首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
In the present study, the question of inelastic p–Δ effects is examined from the perspective of inelastic spectral ordinates. Inelastic acceleration response spectra are generated using a model which includes the effects of gravity; for each spectrum the ductility factor and the gravity load level (characterized by the stability coefficient) are held constant. Amplification spectra are generated by taking the ratio between spectra with and without gravity effects. The results are analysed statistically and a formula for the amplification factor as a function of the relevant parameters is obtained. Special care is taken to present a formulation that is simple and useful in the context of practical earthquake engineering design. Some currently used p–Δ amplification factors are discussed in the light of the results obtained.  相似文献   

2.
A procedure for treating the P– Δ effect in the direct displacement‐based seismic design of regular steel moment resisting frames with ideal elastoplastic material behaviour is proposed. A simple formula for the yield displacement amplification factor as a function of ductility and the stability coefficient is derived on the basis of the seismic response of an inelastic single degree‐of‐freedom system taking into account the P– Δ effect. Extensive parametric seismic inelastic analyses of plane moment resisting steel frames result in a simple formula for the dynamic stability coefficient as a function of the number of stories of a frame and the column to beam stiffness ratio. Thus, the P– Δ effect can be easily taken into account in a direct displacement‐based seismic design through the stability coefficient and the yield displacement amplification factor. A simple design example serves to illustrate the application of the proposed method and demonstrate its merits. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

3.
In this paper, torsional response of nonductile structures with soft‐first‐storey subjected to bidirectional ground motions is studied using a simplified two‐storey model with two‐way eccentricities. The stiffness ratio of second storey to first storey is varied to create different levels of soft‐first‐storey effect, while the stiffness eccentricity is varied to create torsional effects. Different overstrength ratios are used in the simplified models to study the response of structure with different structural capacity. Hysteretic model with strength deterioration and stiffness degradation properties is used to capture the deterioration of element stiffness and strength. Ductility capacity of 2.0 is used as the models are for nonductile structures. In general, displacement amplification of irregular model with respect to regular model increases as stiffness ratio increases, while no consistent trend of changes in displacement amplification is found with increase in stiffness eccentricity. It is found that the displacement amplification due to only soft‐first‐storey effect can be conservatively taken as 1.5. Coupling of torsional and soft‐first‐storey effects is more significant in affecting the displacement amplification of elements at flexible side. The trend of changes in displacement amplification of elastic system is similar to that of inelastic system. The displacement amplification of elements at the flexible side is larger than that at the stiff side. The elements at the flexible side in the direction of shorter uncoupled lateral period have larger displacement response than those in the orthogonal direction. Ductility demand–capacity curves subsequently constructed can be used to approximately assess the seismic performance of existing structures and as guidelines for designing structures in Singapore to withstand the maximum credible earthquake considering the coupling of torsional and soft‐first‐storey effects. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

4.
Air Traffic Control (ATC) towers are among the most vital structures in each airport. Due to inadequate information regarding the seismic design and assessment of these types of structures, practicing engineers may refer to building codes. However, taking into account the special dynamic behavior of ATC towers, instructions and recommendations provided in building codes often do not comply with the required seismic performance levels of ATC towers. In this study, seismic behaviors of three in-service ATC towers with a dual concrete core lateral load resisting system were studied through pushover and incremental dynamic collapse analysis. Seismic design response factors of the reference towers were calculated. It was found that seismic design response factors adopted by the design code did not provide a uniform safety margin for all reference ATC towers. It was also observed that shorter towers have significantly higher response modification factors compared to taller towers. For the studied towers, a structural over-strength factor of 2.4 and a displacement amplification factor of 4 were obtained.  相似文献   

5.
This paper explores the effectiveness of a new approach to foundation seismic design. Instead of the present practice of over‐design, the foundations are intentionally under‐dimensioned so as to uplift and mobilize the strength of the supporting (stiff) soil, in the hope that they will thus act as a rocking–isolation mechanism, limiting the inertia transmitted to the superstructure, and guiding plastic ‘hinging’ into soil and the foundation–soil interface. An idealized simple but realistic one‐bay two‐story reinforced concrete moment resisting frame serves as an example to compare the two alternatives. The problem is analyzed employing the finite element method, taking account of material (soil and superstructure) and geometric (uplifting and P–Δ effects) nonlinearities. The response is first investigated through static pushover analysis. It is shown that the axial forces N acting on the footings and the moment to shear (M/Q) ratio fluctuate substantially during shaking, leading to significant changes in footing moment‐rotation response. The seismic performance is explored through dynamic time history analyses, using a wide range of unscaled seismic records as excitation. It is shown that although the performance of both alternatives is acceptable for moderate seismic shaking, for very strong seismic shaking exceeding the design, the performance of the rocking‐isolated system is advantageous: it survives with no damage to the columns, sustaining non‐negligible but repairable damage to its beams and non‐structural elements (infill walls, etc.). Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

6.
Shaking table tests were conducted on a scaled reinforced concrete waffle–flat plate structure. It represented a conventional construction design under current building codes in the Mediterranean area. The test structure was subjected to a sequence of four seismic simulations of increasing magnitude. Each simulation was associated with a seismic hazard level characterized by the mean return period PR. The test structure performed well for the simulations associated with PR = 95, 475 and 975 years but collapsed under the maximum considered earthquake of PR = 2475 years. Damage concentrated at column bases, where the maximum chord rotation reached 93% of the ultimate capacity, and at the transverse beams of the exterior plate‐to‐column connection that failed in torsion. It is shown that most (from 85% to 90%) of the energy input by the earthquake that contributes to damage is dissipated by the plate. The capacity curve of the tested structure estimated from the experimental base shear vs. top displacement relationship allowed us to compute the overstrength (1.4). It is close to the maximum established by European code EN 1998‐1 (1.5). Based on a detailed study of the test results, potential updates to current codes and design recommendations are suggested. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

7.
  相似文献   

8.
Uneven distribution of seismic demand in asymmetric-plan structures is a critical concern in earthquake-resistant design. Contemporary seismic design strategies that are based on linear elastic response, single load reduction factor, and uniform ductility demand throughout an asymmetric system generally lead to unsatisfactory performance in terms of realized ductilities and nonuniform damage distribution due to strong torsional coupling associated with asymmetric-plan systems. In many cases, actual nonlinear behavior of the structure displays significant deviation from what is estimated by a linear elastic, force-based seismic design approach. This study investigates the prediction of seismic demand distribution among structural members of a single-story, torsionally stiff asymmetric-plan system. The focus is on the effect of inherent unbalanced overstrength, resulting from current force-based design practices, on the seismic response of code-designed single-story asymmetric structures. The results obtained are utilized to compile unsymmetrical response spectra and uniform ductility spectra, which are proposed as assessment and preliminary design tools for estimating the seismic performance of multistory asymmetric structures. A simple design strategy is further suggested for improving the inelastic torsional performance of asymmetric systems. Providing additional strength to stiff edge members over their nominal design strength demands leads to a more balanced ductility distribution. Finally, seismic responses of several asymmetric case study structures designed with the aid of the proposed strategy are assessed for validating their improved performance.  相似文献   

9.
This paper summarizes the results of an extensive study on the inelastic seismic response of X‐braced steel buildings. More than 100 regular multi‐storey tension‐compression X‐braced steel frames are subjected to an ensemble of 30 ordinary (i.e. without near fault effects) ground motions. The records are scaled to different intensities in order to drive the structures to different levels of inelastic deformation. The statistical analysis of the created response databank indicates that the number of stories, period of vibration, brace slenderness ratio and column stiffness strongly influence the amplitude and heightwise distribution of inelastic deformation. Nonlinear regression analysis is employed in order to derive simple formulae which reflect the aforementioned influences and offer a direct estimation of drift and ductility demands. The uncertainty of this estimation due to the record‐to‐record variability is discussed in detail. More specifically, given the strength (or behaviour) reduction factor, the proposed formulae provide reliable estimates of the maximum roof displacement, the maximum interstorey drift ratio and the maximum cyclic ductility of the diagonals along the height of the structure. The strength reduction factor refers to the point of the first buckling of the diagonals in the building and thus, pushover analysis and estimation of the overstrength factor are not required. This design‐oriented feature enables both the rapid seismic assessment of existing structures and the direct deformation‐controlled seismic design of new ones. A comparison of the proposed method with the procedures adopted in current seismic design codes reveals the accuracy and efficiency of the former. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

10.
Recent researches have revealed that the seismic ground response above tunnels can be different from the free-field motion during earthquakes. Nevertheless, to the best of the authors׳ knowledge, neither building codes nor seismic microzonation guidelines have yet considered this matter. In the present study, the seismic response of a linear elastic medium including a buried unlined tunnel subjected to vertically propagating incident SV and P waves are addressed. For analysis purposes, a numerical time-domain analysis is performed by utilizing a robust numerical algorithm working based on the boundary element method. It is observed that the amplification of the ground surface underlain by a tunnel is increased in long periods. The variation of the amplification factor and characteristic period of the medium versus the buried depth of the tunnel are depicted as the major results of this study. Some simple and useful relations are proposed for estimating the seismic microzonation of the areas underlain by tunnels. These relations can also be used for the preliminary seismic design of structures located on underground structures.  相似文献   

11.
在综述国内核电厂高边坡案例研究进展的基础上,以某核电厂含泥化夹层顺层软岩高边坡为例,探讨应用多种方法综合分析其在地震作用下的动态放大效应、抗震性能和加固效果的分析思路。首先,基于典型二维计算剖面,采用拟静力法对边坡进行初步加固设计;然后,基于振动台试验和数值计算,研究边坡动态放大效应、支挡结构的动力响应和抗震性能,探讨边坡抗震参数优化取值。研究结果表明:(1)原位边坡坡面的加速度放大系数随高度增加而增大,且泥化夹层饱水后的放大系数要大于饱水前,水平向最大值为1.90;(2)加固后边坡动态放大系数显著降低,水平最大放大系数为1.31,垂直向基本为1.0,锚索抗滑桩以下边坡基本不存在放大效应,说明软岩高边坡采用锚索抗滑桩和锚索框架抗震性能较好;(3)PGA=0.21g时,边坡整体抗震性能较好,仅最顶部锚杆和中部锚索受力超过设计锚固力,分别超过设计值的20%和5%,适当加强即可;(4)数值计算得到的加速度放大系数分布规律与振动台试验结果较为一致,且两者得到的坡顶加速度放大系数也十分接近。研究成果可为核电厂软岩高陡边坡抗震安全评价和工程设计提供技术支持。  相似文献   

12.
The earthquake of 26 September 1997 in Central Italy is one of the largest seismic events of the last 20 years in Italy. Two main events that caused significant damage in a large area of Umbria and Marche regions and site amplification phenomena were recorded even at large distances from the epicenter. After the emergency period, a detailed study of the surface effects was necessary for the post-earthquake reconstruction, but in a way it should be carried out rapidly enough to allow urban planners to give instructions and codes to public administrators. A team of surveyors were trained to collect field information such as geologic and geomorphologic features and, where possible, pre-existing geotechnic or geophysic information. Information was collected and analyzed with the aid of dynamic codes to calculate the possible local site effects. A one-dimensional code, analyzing single soil columns, [Schabel PB, Lysmer J, Seed HB. , a computer program for earthquake response analysis of horizontally layered sites. College of Engineering, University of California, Berkeley, Report PB-220. 1972, p. 207], as well as the two-dimensional codes working with finite or boundary elements, [Idriss IM, Lysmer J, Hwang R, Seed HB. , a computer program for evaluating the seismic response of the soil structure by variable damping finite element procedures. UCB EERC Report No. 73-16. 1973.] and [Sanò T. , un programma per il calcolo della propagazione delle onde sismiche. Technical Report SSN/RT/96/9. 1996, pp. 51.], were used and the results are presented as response spectra or amplification coefficients.  相似文献   

13.
—?Modern seismic codes usually include provisions for site effects by considering different coefficients chosen on the basis of soil properties at the surface and an estimate of the depth of bedrock. However, complex local geology may generate site amplification on soft soils significantly larger than what would be expected if we assume that the subsoil consists of plane soil layers overlaying a homogeneous half-space. This paper takes advantage of the large number of previous studies of site effects done at Euroseistest (northern Greece). Those studies have supplied a very detailed knowledge of the geometry and properties of the materials filling this shallow valley. In this paper we discuss the differences between site effects evaluated at the surface using simple 1-D computations and those evaluated using a very detailed 2-D model of the subsoil structure. The 2-D model produces an additional amplification in response spectra that cannot be accounted for without reference to the lateral heterogeneity of the valley structure. Our numerical results are extensively compared with observations, which show that the additional amplification computed from the 2-D model is real and affects by a significant factor response spectra, and thus suggests that some kind of aggravation factor due to the complexity of local geology is worthy of consideration in microzonation studies and seismic codes.  相似文献   

14.
A three-dimensional method of analysis is presented for the seismic response of structures constructed on pile foundations. An analysis is formulated in the time domain and the effects of material nonlinearity of soil on the seismic response are investigated. A subsystem model consisting of a structure subsystem and a pile-foundation subsystem is used. Seismic response of the system is found using a successive-coupling incremental solution scheme. Both subsystems are assumed to be coupled at each time step. Material nonlinearity is accounted for by incorporating an advanced plasticity-based soil model, HiSS, in the finite element formulation. Both single piles and pile groups are considered and the effects of kinematic and inertial interaction on seismic response are investigated while considering harmonic and transient excitations. It is seen that nonlinearity significantly affects seismic response of pile foundations as well as that of structures. Effects of nonlinearity on response are dependent on the frequency of excitation with nonlinearity causing an increase in response at low frequencies of excitation.  相似文献   

15.
In order to achieve a low isolation frequency, elastomeric bearings used for base isolation of buildings usually have low shear rigidity which leads to a significant reduction in the buckling load. The effects of compression load on the bearing behaviour are therefore an important consideration. A study of seismic response of base isolated buildings is presented in this paper, fully accounting for the P-Δ effects of isolation bearings. An analytical procedure is formulated that treats separately the superstructure and the supporting bearings and assembles the governing equations via the interaction forces at the base deck. The resulting equations are then solved step-by-step numerically. Numerical results obtained for a base isolated five-storey shear building show that neglecting the P-Δ effects can lead to considerable errors in the computed seismic response when the buckling safety factor of bearings is low.  相似文献   

16.
Seismic codes estimate the maximum displacements of building structures under the design-basis earthquakes by amplifying the elastic displacements under the reduced seismic design forces with a deflection amplification factor(DAF). The value of DAF is often estimated as ρ× R, where R is the force reduction factor and ρ is the inelastic displacement ratio that accounts for the inelastic action of the structure according to the definition presented by FEMA P695. The purpose of this study is to estimate the ρ-ratio of moment resisting steel frames(MRSFs) designed according to the Egyptian code. This is achieved by conducting a series of elastic and inelastic time-history analyses by two sets of earthquakes on four MRSFs designed according to the Egyptian code and having 2, 4, 8 and 12 stories. The earthquakes are scaled to produce maximum story drift ratios(MSDRs) of 1.0%, 1.5%, 2.0% and 2.5%. The mean values of the ρ-ratio are calculated based on the displacement responses of the investigated frames. The results obtained in this study indicate that the consideration of ρ for both the roof drift ratios(RDRs) and the MSDRs equal to 1.0 is a reasonable estimation for MRSFs designed according to the Egyptian code.  相似文献   

17.
A continuum model for the interaction analysis of a fully coupled soil–pile–structure system under seismic excitation is presented in this paper. Only horizontal shaking induced by harmonic SH waves is considered so that the soil–pile–structure system is under anti‐plane deformation. The soil mass, pile and superstructure were all considered as elastic with hysteretic damping, while geometrically both pile and structures were simplified as a beam model. Buildings of various heights in Hong Kong designed to resist wind load were analysed using the present model. It was discovered that the acceleration of the piled‐structures at ground level can, in general, be larger than that of a free‐field shaking of the soil site, depending on the excitation frequency. For typical piled‐structures in Hong Kong, the amplification factor of shaking at the ground level does not show simple trends with the number of storeys of the superstructure, the thickness and the stiffness of soil, and the stiffness of the superstructure if number of storeys is fixed. The effect of pile stiffness on the amplification factor of shaking is, however, insignificant. Thus, simply increasing the pile size or the superstructure stiffness does not necessarily improve the seismic resistance of the soil–pile–structure system; on the contrary, it may lead to excessive amplification of shaking for the whole system. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

18.
An analytical methodology for estimating the values of behaviour factors (q-factors), suggested in a companion paper, is applied to two typical reinforced concrete structures, a frame and a dual system, designed according to current European trends in seismic design. The two case studies clearly indicated the need to consider both local and overall collapse criteria, since either one can be critical for the determination of the behaviour factor, depending on the type of the structure. The calculated values of q-factors were close to, but higher than those specified by modern seismic codes, the larger discrepancy occurring in the case of the dual system.  相似文献   

19.
Post‐tensioned technologies for concrete seismic resistant buildings were first developed in the 1990s during the PREcast Seismic Structural Systems program. Among different solutions, the hybrid system proved to be the most resilient solution providing a combination of re‐centering and energy dissipative contributions respectively by using post‐tensioned tendons and mild steel reinforcement. The system, while providing significant strength and energy dissipation, reduces structural element damage and limits post‐earthquake residual displacements. More recently, the technology was extended to laminated veneer lumber (LVL) structural members, and extensive experimental and numerical work was carried out and allowed the development of reliable analytical and numerical models as well as design guidelines. On the basis of the experimental and numerical outcomes, this paper presents the evaluation of the seismic performance factors for post‐tensioned rocking LVL walls using the FEMA P‐695 procedure. Several archetype buildings were designed considering different parameters such as the building and story height, the type of seismic resistant system, the magnitude of gravity loads and the seismic design category. Lumped plasticity models were developed for each index archetype to simulate the behavioral aspects and collapse mechanisms. Non‐linear quasi‐static analyses were carried out to evaluate the system over‐strength factor; moreover, non‐linear time history analyses were performed using the incremental dynamic analysis concept to assess the collapse of each building. From the results of quasi‐static and dynamic analyses the response modification factor, R, system over‐strength factor, Ω0, and deflection amplification factor, Cd, values of, respectively, 7, 3.5 and 7.5 are recommended. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

20.
Complex seismic behaviour of soil–foundation–structure (SFS) systems together with uncertainties in system parameters and variability in earthquake ground motions result in a significant debate over the effects of soil–foundation–structure interaction (SFSI) on structural response. The aim of this study is to evaluate the influence of foundation flexibility on the structural seismic response by considering the variability in the system and uncertainties in the ground motion characteristics through comprehensive numerical simulations. An established rheological soil‐shallow foundation–structure model with equivalent linear soil behaviour and nonlinear behaviour of the superstructure has been used. A large number of models incorporating wide range of soil, foundation and structural parameters were generated using a robust Monte‐Carlo simulation. In total, 4.08 million time‐history analyses were performed over the adopted models using an ensemble of 40 earthquake ground motions as seismic input. The results of the analyses are used to rigorously quantify the effects of foundation flexibility on the structural distortion and total displacement of the superstructure through comparisons between the responses of SFS models and corresponding fixed‐base (FB) models. The effects of predominant period of the FB system, linear vs nonlinear modelling of the superstructure, type of nonlinear model used and key system parameters are quantified in terms of different probability levels for SFSI effects to cause an increase in the structural response and the level of amplification of the response in such cases. The results clearly illustrate the risk of underestimating the structural response associated with simplified approaches in which SFSI and nonlinear effects are ignored. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号