Vibration characteristics of a suspension bridge under traffic and no traffic conditions

This paper presents the variations of vibration under different traffic conditions on the Fatih Sultan Mehmet suspension bridge in Istanbul, Turkey.The main intention is to determine the vibration amplifications under heavy-traffic as opposed to no-traffic conditions. This is the first study in this particular area that has been performed on this bridge, over which an average of 200,000 cars pass daily. Two full-scale ambient vibration surveys were carried out on two different days to determine the response of the bridge to diverse traffic conditions. Initial measurements were taken as the bridge experienced heavy stress conditions caused by rush-hour traffic. Secondary measurements were recorded after closing the bridge to traffic. The data were analyzed to gauge the vibration effects of heavy-traffic conditions on the bridge and to determine the effects of different traffic conditions on the free vibration characteristics of the bridge. The analyses were performed utilizing different amplification methods. Results show that there are important differences in the amplifications of the vibration amplitudes. Especially heavy-traffic on the bridge causes the vibration response of the bridge to be intensified in comparison to no-traffic conditions. Additionally, predominant frequencies are shifted as a direct result of traffic load acting on the bridge. Even more importantly and is probably analogous for all long-span bridges, is the fact that any movement causing vibration on the bridge is carried and amplified along its length. These significant amplifications indicate the important effect of varying traffic loads and how the bridge responds to the diverse movements it experiences. Copyright © 2011 John Wiley & Sons, Ltd.     

Vibration studies and tests of liquid storage tanks

Theoretical and experimental investigations of the dynamic behaviour of ground-supported, deformable, cylindrical liquid storage tanks were conducted. The study was carried out in three phases: (I) a detailed theoretical treatment of the coupled liquid-shell system for tanks rigidly anchored to their foundations; (II) an experimental investigation of the dynamic characteristics of full-scale tanks; and (III) a development of an improved seismic design procedure.     

Vibration tests and analysis of a model arch dam

Vibration tests were conducted on a 1/24-scale model of the North Fork Dam, a double-curvature arch dam, to determine natural frequencies, mode shapes and hydrodynamic pressures. The mode shapes and natural frequencies were determined from tests using two vibrators mounted on the crest of the dam. Hydrodynamic pressures at the dam/reservoir interface were determined from tests in which the vibrator was attached to the downstream foundation of the dam. The hydrodynamic pressures calculated using Westergaard's theory and a theory for arch dams developed by Perumalswami and Kar accurately predicted the measured pressure at frequencies below the first mode frequency of the dam. The differences in the two theories were insignificant. The Structural Analysis Program (SAP), a linear three-dimensional (3-D) finite element code, was used to compute mode shapes and frequencies for the dam with its base fixed and with a foundation. Numerical solution schemes used in the finite element analysis consisted of a Ritz analysis and a subspace iteration method. Calculations were conducted for both full and empty reservoir conditions. The accuracy of the Ritz analysis improved considerably as more nodes in flexible regions of the dam were loaded. However, the lowest eigenvalues were computed using the subspace iteration method. For the full reservoir, the natural frequencies decreased by 20-30 per cent when the foundation was included in the finite element model. The difference was less when the reservoir was empty. The calculations using the subspace iteration scheme and including the foundation agreed closely with experimental mode shapes and corresponding natural frequencies.     

Dynamic investigation of a hybrid suspension and cable‐stayed bridge

Ambient and forced vibration tests were carried out on the Beauharnois bridge, a unique, 177‐m combined suspension and cable‐stayed structure near Montreal, Canada. A rehabilitation program was completed on the bridge during which the deck was completely rebuilt with an orthotropic slab on two steel trusses. The rehabilitation program also included the addition of two pairs of stay cables on both towers, creating a hybrid suspension system. The paper presents a series of dynamic tests performed to evaluate the dynamic properties and the dynamic amplification factor (DAF) for the rehabilitated bridge. The experimental program involved the measurement of vertical, transverse, and longitudinal acceleration responses of the deck and tower under ambient and controlled traffic loads. Displacement, strain, and integrated acceleration DAFs were computed under different loading conditions. Modal properties were evaluated and used to correlate a three‐dimensional finite element model for the bridge, including non‐linear cable behaviour. The paper discusses the experimental setup as well as the techniques used to evaluate vibration frequencies, mode shapes, and the DAF. Correlation of numerical dynamic properties and experimental results is also presented. Copyright © 2000 John Wiley & Sons, Ltd.     

Spatially coupled vibrations of a suspension bridge under random highway traffic

The problem of spatial dynamic response of a suspension bridge to the passage of trains of concentrated forces with random values is considered. The arrival of forces at the bridge is assumed to constitute a Poisson process of events. Such an excitation process is an appropriate model of vehicular traffic loads acting on the bridge. The bridge is idealized by a single-span thin-walled beam underslung to two cables. The response of the bridge in the space-time domain is described by a coupled system of non-linear, integro-differential equations. The dynamic influence functions of vertical and horizontal deflections at each cross-section point are obtained for the linear case. Cumulants and probability density functions of response are determined. Numerical methods have been used to develop a computer-oriented algorithm aimed at the numerical solution of the problem. As examples, numerical results for a particular bridge with some practical load cases are presented and illustrated by graphs.     

The estimation of earthquake-generated additional tension in a suspension bridge cable

A simple formula, equation (15), is derived for the peak additional cable tension that can be expected in a suspension bridge undergoing earthquake excitation. The method involves application of the response spectrum technique and rests on several plausible assumptions. The formula gives a reasonable upper bound, suitable for preliminary design estimates, irrespective of whether the ground motions at each end are in-phase or, as is probable with a long span bridge, whether they are out-of-phase.     

Analysis and prevention of suspension bridge flutter in construction

It is known that the maximum danger of flutter-type aerodynamic instability for suspension bridges occurs during the early erection phases of the deck. The effectiveness of two different provisional measures for increasing the instability critical windspeed in such conditions is evaluated for a long span suspension bridge with flat box stiffening girder.     

某悬索桥振动特性现场测试及数值模拟

振动频率是结构的固有特性,由结构材料和构造所决定,是结构设计阶段验算的重要参数。结构现场振动频率的测试结果既可用来校核设计质量也可作为结构健康诊断的重要依据。通过脉动和人工激励,获得了某悬索跨越结构整体前三阶振动频率,同时依据结构健康诊断的对称信号法,测试了主索、锚固索等局部结构的振动频率。测试结果与模型实验和有限元分析结果接近,并且证明对称的局部结构具有对称的振动频率。     

自锚式悬索桥主桥动力特性分析

以某自锚式悬索桥主桥为研究对象,采用M IDAS有限元程序,建立了该桥的空间力学计算模型,利用子空间迭代法计算了该桥梁结构的自振频率和振型,结合计算结果对桥梁的动力特性和刚度特点进行了讨论。计算结果可为该类型桥梁的设计、施工以及使用阶段的健康检测和维护提供技术参数和依据。     

Ambient vibration survey of the bosporus suspension bridge

Traffic and wind excitation has been used to obtain the dynamic characteristics of the first Bosporus (Bogazici) Suspension Bridge. Structural symmetry and the absence of suspended side-spans allowed attention to be focused on the main span and the Asian tower. For the main span, 18 vertical and 20 lateral modes were obtained, including torsional modes. For the tower, 12 longitudinal and 12 lateral modes were identified. All these models lie in the range 0–1-1 Hz. A detailed comparison is given between these modes and corresponding calculated modes, obtained by use of a three-dimensional finite element model which includes a geometric stiffness matrix. Of particular interest is the validity of the theoretical model used for the box-deck, because of its subsequent use in response studies of asynchronous seismic excitation. Comparison with a more limited study made in 1973 shows that the bridge continues to behave as it was designed to behave, particularly with regard to the deck-tower interface. From natural frequency measurements of two hangers, the load which they carry was assessed.     

哪吒大桥地震响应分析

建立了哪吒大桥———钢桁加劲梁悬索桥的三维有限元计算模型,对该桥进行了振动特性和地震响应研究,对比分析了一致激励下和考虑局部场地效应、行波效应非一致激励下的桥梁地震时程响应的差异。计算结果表明,由于悬索桥的钢桁加劲梁采用半飘浮体系,纵向约束较弱,桥梁第1阶振型为钢桁加劲梁的纵飘;各阶振型的频率呈现出密集分布的形态;考虑地震波传播速度产生的行波效应的影响,桥梁的地震响应与一致激励下的地震响应有一定差异;局部场地效应对哪吒大桥的地震响应影响较大,特别是对大缆应力及结构变形影响很大;哪吒大桥在7度设防(地震波峰值加速度不超过0.15g)时,大桥的抗震安全性是可以得到保证的。     

结构参数对双链式悬索桥地震响应的影响

以某双链式悬索桥为工程背景,建立了空间有限元计算模型.运用几何非线性时程分析方法,分析并探讨了不同垂跨比、阻尼比,塔桥不同连接方式和双链位置对双链式悬索桥地震响应的影响.分析表明,竖向地震动分量使桥塔轴力迅速增大,加劲梁采用钢筋混凝土边梁可有效减小地震响应;塔梁间采用简支连接方案其抗震性能比固定支座连接方案、滑动支座连接方案要好;双链式悬索桥横向抗震性能优于单链式悬索桥.对双链式悬索桥抗震性能的研究,为此类悬索结构的设计选型,乃至减震、隔震及抗震加固提供了参考.     

墩底可抬升刚构桥模型振动台试验研究

桥墩基础抬升可以切断地震动向上部结构传播,基础与地基碰撞还可以耗散地震能量,然而已有研究大多集中在数值模拟或理论分析,缺乏大型振动台试验验证.文中设计制作了一座基础固接和基础可抬升的连续刚构桥模型并进行振动台试验.对2种模型的动力特性、加速度响应和位移响应等进行对比分析.结果表明:基础可抬升桥梁较基础固接桥梁固有频率小...     

大跨度悬索桥振动控制的双向TMD参数研究

用时域内的逐步积分法对无ＴＭＤ和带有不同参数ＴＭＤ的悬索桥进行了时程分析,结果表明,ＴＭＤ的参数对结构的动力特性的抑振效果均有较大的影响,ＴＭＤ的频率不仅影响振动控制的效果也影响结构的表观阻尼,扭转振动的控制可由侧向ＴＭＤ来实现。     

钢筋混凝土加劲桁架悬索桥的模态分析

采用ANSYS软件对钢筋混凝土土加劲架悬索桥进行有限元分析，提出较为准确的建模方法和最优初始应变的取值条件，使计算结果能满足工程分析精度要求。在此基础上进行了模态分析，计算了钢筋混凝土加劲杵架悬索桥的动力特征及主要结构参数对动力学特性的影响，并对福建三明下洋悬索桥进行实桥测试与分析比较，研究结果有助于对该类悬索桥力学性能的认识。     

Shaking table tests of a reinforced concrete bridge pier with a low-cost sliding pendulum system

After the occurrence of various destructive earthquakes in Japan, extensive efforts have been made to improve the seismic performance of bridges. Although improvements to the ductile capacities of reinforced concrete (RC) bridge piers have been developed over the past few decades, seismic resilience has not been adequately ensured. Simple ductile structures are not robust and exhibit a certain level of damage under extremely strong earthquakes, leading to large residual displacements and higher repair costs, which incur in societies with less-effective disaster response and recovery measures. To ensure the seismic resilience of bridges, it is necessary to continue developing the seismic design methodology of RC bridges by exploring new concepts while avoiding the use of expensive materials. Therefore, to maximize the postevent operability, a novel RC bridge pier with a low-cost sliding pendulum system is proposed. The seismic force is reduced as the upper component moves along a concave sliding surface atop the lower component of the RC bridge pier. No replaceable seismic devices are included to lengthen the natural period; only conventional concrete and steel are used to achieve low-cost design solutions. The seismic performance was evaluated through unidirectional shaking table tests. The experimental results demonstrated a reduction in the shear force transmitted to the substructure, and the residual displacement decreased by establishing an adequate radius of the sliding surface. Finally, a nonlinear dynamic analysis was performed to estimate the seismic response of the proposed RC bridge pier.     

Ambient vibration test of an anchorage of South Bisan-seto suspension bridge

Dynamical properties of an anchorage of South Bisan-seto Suspension Bridge are determined by means of in situ measurement of the ambient vibrations. This anchorage, being located in the sea, is tall and slender; its natural frequencies are expected to be low. On the other hand, the elasticity of the ground estimated from the conventional geological data is highly dispersed. It even suggests the possibility of resonance between the towers and this anchorage. It is therefore necessary to know its natural frequencies precisely. Seismic motions and microtremors are acquired and used properly. The observation is repeated every year during the period of construction of the anchorage so that its inertial properties change significantly. First modes of the longitudinal and the transversal vibrations of the anchorage are identified and their frequencies determined. The elasticity modulus estimated from these natural frequencies is about five times as large as that of the geological survey while it is consistent with that obtained from the velocity of the elastic waves of the ground.     

钢-混凝土叠合梁自锚式悬索桥动力特性测试分析

针对钢-混凝土叠合梁自锚式悬索桥的结构特点,在总结以往大跨径桥梁有限元模型的基础上,采用组合有限元法以梁、板、杆、索单元建立邵阳市桂花大桥三维有限元模型,计算分析获得结构的主要频率和振型。通过现场测试获得大桥环境激励和行车激励作用下的响应时程,分析得到结构的主要频率、振型和阻尼比,引入模态置信因子对计算模态信息和实测模态信息进行比较分析,以验证数值模型和实测结果的准确性。研究结果可为全面了解钢-混凝土叠合梁自锚式悬索桥的动力特性、准确预测结构的动力响应以及类似桥梁的抗震、抗风设计提供重要依据。     

System identification of the Vincent Thomas suspension bridge using earthquake records

The Vincent Thomas Bridge in the Los Angeles metropolitan area, is a critical artery for commercial traffic flow in and out of the Los Angeles Harbor, and is at risk in the seismically active Southern California region, particularly because it straddles the Palos Verdes fault zone. A combination of linear and non‐linear system identification techniques is employed to obtain a complete reduced‐order, multi‐input–multi‐output (MIMO) dynamic model of the Vincent Thomas Bridge based on the dynamic response of the structure to the 1987 Whittier and 1994 Northridge earthquakes. Starting with the available acceleration measurements (which consists of 15 accelerometers on the bridge structure and 10 accelerometers at various locations on its base), an efficient least‐squares‐based time‐domain identification procedure is applied to the data set to develop a reduced‐order, equivalent linear, multi‐degree‐of‐freedom model. Although not the main focus of this study, the linear system identification method is also combined with a non‐parametric identification technique, to generate a reduced‐order non‐linear mathematical model suitable for use in subsequent studies to predict, with good fidelity, the total response of the bridge under arbitrary dynamic environments. Results of this study yield measurements of the equivalent linear modal properties (frequencies, mode shapes and non‐proportional damping) as well as quantitative measures of the extent and nature of non‐linear interaction forces arising from strong ground shaking. It is shown that, for the particular subset of observations used in the identification procedure, the apparent non‐linearities in the system restoring forces are quite significant, and they contribute substantially to the improved fidelity of the model. Also shown is the potential of the identification technique under discussion to detect slight changes in the structure's influence coefficients, which may be indicators of damage and degradation in the structure being monitored. Difficulties associated with accurately estimating damping for lightly damped long‐span structures from their earthquake response are discussed. The technical issues raised in this paper indicate the need for added spatial resolution in sensor instrumentation to obtain identified mathematical models of structural systems with the broadest range of validity. Copyright © 2003 John Wiley & Sons, Ltd.