大跨度球面网壳结构的风振系数研究

大跨度球面网壳结构应用日趋广泛,其结构特点使风荷载常常起主要甚至是控制作用,风振动力响应研究日益受到关注与重视。本文讨论了网壳结构风振响应时程分析计算方法,并采用节点位移风振系数来衡量网壳结构风振特性。对角锥型双层球面网壳结构进行了不同几何参数即跨度、矢高、球壳厚度等多种情况下的位移风振系数的研究,得出该类网壳的风振系数随几何参数的变化规律,并回归出计算公式,为双层球面网壳结构的抗风设计提供参考。     

The near-field method: a modified equivalent linear method for dynamic soil–structure interaction analysis. Part II: verification and example application

Usually for modeling of soil in a direct soil–structure interaction (SSI) problem, the equivalent linear soil properties are used. However, this approach is not valid in the vicinity of a foundation, where the soil experiences large strains and a high level of nonlinearity because of structural vibrations. The near-field method was developed and described in a companion paper to overcome this limitation. This method considers the effects of large strains and suggests a shear modulus and a damping ratio further modified in the near-field of a foundation. Validity and performance of this approach are evaluated, application examples are explained and the results of a parametric study about the role of soil and structure parameters in the extent of SSI effects on the nonlinear seismic response of structures are presented in this paper. One real existing and five, ten, fifteen and twenty story moment-resisting frame steel buildings with two different site conditions corresponding to firm and soft soils are considered and the responses obtained from the near-field method are compared with the recorded and rigorous responses. Moreover, various SSI modeling techniques are employed to investigate the accuracy and performance of each approach. The results show that the near-field method is a simple yet accurate enough approach for analysis of direct SSI problems.     

The numerical and empirical evaluation of chimneys considering soil structure interaction and high-temperature effects

During the past strong ground motions, chimneys constructed according to international standards are representative of similar structures at industrial areas throughout the world, including those collapsed or moderately damaged in earthquake-prone regions. This is due to the specialty of structural characteristics and the special loads acting on the structure such as earthquakes, wind and differences in the level of temperature, etc. In this context, the researchers and designers should focus on the dynamic behavior of chimneys especially under high temperature and seismic effects. For this purpose, the main focus of this study is to evaluate the dynamic response of a chimney under the above-mentioned effects considering soil-structure interaction (SSI). A 52 m steel chimney in Yeşilyurt township of Samsun City in Turkey was studied. The in-situ model testing and numerical models were compared. Before the commissioning of the chimney, a series of tests was realized to define its dynamic characteristics in case of no-heat and after the fabric got to work, the same tests were repeated for the same sensor locations to understand the heat effect on the dynamic response of the chimney. The ambient vibration tests are proven to be fast and practical procedures to identify the dynamic characteristics of those structures. The dynamic testing of the towers promises a widespread use, as the identification of seismic vulnerability of such structures becomes increasingly important. The data presented in this study are considered to be useful for the researchers and engineers, for whom the temperature and SSI effects on steel chimneys are a concern. Using the modal analysis techniques, presented finite element simulation for the soil/pile foundation-chimney interaction system is verified. The results of modal analyses using numerical solutions are shown to have acceptable accuracy compared with results obtained by in-situ test. The present study also aims to provide designers with material examples about the influence of these on the seismic performance of steel chimneys by means of reflecting the changes in the dynamic behavior.     

Modal parameter identification and damping ratio estimation from the full-scale measurements of a typical Tibetan wooden structure

Tibetan heritage buildings have a high historical and cultural value. They have endured adverse environmental loadings over hundreds of years without significant damage. However, there are few reports on their structural characteristics under normal environmental loadings and their behavior under dynamic loadings. In this research, a typical Tibetan wooden wall-frame building is selected to study its dynamic characteristics. Field measurements of the structure were conducted under environmental excitation to collect acceleration responses. The stochastic subspace identification (SSI) method was adopted to calculate the structural modal parameters and obtain the out-of-plane vibration characteristics of the slab and frames. The results indicated that the wall-frame structure had a lower out-of-plane stiffness and greater in-plane stiffness due to the presence of stone walls. Due to poor identified damping ratio estimates from the SSI method, a method based on the variance upper bound was proposed to complement the existing variance lower bound method for estimating the modal damping ratio to address the significant damping variability obtained from different points and measurements. The feasibility of the proposed method was illustrated with the measured data from the floor slab of the structure. The variance lower and upper bound methods both provided consistent results compared to those from the traditional SSI method.     

地震动差动作用下大跨度空间网壳结构的反应分析

选取100m跨度的双层柱面网壳结构为研究对象,采用时程分析法,分别进行了结构在单向和三向地震行波输入作用下的反应分析,并针对多种视波速情况进行了研究,考查了地震动不同输入情况下结构杆件内力的分布特点,对其进行了对比分析,为大跨网壳结构的抗震设计提供了理论依据。研究表明,考虑行波效应会使结构部分构件内力有一定程度的提高,多维地震作用比单维地震作用下结构的杆件内力大。由此得出结论：对于大跨度空间网壳结构,应该进行多维非一致输入下的地震反应分析;为保证抗震安全,应对可能出现的地面视波速进行全面分析。     

Natural vibration frequency and damping of slender structures founded on monopiles

Offshore wind turbine (OWT) is a typical example of a slender engineering structure founded on large diameter rigid piles (monopiles). The natural vibration characteristics of these structures are of primary interest since the dominant loading conditions are dynamic. A rigorous analytical solution of the modified SSI eigenfrequency and damping is presented, which accounts for the cross coupling stiffness and damping terms of the soil–pile system and is applicable but not restrictive to OWTs. A parametric study was performed to illustrate the sensitivity of the eigenfrequency and damping on the foundation properties, the latter being expressed using the notion of dimensionless parameters (slenderness ratio and flexibility factor). The application of the approximate solution that disregards the off diagonal terms of the dynamic impedance matrix was found to overestimate the eigenfrequency and underestimate the damping. The modified SSI eigenfrequency and damping was mostly affected by the soil–pile properties, when the structural eigenfrequency was set between the first and second eigenfrequency of the soil layer. Caution is suggested when selecting one of the popular design approaches for OWTs, since the dynamic SSI effects may drive even a conservative design to restrictive frequency ranges, nonetheless along with advantageous – from a designers perspective – increased damping.     

基于粘滞阻尼器的元江大桥减震设计方案

为了提高大跨度桥梁的抗震性能水平,基于粘滞阻尼器的结构减震控制技术成为专家学者研究的重点和工程设计人员首先考虑的抗震设防措施。现有成果主要集中在大跨度公路斜拉桥的研究和应用,在山区的非规则高墩钢桁连续梁桥研究的较少。以云南省元江大桥为例,采用Midas/civil建立弹性分析的有限元模型,对元江大桥进行了动力特性分析。采用快速非线性时程分析方法对粘滞阻尼器参数进行了优化分析,并且总结了粘滞阻尼器参数对高墩连续梁桥的减震作用规律。最后,通过有控和无控结构的地震动响应对比分析,评价了安装阻尼器后的结构主控部位的减震效果。结果表明:减震控制提高了结构的安全性。     

拱形波纹钢屋盖动力特性测试及分析

拱形波纹钢屋盖是一种新颖的金属拱形薄壳结构，在我国已经得到了广泛的应用。由于结构的复杂性，目前有关的理论研究尚不够深入，动力研究还很少有人涉及。以18m和30m两种跨度的W666型拱形波纹钢屋盖为研究对象，测试了拱在竖向平面内振动时的频率、振型及阻尼。在所提简化计算模型的基础上，采用有限元方法计算了结构的动力特性，其结果与试验数据符合较好。测试和分析结果为该结构的动力反应研究提供了方便。     

抗震约束下大跨度空间钢结构设计中支吊架的合理选取

大跨度空间钢结构设计中,合理选取支吊架对建筑的抗震性有着重要的影响,当前设计过程中未考虑支吊架组合后的抗震性能。综合选取适用于大跨度空间钢结构的支吊架结构,使其具有更优的抗震性能。首先给出目前应用较多的三种结构,其次给出一定的抗震约束条件,如承载力、应力应变与拉伸变形等。在条件约束下,给出具有抗震能力的支吊架选取模型并给出具体计算示例,选取和组合具有一定抗震能力的大跨度空间钢结构支吊架。结果表明,相较于传统的随机支吊架选取方式,具有抗震能力的模型下支吊架核心钢结构抗震水平明显提升。     

Damping coefficients for soil–structure systems and evaluation of FEMA 440 subjected to pulse-like near-fault earthquakes

In this study, attempts are made to investigate the effects of inertial soil–structure interaction (SSI) on damping coefficients subjected to pulse-like near-fault ground motions. To this end, a suit of 91 pulse-like near-fault ground motions is adopted. The soil and superstructure are idealized employing cone model and single-degree-of-freedom (SDOF) oscillator, respectively. The results demonstrate that soil flexibility reduces and amplifies the damping coefficients for structural viscous damping levels higher and lower than 5%, respectively. The coefficients reach one for both acceleration and displacement responses in cases of dominant SSI effects. The effect of structure dimensions on damping confidents are found insignificant. Moreover, damping coefficients of displacement responses are higher than those of acceleration responses for both fixed-base and flexible-base systems. Evaluation of damping correction factor introduced by FEMA 440 shows its inefficiency to predict acceleration response of soil–structure systems under pulse-like near-fault ground motions. Soil flexibility makes the damping correction factor of moderate earthquakes more pronounced and a distinctive peak value is reported for cases with dominant SSI effects.     

Automatic modal parameter identification of high arch dams: feasibility verification

Modal parameters, including fundamental frequencies, damping ratios, and mode shapes, could be used to evaluate the health condition of structures. Automatic modal parameter identification, which plays an essential role in realtime structural health monitoring, has become a popular topic in recent years. In this study, an automatic modal parameter identification procedure for high arch dams is proposed. The proposed procedure is implemented by combining the density-based spatial clustering of applications with noise (DBSCAN) algorithm and the stochastic subspace identification (SSI). The 210-m-high Dagangshan Dam is investigated as an example to verify the feasibility of the procedure. The results show that the DBSCAN algorithm is robust enough to interpret the stabilization diagram from SSI and may avoid outline modes. This leads to the proposed procedure obtaining a better performance than the partitioned clustering and hierarchical clustering algorithms. In addition, the errors of the identified frequencies of the arch dam are within 4%, and the identified mode shapes are in agreement with those obtained from the finite element model, which implies that the proposed procedure is accurate enough to use in modal parameter identification. The procedure is feasible for online modal parameter identification and modal tracking of arch dams.

     

大跨度桥梁风场模拟方法对比研究

本文将基于线性滤波器的ARMA模型应用于大跨度桥梁的风场模拟,推导出自回归(AR)阶数P和滑动回归(MA)阶数q不等情况下,ARMA模型用于模拟多变量稳态随机过程的公式,将ARMA风场模拟方法与目前广泛应用于大跨度桥梁风场模拟的谐波合成法应用于一座实际大跨度斜拉桥的风场模拟,通过对比研究得出一些有意义的结论,并证实了ARMA法能够在保证模拟精度的前提下,大大提高风场模拟的效率。     

Seismic response of base-isolated buildings including soil–structure interaction

This study investigates the effect of soil–structure interaction (SSI) on the response of base-isolated buildings. The equations of motion are formulated in the frequency domain, assuming frequency-independent soil stiffness and damping constants. An equivalent fixed-base system is developed that accounts for soil compliance and damping characteristics of the base-isolated building. Closed-form expressions are derived, followed by a thorough parametric study involving the pertinent system parameters. For preliminary design, the methodology can serve as a means to assess effective use of base isolation on building structures accounting for SSI. This study concludes that the effects of SSI are more pronounced on the modal properties of the system, especially for the case of squat and stiff base-isolated structures.     

Non-linear buffeting response analysis of long-span suspension bridges with central buckle

The rigid central buckle employed in the Runyang Suspension Bridge (RSB) was the first time it was used in a suspension bridge in China. By using a spectral representation method and FFT technique combined with measured data,a 3D fluctuating wind field considering the tower wind effect is simulated. A novel FE model for buffeting analysis is then presented,in which a specific user-defined Matrix27 element in ANSYS is employed to simulate the aeroelastic forces and its stiffness or damping matrices are parameterized by wind velocity and vibration frequency. A nonlinear time history analysis is carried out to study the influence of the rigid central buckle on the wind-induced buffeting response of a long-span suspension bridge. The results can be used as a reference for wind resistance design of long-span suspension bridges with a rigid central buckle in the future.     

高层建筑结构-地基动力相互作用效果的振动台试验对比研究

本文通过对高层建筑结构－地基动力相互作用体系和刚性地基上高层建筑结构的振动台模型试验成果的对比分析，研究了相互作用对结构动力特性和地震反应的影响。结果－地基动力相互作用使结构频率减小，阻尼增大；相互作用体系的振型曲线与刚性地基上结构的振型曲线不同，基础处存在平动和转动；在地震动作用下考虑相互作用的结构加速度、层间剪力、弯矩以及应变通常比刚性地基上的情况小，而位移则比刚性地基上的情况大。     

非经典振型分解法求混凝土房屋及其上钢塔的地震响应

钢结构与混凝土结构阻尼比不同,混凝土房屋与其顶上钢塔组成了非比例阻尼结构系统。本文用非经典振型分解法求解该类结构系统的线弹性地震响应,发现只用前几阶振型响应迭加的结果即可逼近直接积分法的精确度。     

Considering dynamic soil structure interaction (SSI) effects on seismic isolation retrofit efficiency and the importance of natural frequency ratio

This paper utilizes and expands on existing coupled BEM–FEM (finite element method) methods for the investigation of the effects of soil structure interaction (SSI) on both an un-retrofitted and seismically isolated typical bridge structure. A simple numerical model of the bridge and surrounding soil is formulated and excited by an earthquake excitation. Utilizing Newmark's β FEM solution method along with the closed form B-spline BIRF method, the structural damped period, composite damping ratio, pier relative displacement, and base shear demand are monitored. From these results, the effects of SSI on this structure are identified. Additionally, the importance of the relative rigidity between the soil-foundation system and the bridge structure is also investigated. The results of the studies indicate that the response of the complete structure system considered is affected by the inclusion of SSI effects. Furthermore, the efficiency of the isolation measures designed using fixed base conditions is decreased by considering SSI over a certain relative rigidity range that is quantified using the structure to soil-foundation natural frequency ratio.     

Seismic design of yielding structures on flexible foundations

This paper introduces a simple method to consider the effects of inertial soil–structure interaction (SSI) on the seismic demands of a yielding single‐degree‐of‐freedom structure. This involves idealizing the yielding soil–structure system as an effective substitute oscillator having a modified period, damping ratio, and ductility. A parametric study is conducted to obtain the ratio between the displacement ductility demand of a flexible‐base system and that of the corresponding fixed‐base system. It is shown that while additional foundation damping can reduce the overall response, the effects of SSI may also increase the ductility demand of some structures, mostly being ductile and having large structural aspect ratio, up to 15%. Finally, a design procedure is provided for incorporation of the SSI effects on structural response. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaluation of FEMA-440 for including soil-structure interaction

Replacing the entire soil-structure system with a fixed base oscillator to consider the effect of soil-structure interaction (SSI) is a common analysis method in seismic design. This technique has been included in design procedures such as NEHRP, ASCE, etc. by defining an equivalent fundamental period and damping ratio that can modify the response of the structure. However, recent studies indicate that the effects of SSI should be reconsidered when a structure undergoes a nonlinear displacement demand. In recent documents on Nonlinear Static Procedures (NSPs), FEMA-440 (2005), a modified damping ratio of the replacement oscillator was proposed by introducing the ductility of the soil-structure system obtained from pushover analysis. In this paper, the damping defined in FEMA-440 to include the soil-structure interaction effect is evaluated, and the accuracy of the Coefficient Method given in FEMA-440 and the Equivalent Linearization Method is studied. Although the improvements for Nonlinear Static Procedures (NSPs) in FEMA-440 are achieved for a fixed base SDOF structure, the soil effects are not perfectly obtained. Furthermore, the damping definition of a soil-structure system is extended to structures to consider bilinear behavior.     

基于土-结构体系对建筑结构响应的减震效果评估

提出一种基于土-结构体系地震记录的土-结构相互作用(SSI)的减震评估方法。该方法采用简化的SSI模型,通过系统辨识确定模型参数。将上部建筑结构地震反应的SSI减震效应分解为惯性相互作用和运动相互作用,同时还提出由惯性相互作用和运动相互作用单独降低结构响应的方法。将2011年东北地震太平洋沿岸期间两栋中层建筑用此方法进行分析,结果表明:当建筑物结构响应进入非弹性范围时,惯性相互作用的减震效果降低。