on the improved multimode pushover analysis method can achieve the expected purpose. Furthermore, the method can lead to significantly reduced structural torsional displacement ratio and highly improved structural seismic performance.

Seismic Margin Assessment for earthquake beyond design basis – Simplified practical approach

Accentuated by recent Fukushima accident, and earthquake events that affected Kashiwazaki Kariwa and North Anna Nuclear power plants, vulnerability assessment of Structures Systems and Components (SSCs) of nuclear facilities for earthquake ground motion exceeding the design basis has become an important exercise to ensure safety in case of highly improbable but possible extreme earthquake event beyond design basis.For seismic performance assessment of structures, three methods viz. response reduction factor based approach, nonlinear response history analysis, and nonlinear static response analysis are available (IAEA, 2011). While the first two methods are either approximate or complex, Nonlinear Static Procedure (NSP) has ability for offering a practical and accurate solution to the problem of Seismic Margin Assessment (SMA) and is simple to adopt at design office.In the present work, step by step approach is presented for Seismic Margin Assessment of structures using nonlinear static analysis procedures. In this approach, at first, pushover analysis is performed on the structure with the appropriate load pattern (e.g. proportional to fundamental mode shape) and effect of higher modes is evaluated using an elastic response analysis. Response of higher mode is then combined to the response from pushover analysis using Modified Modal Pushover Analysis (MMPA) method (Chopra and Goel, 2004). In the second step, demand spectra for different performance objectives is evaluated by augmenting the design spectra. Hence, seismic margin of the structure is evaluated in terms of scaling factor for design spectra for different performance objectives. Material models used in this approach for pushover analysis is validated using structural response data from tests conducted on different types of structures. Application of the proposed approach is also demonstrated for Seismic Margin Assessment of a typical NPP structure.

Applicability of nonlinear static procedures for low-rise unreinforced masonry buildings with flexible diaphragms

The applicability of the N2 method, the modal pushover analysis (MPA) and an adaptive pushover analysis method are investigated for estimating the peak seismic responses of unreinforced masonry buildings with flexible diaphragms. The performance of nonlinear static procedures are compared against the nonlinear time-history analyses of three low-rise building models with various levels of stiffness eccentricity, type of failure mechanisms of piers (rocking or shear), and a range of diaphragm stiffness representing timber floor and roof systems. The results indicate that the MPA is unsuitable for unreinforced masonry buildings with flexible diaphragms, if the building response is shear-dominated. The adaptive method provides the most accurate estimates when the diaphragms are relatively stiff. When the diaphragms are relatively flexible, none of these methods can provide accurate predictions of peak seismic demands. However, conservative results may be obtained with the N2 method, by taking the envelope of pushover analyses carried out using the force distributions proportional to the uniform and linear displacement shapes along the height of the building. The present study has also identified the most suitable parameters/methods for the use of various nonlinear static procedures, such as the location of the control node used in the N2 method and the modal combination rule used in the MPA, for unreinforced masonry buildings with flexible diaphragms.

Collapse safety reserve of jacket offshore platforms subjected to rare intense earthquakes

The majority of jacket platforms are located in areas of high seismic risk. The seismic performance and overall safety reserve of jacket offshore platforms during rare intense earthquakes must be investigated. This study compares the collapse margin ratio (CMR), reserve strength ratio (RSR) and ductility factor (μ), three frequently used deserve measures. The CMR can comprehensively and simultaneously reflect the contributions of strength and deformation reserve to structural collapse resistance. The results of a case study indicate the CMR increases nonlinearly with increases in the RSR and μ. Comparatively, the collapse safety margin of aseismic jacket offshore platforms is associated more closely with their overall lateral deformability reserve. To satisfy the requirement on the acceptable lower limit of the CMR, as suggested in the ATC-63 report in the case of a structural collapse probability of 10% under rare-level earthquakes, the minimum μ should be met. Furthermore, a simplified method is proposed for the CMR. With the simplified procedures, the computational efficiency of the pushover analysis method used in the assessment of the collapse safety margin of aseismic jacket offshore platforms has been substantially improved without a motion-to-motion evaluation of the collapse-level earthquake intensity required in ordinary pushover analysis.

Multifactor Influence Analysis of Seismic Performance of RC Frame Structure with Cast-in-site Slabs

Abstract Pushover and nonlinear dynamic analysis method are applied to analyze the seismic performance of RC frame structure with or without slabs under the action of multiple factors in this thesis. Meanwhile, the structural response exerted by oblique seismic action is summarized from the theoretical formula derivation. By analyzing and comparing two kinds of established models, the results demonstrate that with various axial compression ratio and moment magnifying factors of columns, the seismic performance of the structure will be degraded when the influence of the floor is taken into account. And the increment of the values of axial compression ratio μ and moment magnifying factor of column η c with the floor exceeds that without the floor. Moreover, Earthquakes input along the principal axis can realize the “strong column weak beam” of the structure. Considering the influence of floor, when the value of axial compression ratio is higher than 0.56 or moment magnifying factors with the floor increases above 2.0, frame structure under oblique seismic action can form “strong column weak beam” yield mechanism.

Assessment of nonlinear static and incremental dynamic analyses for RC structures

In this study, seismic behaviour of reinforced concrete buildings using the pushover and incremental dynamic analysis method was investigated. A numerical study was performed for a reinforced concrete frame building. Pushover analysis according to uniform and triangular load shapes and incremental dynamic analyses were performed for selected building. For the nonlinear analysis, three ground motion records were selected to ensure compatibility with the design spectrum defined in the Turkish Seismic Code. The maximum response, dynamic pushover curve, capacity curves, interstorey drifts and moment rotation curves for various element ends of the selected building were obtained. Results were compared each other and good correlation was obtained between the dynamic analyses envelope with static pushover curves for the building.

Review of Different Pushover Analysis Methods Applied to Masonry Buildings and Comparison with Nonlinear Dynamic Analysis

This article presents the comparison among different nonlinear seismic analysis methods applied to masonry buildings, i.e., pushover analyses with invariant lateral force distributions, adaptive pushover analysis and nonlinear dynamic analysis. The study focuses on the influence of lateral force distribution on the results of the pushover analysis. Two simple benchmark case studies are considered for the purpose of the research, i.e., a four-wall masonry building prototype without floor rigid diaphragms and a two-wall system with a cross-vault. The comparative study offers a useful review of pushover analysis methods for masonry structures and shows advantages and possible limitations of each approach.

A Multi-mode Pushover Analysis Procedure to Estimate the Seismic Demands of Medium-rise Unsymmetric-plan Buildings

In this study a multi-mode, adaptive, spectra based pushover procedure is presented in order to estimate the structural performances of 3D medium-rise unsymmetric-plan buildings. Response Spectrum Analysis (RSA) procedure has become a standard analysis tool for buildings, however single-mode pushover analysis can be applied to only regular low-rise building structures. This presented procedure (IRSA) is an advanced multi-mode pushover analysis that takes into account the effects of higher modes and can be applied effectively to irregular buildings. IRSA procedure is based on using RSA at each piecewise linear incremental step between the formation of consecutive plastic hinges. For the purpose of comparison, storey displacements of unsymmetric-plan medium-rise buildings are computed using nonlinear Response History Analysis (RHA) and the results are compared with the presented method (IRSA) together with the Modal Pushover Analysis (MPA) method. It is illustrated that the proposed method can compute-estimate the seismic demands of the buildings with reasonable accuracy.

Considering Soil-Nonlinearity Effect on Seismic Performance Evaluation of Structures based on Pushover Analysis

According to the most of current seismic codes, nonlinear soil behavior is commonly ignored in seismic evaluation procedure of the structures. To contribute on this matter, a pushover analysis method incorporating the probabilistic seismic hazard analysis (PSHA) is proposed to evaluate the effect of nonlinear soil response on seismic performance of a structure. The PSHA outcomes considering soil nonlinearity effect is involved in the analysis procedures by modifying the site-specific response spectrum. Results showed that incorporation of nonlinear soil behavior leads to an increase in displacement demand of structures which should accurately be considered in seismic design/assessment procedure. Results of implemented procedure are confirmed with the estimated displacement demand including soil-structure interaction (SSI).

Seismic evaluation of geometrically irregular steel moment resisting frames with setbacks considering their dynamic characteristics

A setback building has a sudden discontinuity in the frame geometry along the height. This kind of irregularity causes an abrupt discontinuity in stiffness, strength and mass of the building frame. In this study, a total of nineteen mid-rise 9-story steel moment resisting frames with setbacks including the broad range of different geometrical configurations were studied. An eigenvalue analysis was performed to evaluate and scrutinize the dynamic characteristics of setback structures. The effect of geometrical configurations on the seismic responses of setback frames was studied by means of nonlinear response history analysis using a set of far-field ground motion records. Moreover, due to the rapidly increasing use of pushover analysis for the seismic evaluation of structures in recent years, enhanced pushover analyses (EPAs) including the modal pushover analysis, the upper bound pushover analysis, the consecutive modal pushover and the extended N2 methods were implemented as a main part of this study. The findings show that two factors including the location of setback and the degree of setback are of key importance and influence the dynamic characteristics and seismic responses of setback structures. The degree of accuracy of the enhanced pushover analysis methods generally depends on the dynamic characteristics (geometrical configuration) of the setback frames. The largest error in the EPAs in predicting the story drifts generally occurs in a setback frame with a larger amount of the ratio between the effective modal participating mass ratio of the higher modes and that of the first mode.

Seismic response estimation of steel plate shear walls using nonlinear static methods

One of the major components for performance based seismic design is accurate estimation of critical seismic demand parameters. While nonlinear seismic analysis is the most appropriate analysis method for estimation of seismic demand parameters, this method is very time consuming and complex. Single mode pushover analysis method, N2 method and multi-mode pushover analysis method, modal pushover analysis (MPA) are two nonlinear static methods that have recently been used for seismic performance evaluation of few lateral load-resisting systems. This paper further investigates the applicability of N2 and MPA methods for estimating the seismic demands of ductile unstiffened steel plate shear walls (SPSWs). Three different unstiffened SPSWs (4-, 8-, and 15-storey) designed according to capacity design approach were analysed under artificial and real ground motions for Vancouver. A comparison of seismic response quantities such as, height-wise distribution of floor displacements, storey drifts estimated using N2 and MPA methods with more accurate nonlinear seismic analysis indicates that both N2 and MPA procedures can reasonably estimates the peak top displacements for low-rise SPSW buildings. In addition, MPA procedure provides better predictions of inter-storey drifts for taller SPSW. The MPA procedure has been extended to provide better estimate of base shear of SPSW.

Evaluation of Regular Multistory Buildings Using IBC2009 Code and ESEE Regulations by Pushover Analysis Method

For structural design and assessment of reinforced concrete members, the nonlinear analysis has become an important tool. The purpose of the pushover analysis is to assess the structural performance by estimating the strength and deformation capacities using static, nonlinear analysis and comparing these capacities with the demands at the corresponding performance levels. This paper aims to compare the results given by IBC2009 code and ESEE regulations. In this paper, four RC frames having 5, 15, 20 and 30 storeys were designed for seismicity according to both the recently adopted seismic code in Abu Dhabi (IBC2009) and the ESEE regulations. A pushover analysis is carried out for these buildings using SAP2000 (Ver. 15) and the ultimate capacities of the buildings are established. The obtained pushover curves and plastic hinges distributions are used to compare between the IBC2009 code and ESEE regulations. The comparison showed that there was variation in the obtained results by the two codes and the buildings designed by IBC2009 code were more vulnerable.

Research on Failure Modes and Key Parameters of Corrugated Steel Shear Walls

The corrugated steel plate shear wall is an effective and economical lateral load resisting system. And the thin steel plate is easy to buckle. And it is necessary to use much more thickness or ribs to prevent the buckle at out of plane direction, which is not economical or complicated to construct. A number of corrugated shear walls are analyzed by nonlinear static pushover analysis method. And failure modes, buckling, the ultimate capacity, and shear force-drift curves are studied. The results showed that the corrugated steel plate can be designed according to the failure modes, in order to have a desirable failure mode that the corrugated steel plate yield before buckling.

Modal pushover and response history analyses of a masonry chimney before and after shortening

The 50m high masonry chimney located in the old industrial facility that houses the School of Engineering of the University of Ferrara, Italy, suffered severe damages during the 2012 Emilia seismic sequence. Afterward, for security reasons, the upper damaged 12.40m were disassembled. Both before and after shortening, the ratio between the effective mass of the fundamental mode and the total mass is approximately 20%, leading standard pushover analysis methods not to be appropriate for estimating the seismic demand. Using a single, consistent 3D FE formulation, the results of a Modal Pushover Analysis (MPA) and four nonlinear Response History Analyses (RHA) for the shortened and the original chimney were presented in the paper. The ground motions considered in the simulations are accelerograms recorded during recent and less recent devastating seismic events in Italy, New Zealand, and Japan.For both chimneys, a very good agreement between MPA and RHA was observed in terms of lateral displacements. Moreover, for the 50m high chimney, a strong similarity was observed between the damage maps deriving from the MPA and those obtained with the RHA. All analyses confirmed a significant contribution of the higher modes. For the shortened chimney, the MPA revealed damages in the lower part of the stack (8–21m), because of a prevailing influence of the fundamental mode. In the RHA, a more evident contribution of the higher modes was observed, probably because of the effect of the vertical component of the ground motion, not accounted for in the MPA.

SEISMIC PERFORMANCE OF STRUCTURE WITH FIXED BASE, BASE ISOLATED STRUCTURE AND STRUCTURE WITH VISCOUS DAMPER

Earthquakes can create serious damage to structures. The structures already built are vulnerable to future earthquakes. The damage to structures causes deaths, injuries, economic loss, and loss of functions. Damage to the structure causes due to ground acceleration. This can be minimizing by increasing strength of the structure. Sometimes it is not possible to increase the strength of the structure indefinitely. So it should be tried to increase the capacity of structure using seismic improvement techniques. In this study, there are three different types of G+10 structures with fixed base, structure with viscous damper and structure with high damping rubber bearing are considered. These structures are modeled using finite element software SAP2000v16. These structures have same plan area and are assumed to be situated in seismic zone IV and having medium soil condition. The analysis of these structures is done by performance based pushover analysis method and response spectrum method. The pushover analysis method is used to find out the performance point and capacity of structure. High damping rubber bearing isolators (HDRB) and viscous damper (VD) are use for seismic improvement of RC structure. The procedure of analysis adopted for fixed base structure is same repeated for structure with viscous damper (VD) and base isolated (HDRB) structure. So it will help in comparative parametric study. The results of analysis are compared in terms of storey displacements, modal time period, storey acceleration, performance point, storey drift. The comparative study shows that storey displacement, storey acceleration, storey drift is reduced substantially for structure with high damping rubber bearing (base isolated structure).