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1.
The seismic evaluation of existing buildings is a more difficult task than the seismic design of new buildings. Non-linear
methods are needed if realistic results are to be obtained. However, the application to real complex structures of various
evaluation procedures, which have usually been tested on highly idealized structural models, is by no means straightforward.
In the paper, a practice-oriented procedure for the seismic evaluation of building structures, based on the N2 method, is
presented, together with the application of this method to an existing multi-storey reinforced concrete building. This building,
which is asymmetric in plan and irregular in elevation, consists of structural walls and frames. It was designed in 1962 for
gravity loads and a minimum horizontal loading (2% of the total weight). The main results presented in terms of the global
and local seismic demands are compared with the results of non-linear dynamic response-history analyses. As expected, the
structure would fail if subjected to the design seismic action according to Eurocode 8. The shear capacity of the structural
walls is the most critical. If the shear capacity of these elements was adequate, the structure would be able to survive the
design ground motion according to Eurocode 8, in spite of the very low level of design horizontal forces. The applied approach
proved to be a feasible tool for the seismic evaluation of complex structures. However, due to the large randomness and uncertainty
which are involved in the determination of both the seismic demand and the seismic capacity, only rough estimates of the seismic
behaviour of such structures can be obtained. 相似文献
2.
3.
Performance-Based Seismic Design is now widely recognized as the pre-eminent seismic design and assessment methodology for
building structures. In recognition of this, seismic codes may require that buildings achieve multiple performance objectives
such as withstanding moderate, yet frequently occurring earthquakes with minimal structural and non-structural damage, while
withstanding severe, but rare earthquakes without collapse and loss of life. These objectives are presumed to be satisfied
by some codes if the force-based design procedures are followed. This paper investigates the efficacy of the Eurocode 8 force-based
design provisions with respect to RC frame building design and expected seismic performance. Four, eight, and 16-storey moment
frame buildings were designed and analyzed using the code modal response spectrum analysis provisions. Non-linear time-history
analyses were subsequently performed to determine the simulated seismic response of the structures and to validate the Eurocode
8 force-based designs. The results indicate the design of flexural members in medium-to-long period structures is not significantly
influenced by the choice of effective member stiffness; however, calculated interstorey drift demands are significantly affected.
This finding was primarily attributed to the code’s enforcement of a minimum spectral ordinate on the design spectrum. Furthermore,
design storey forces and interstorey drift demand estimates (and therefore damage), obtained by application of the code force-based
design procedure varied substantially from those found through non-linear time-history analysis. Overall, the results suggest
that though the Eurocode 8 may yield life-safe designs, the seismic performance of frame buildings of the same type and ductility
class can be highly non-uniform. 相似文献
4.
The paper deals with the topic of analyses performed according to modern code provisions, in particular Eurocode 8 (EC8) rules. Non linear static and non linear dynamic analyses of a plan irregular multi-storey r/c frame building designed according to Eurocode 2 (EC2) and EC8 provisions are carried out.The extension of the N2 method to torsionally flexible structures, as applied in previous papers, does not consider the accidental eccentricity, which is prescribed by EC8 also in the case of non linear static analysis. In this paper, three methods combining the accidental eccentricity prescribed by EC8 to the procedure which extends the N2 method to torsionally flexible structures are proposed and discussed. Each of them provides four modal response spectrum analyses (one for each model, corresponding to each position of centre of mass) and eight non linear static analyses (two signs for four models). NLSA(meth. n.2) seems to be the more reliable method of the three proposed, because it better fits absolute displacements obtained by non linear dynamic analysis.In the paper it is also observed that the value of the behaviour factor assigned by EC8 to torsionally flexible systems seems too conservative. 相似文献
5.
In Europe, computation of displacement demand for seismic assessment of existing buildings is essentially based on a simplified formulation of the N2 method as prescribed by Eurocode 8(EC8). However, a lack of accuracy of the N2 method in certain conditions has been pointed out by several studies. This paper addresses the assessment of effectiveness of the N2 method in seismic displacement demand determination in non-linear domain. The objective of this work is to investigate the accuracy of the N2 method through comparison with displacement demands computed using non-linear timehistory analysis(NLTHA). Results show that the original N2 method may lead to overestimation or underestimation of displacement demand predictions. This may affect results of mechanical model-based assessment of seismic vulnerability at an urban scale. Hence, the second part of this paper addresses an improvement of the N2 method formula by empirical evaluation of NLTHA results based on EC8 ground-classes. This task is formulated as a mathematical programming problem in which coefficients are obtained by minimizing the overall discrepancy between NLTHA and modified formula results. Various settings of the mathematical programming problem have been solved using a global optimization metaheuristic. An extensive comparison between the original N2 method formulation and optimized formulae highlights benefits of the strategy. 相似文献
6.
The use of nonlinear static procedures for performance‐based seismic design (PBSD) and assessment is a well‐established practice, which has found its way into modern codes for quite some time. On the other hand, near‐source (NS) ground motions are receiving increasing attention, because they can carry seismic demand systematically different and larger than that of the so‐called ordinary records. This is due to phenomena such as rupture forward directivity (FD), which can lead to distinct pulses appearing in the velocity time‐history of the ground motion. The framework necessary for taking FD into account in probabilistic seismic hazard analysis (PSHA) has recently been established. The objective of the present study is to discuss the extension of nonlinear static procedures, specifically the displacement coefficient method (DCM), with respect to the inelastic demand associated with FD. In this context, a methodology is presented for the implementation of the DCM toward estimating NS seismic demand, by making use of the results of NS‐PSHA and a semi‐empirical equation for NS‐FD inelastic displacement ratio. An illustrative application of the DCM, with explicit inclusion of NS‐pulse‐like effects, is given for a set of typical plane R/C frames designed under Eurocode provisions. Different scenarios are considered in the application and nonlinear dynamic analysis results are obtained and discussed with respect to the static procedure estimates. Conclusions drawn from the results may help to assess the importance of incorporating NS effects in PBSD. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
7.
Extensive studies have confirmed the good performance of the N2 method, recommended by Eurocode8, when performing pushover
analyses in regular structures. However, this procedure shows lack of accuracy in predicting the torsional motion of plan-asymmetric
buildings. In order to overcome this problem, Peter Fajfar and his team have proposed an extension of the method based on
a combination of a pushover analysis and of an elastic response spectrum analysis. Since definitive answers about this topic
have not yet been reached, this paper intends to proceed the study applying the extended N2 method to real existing RC buildings.
Three real plan-asymmetric buildings with three, five and eight storeys were assessed. The results obtained with the extended
N2 method were compared with the ones evaluated by means of the original N2 and with the nonlinear dynamic analysis through
the use of semi-artificial ground motions. The analyses were performed for different seismic intensities in order to evaluate
the torsional response of the building through different stages of structural inelasticity. The results obtained show that
the extended N2 method generally reproduces in a very good fashion the real torsional behavior of the analyzed buildings.
The conclusions herein outlined, added to the ones already published by the aforementioned authors, seem to confirm that the
extended N2 method can be introduced in the next version of Eurocode8 as a nonlinear static procedure capable of accurately
predicting the torsional response of plan-asymmetric buildings. 相似文献
8.
Estimating seismic demands on structures, to predict their performance level with confidence, requires explicit consideration
of the structural inelastic behaviour: to this end, the use of nonlinear static procedures is inevitably going to be favoured
over complex nonlinear time-history methods.
The currently available assessment procedures have been tested predominantly against building frames. A newly derived assessment
procedure is proposed within the scope of bridge applications, based on an innovative displacement-based adaptive pushover
technique. The procedure, which can be incorporated into a performance-based engineering philosophy, is applicable to MDOF
continuous span bridges with flexible or rigid superstructures, and for varying degrees of abutment restraint.
As a first application to determine the viability of the proposed procedure, a parametric study is conducted on a ensemble
of bridges subjected to earthquake motion. It is shown that, compared to the seismic demand estimated by means of the more
accurate nonlinear dynamic analysis tool, the novel static assessment method can lead to the attainment of satisfactory predictions,
both in terms of displacement as well as moment demand on members. 相似文献
9.
Ground-Motion Prediction Equations (GMPEs) for inelastic displacement and ductility demands of constant-strength SDOF systems 总被引：1，自引：1，他引：0
The objective of this paper is to present ground-motion prediction equations for ductility demand and inelastic spectral displacement
of constant-strength perfectly elasto-plastic single-degree-of-freedom (SDOF) oscillators. Empirical equations have been developed
to compute the ductility demand as a function of two earthquake parameters; moment magnitude, and source-to-site distance;
one site parameter, the ground type; and three oscillator parameters, an undamped natural period, critical damping ratio,
and the mass-normalized yield strength. In addition, a comparative study of the proposed model with selected previous studies
and recommendations of Eurocode 8 is presented. Proposed equations can easily be incorporated in existing probabilistic seismic
hazard analysis (PSHA) software packages with the introduction of an additional parameter. This leads to hazard curves for
inelastic spectral displacement, which can provide better estimates of target displacement for nonlinear static procedures
and an efficient intensity measure for probabilistic seismic demand analysis (PSDA). Proposed equations will be useful in
performance evaluation of existing structures. 相似文献
10.
Gennaro Magliulo Vittorio Capozzi Roberto Ramasco 《Bulletin of Earthquake Engineering》2012,10(2):679-694
The paper presents the results of a research study concerning the seismic response and design of r/c frames with overstrength
discontinuities in elevation. The discontinuities are obtained assigning overstrengths either to the beams or to the columns
of a “regular frame” (assumed as reference). Two “regular frames” are designed: one according to the Eurocode 8 (EC8) medium
ductility class (DCM) rules and the other one according to the EC8 high ductility class (DCH) rules. For all frames the criteria
of vertical strength irregularity of many international seismic codes are applied. Non linear static and dynamic analyses
are performed; mechanical non linearity is concentrated at the element ends. These analyses are carried out according to EC8
provisions: for non linear static analysis the N2 method is applied; in the case of non linear time-history analyses, seven
real earthquakes, selected in order to fit on average the elastic design spectrum, are used as input. The seismic response
of frames characterised by the assigned overstrength is not very different with respect to the “regular frame” one; furthermore
all the frames satisfy the Ultimate Limit State, verified by the application of non linear static and dynamic analyses. This
demonstrates that the sensitivity of frames, designed according to EC8 medium and high ductility classes, to overstrength
vertical variations is low. Consequently, international code provisions on vertical strength regularity should be reviewed. 相似文献
11.
Evaluation of displacement coefficient method for seismically retrofitted buildings with various ductility capacities 
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下载免费PDF全文 This research study is aimed at evaluating the accuracy of the displacement coefficient method (DCM) of FEMA 440 and associated nonlinear static procedure (NLSP) for actual buildings with soft story mechanism and various ductility capacities. The DCM and associated NLSP are evaluated using two existing seismically vulnerable buildings with soft story mechanism. The buildings are first retrofitted using a ductile steel‐brace‐link system to represent those with good ductility capacity and then retrofitted with RC squat infill shear panels (SISPs) to represent those with relatively poor ductility capacity. The evaluation of the DCM of FEMA 440 and associated NLSP is then performed by comparing the roof displacements (target displacements), maximum interstory drifts, and maximum plastic hinge rotations of the original and retrofitted buildings obtained from NLSP (at the target displacement level of DCM) with those obtained from nonlinear response history (NRH) analyses for three different seismic performance levels. It is observed that the DCM, and hence, the NLSP fail to accurately predict the NRH analyses results mainly due to uncertainties in the coefficient C1 of the DCM in the short period range, the inability of the DCM to capture the failure of structural members beyond a certain lateral displacement or plastic rotation limit and associated soft story mechanism. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
12.
For the performance‐based seismic design of buildings, both the displacement coefficient method used by FEMA‐273 and the capacity spectrum method adopted by ATC‐40 are non‐linear static procedures. The pushover curves of structures need to be established during processing of these two methods. They are applied to evaluation and rehabilitation of existing structures. This paper is concerned with experimental studies on the accuracy of both methods. Through carrying out the pseudo‐dynamic tests, cyclic loading tests and pushover tests on three reinforced concrete (RC) columns, the maximum inelastic deformation demands (target displacements) determined by the coefficient method of FEMA‐273 and the capacity spectrum method of ATC‐40 are compared. In addition, a modified capacity spectrum method which is based on the use of inelastic design response spectra is also included in this study. It is shown from the test specimens that the coefficient method overestimates the peak test displacements with an average error of +28% while the capacity spectrum method underestimates them with an average error of ‐20%. If the Kowalsky hysteretic damping model is used in the capacity spectrum method instead of the original damping model, the average errors become ‐11% by ignoring the effect of stiffness degrading and ‐1.2% by slightly including the effect of stiffness degrading. Furthermore, if the Newmark–Hall inelastic design spectrum is implemented in the capacity spectrum method instead of the elastic design spectrum, the average error decreases to ‐6.6% which undervalues, but is close to, the experimental results. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
13.
This work discusses the simplified estimation of earthquake‐induced nonlinear displacement demands as required by nonlinear static procedures, with particular attention on short‐period masonry structures. The study focuses on systems with fundamental periods between 0.1 and 0.5 s, for which inelastic amplification of the elastic displacement demand is more pronounced; hysteretic force‐displacement relationships characteristic of masonry structures are adopted, because these structures are more commonly found within the considered period range. Referring to the results of nonlinear dynamic analyses of single‐degree‐of‐freedom oscillators, some limitations of the Eurocode 8 and Italian Building Code formulations are first discussed, then an improved equation is calibrated that relates inelastic and elastic displacement demands. Numerical values of the equation parameters are obtained, considering the amount of hysteretic energy dissipation associated with various damage mechanisms observed in masonry structures. Safety factors are also calculated to determine several percentiles of the displacement demand. It is shown that the proposed equation can be extended to more dissipative systems. Finally, the same formulation is adapted to the estimation of seismic displacements when elastic analysis procedures are employed. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
14.
在离散波数法（DWN）基础上,计算了武定M6.5地震断层破裂在周围介质中产生的位移时程（位移理论地震图）和动态位移场;进行弹性动力学转换后,求得应变时程和动态应变场;最后得到了武定M6.5地震所产生的动态库仑破裂应力变化量和动态库仑破裂应力变化场,进而研究其与后续余震的关系. 结果表明,动态应力最大峰值和静态应力的正区均呈非对称性分布,两者的分布特征与余震的分布特征基本一致. 在动态应力峰值为正的确定区应力值超过了0.1 MPa触发阈值, 在静态应力值为正的确定区应力值超过了0.01 MPa触发阈值. 这说明动态应力和静态应力均有助于余震的发生. 相似文献
15.
Timothy John Sullivan 《Bulletin of Earthquake Engineering》2013,11(6):2197-2231
This paper details a direct displacement-based design procedure for steel eccentrically braced frame (EBF) structures and gauges its performance by examining the non-linear dynamic response of a series of case study EBF structures designed using the procedure. Analytical expressions are developed for the storey drift at yield and for the storey drift capacity of EBFs, recognising that in addition to link beam deformations, the brace and column axial deformations can provide important contributions to storey drift components. Case study design results indicate that the ductility capacity of EBF systems will tend to be relatively low, despite the large local ductility capacity offered by well detailed links. In addition, it is found that while the ductility capacity of EBF systems will tend to reduce with height, this is not necessarily negative for seismic performance since the displacement capacity for taller EBF systems will tend to be large. To gauge the performance of the proposed DBD methodology, analytical models of the case study design solutions are subject to non-linear time-history analyses with a set of spectrum-compatible accelerograms. The average displacements and drifts obtained from the NLTH analyses are shown to align well with design values, confirming that the new methodology could provide an effective tool for the seismic design of EBF systems. 相似文献
16.
Peter Fajfar 《地震工程与结构动力学》1999,28(9):979-993
By means of a graphical procedure, the capacity spectrum method compares the capacity of a structure with the demands of earthquake ground motion on it. In the present version of the method, highly damped elastic spectra have been used to determine seismic demand. A more straightforward approach for the determination of seismic demand is based on the use of the inelastic strength and displacement spectra which can be obtained directly by time-history analyses of inelastic SDOF systems, or indirectly from elastic spectra. The advantages of the two approaches (i.e. the visual representation of the capacity spectrum method and the superior physical basis of inelastic demand spectra) can be combined. In this paper, the idea of using inelastic demand spectra within the capacity spectrum method has been elaborated and is presented in an easy to use format. The approach represents the so-called N2 method formulated in the format of the capacity spectrum method. By reversing the procedure, a direct displacement-based design can be performed. The application of the modified capacity spectrum method is illustrated by means of two examples. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
17.
This paper evaluates the American FEMA 356 and the Greek GRECO (EC 8 based) procedural assumptions for the assessment of the seismic capacity of existing buildings via pushover analyses. Available experimental results from a four-storeyed building are used to compare the two different sets of assumptions. If the comparison is performed in terms of initial stiffness or plastic deformation capacities, the different partial assumptions of the procedures lead to large discrepancies, while the opposite occurs when the comparison is performed in terms of structural performance levels at target displacements. According to FEMA 356 assumptions, effective yield point rigidities are approximately four times greater than those of EC 8. Both procedures predicted that the structure would behave elastically during low-level excitation and that the structural performance level at target displacement for a high-level excitation would be between the Immediate Occupancy and Life Safety performance levels. 相似文献
18.
Alternative static pushover methods for the seismic design of new structures are assessed with the aid of advanced computational tools. The current state-of-practice static pushover methods as suggested in the provisions of European and American regulations are implemented in this comparative study. In particular the static pushover methods are: the displacement coefficient method of ASCE-41, the ATC-40 capacity spectrum method and the N2 method of Eurocode 8. Such analysis methods are typically recommended for the performance assessment of existing structures, and therefore most of the existing comparative studies are focused on the performance of one or more structures. Therefore, contrary to previous research studies, we use static pushover methods to perform design and we then compare the capacity of the outcome designs with reference to the results of nonlinear response history analysis. This alternative approach pinpoints the pros and cons of each method since the discrepancies between static and dynamic analysis are propagated to the properties of the final structure. All methods are implemented in an optimum performance-based design framework to obtain the lower-bound designs for two regular and two irregular reinforced concrete building configurations. The outcome designs are compared with respect to the maximum interstorey drift and maximum roof drift demand obtained with the Incremental Dynamic Analysis method. To allow the comparison, also the life-cycle cost of each design is calculated; i.e. a parameter that is used to measure the damage cost due to future earthquakes that will occur during the design life of the structure. The problem of finding the lower bound designs is handled with an Evolutionary type optimization algorithm. 相似文献
19.
Accuracy of nonlinear static procedures for the seismic assessment of shear critical structures 下载免费PDF全文
下载免费PDF全文 The nonlinear behavior of reinforced concrete (RC) members represents a key issue in the seismic performance assessment of structures. Many structures constructed in the 1980s or earlier were designed based on force limits; thus they often exhibit brittle failure modes, strength and stiffness degradation, and severe pinching effects. Field surveys and experimental evidence have demonstrated that such inelastic responses affect the global behavior of RC structural systems. Efforts have been made to consider the degrading stiffness and strength in the simplified nonlinear static procedures commonly adopted by practitioners. This paper investigates the accuracy of such procedures for the seismic performance assessment of RC structural systems. Refined finite element models of a shear critical bridge bent and a flexure‐critical bridge pier are used as reference models. The numerical models are validated against experimental results and used to evaluate the inelastic dynamic response of the structures subjected to earthquake ground motions with increasing amplitude. The maximum response from the refined numerical models is compared against the results from the simplified static procedures, namely modified capacity spectrum method and coefficient method in FEMA‐440. The accuracy of the static procedures in estimating the displacement demand of a flexure‐critical system and shear‐critical system is discussed in detail. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
20.
This paper investigates the accuracy of pushover-based methods in predicting the seismic response of slender masonry towers, through comparison with the results from a large number of nonlinear time-history dynamic analyses. In particular, conventional pushover analyses, in both their force- and displacement-based variants, are considered, and seismic assessment through the well-established N2 method is also addressed. The study is conducted by applying a simple non-linear elastic model recently developed and implemented in the computational code MADY to represent slender masonry structures. The model enables both pushover analyses and non-linear dynamic analyses to be performed with a minimum of effort. A multi-record incremental dynamic analysis carried out for a quite large number of structural cases, each of which is subjected to a comprehensive set of dynamic nonlinear analyses, is used to evaluate the accuracy of pushover methods in predicting the global structural response, as represented by the usual capacity curve together with a damage curve, both of which are compared with dynamic envelopes. Local responses, in terms of lateral displacements and the distribution of damage along the tower height are also compared. The results reveal that the key issue in the accuracy of pushover methods is the nature of the lateral load applied, that is, whether it is a force or a displacement. Different ranges of expected deformation are suggested for adopting each type of lateral load to better represent the actual behaviour of masonry towers and their damage under seismic events through pushover methods. 相似文献