大跨度预应力混凝土连续刚构桥的动力特性分析

介绍了福建泉州后渚大桥——大跨度预应力混凝土连续刚构桥的现场环境振动实验．并利用频域中的单模态识别法(SDOFI)、峰值法(PP)和时域中的随机子空间识别法(SSI)分别进行桥梁动力特性识别。利用ANSYS建立了全桥三维有限元模型并进行了理论模态分析,基于参数分析和环境振动测试结果对有限元模型进行了标定,建立了该桥的基准有限元模型,该模型可服务于桥梁长期健康监测与状态评估。     

Identification of modal parameters of a 600-m-high skyscraper from field vibration tests

Accurate prediction of the dynamic responses of a high-rise building subjected to dynamic loads such as earthquake and wind excitations requires the information of its structural dynamic properties such as modal parameters including natural frequencies and damping ratios. This paper presents the identification results of the modal parameters based on field vibration tests on a 600-m high skyscraper. A set of tests, including ambient vibration test (AVT) and free vibration test (FVT), were conducted on the skyscraper to identify its modal parameters. Firstly, this paper presents and discusses the modal parameters of the skyscraper assessed by several identification methods applied to the AVT measurements. These methods include the wavelet transform (WT) method, the stochastic subspace identification (SSI) method, and the random decrement technique (RDT). Secondly, an active mass damper (AMD) system with total mass 1000 tons equipped into the skyscraper was used to excite the building for estimation of the modal parameters by FVT. Thirdly, this paper presents observations on the structural dynamic behavior of the skyscraper with the operation of the AMD system during a typhoon event. The field measurement results show that the AMD system functioned efficiently for suppression of the wind-induced vibrations of the skyscraper during the typhoon. This paper aims to further understand the structural dynamic properties of super-tall buildings and provide useful information for structural design and vibration control of future skyscrapers.     

Attenuation of ground vibrations using in-filled wave barriers

Ground vibrations generated by construction activities can adversely affect the structural health of adjacent buildings and foundations supporting them. Therefore propagation and rate of attenuation of construction induced ground vibrations is important during construction activities, particularly in urban areas where constructions are carried out in the vicinity of existing structures. In practice wave barriers are installed in the ground to mitigate the ground vibration propagation and hence to minimise the effect of ground vibrations on surrounding structures. Different types of fill materials such as bentonite, EPS geofoam and concrete are used in constructing wave barriers. In this study, a three-dimensional finite element model is developed to study the efficiency of different fill materials in attenuating ground vibrations. The model is first verified using data from full scale field experiments, where EPS geofoam has been used as a fill material in wave barriers. Then the same model has been used to evaluate the efficiency of open trenches, water filled wave barriers and EPS geofoam filled wave barriers on attenuation of ground vibrations. EPS geofoam is found to be the most efficient fill material, providing attenuation efficiency closer to open trenches. The efficiency of EPS geofoam and water filled wave barriers can be significantly increased by increasing the depth of the wave barrier.     

The dependence of response spectrum on the tectonic ambient shear stress field

Introduction The acceleration response spectrum and peak ground acceleration are the necessary and im-portant parameters in earthquake-resistant design at present. They are still active research field. With the increase of digital high accurate strong motion observation data, especially the earth-quakes of Loma Prieta (M=7.0) in 1989; Landers (M=7.3) in 1992; Big Bear (M=6.4) in 1994 and Northridge (M=6.7) in 1994 in USA; Kozani (M=6.6) earthquake and afteshocks in 1995 in Greece; Dinar…     

预应力混凝土连续梁桥振动特性变异及舒适度评价

由于预应力损失、混凝土收缩徐变、超载等原因,预应力混凝土桥梁出现了较强的振动。本文通过对福建省福州市闽江三桥全桥环境振动试验和正常运营条件下24小时的监测,研究预应力混凝土连续梁桥的动力特性及其变异和振动舒适度,结果表明:在较为稳定的风和温湿度环境下,固有频率在一天内的最大变化为3.06%,模态阻尼比的最大变化为37.93%,舒适度的狄克曼指标降到“能忍受短时间振动”的区域。     

利用背景噪声进行地震成像的新方法

利用背景地震噪声提取格林函数，即通过一对台站记录的地震噪声进行互相关计算，来近似台站间的格林函数，并进一步通过地震成像获取地下结构的认识，成为最近跨学科研究的热点问题之一.本文首先叙述了其发展背景及过程，然后分别从四个角度，简要介绍了目前对其物理原理的解释.如果该方法能得到进一步的发展，将不再依赖于天然地震以及人工地震，而仅仅利用地震台站记录的地震噪声便可获取高分辨率的地下成像.该方法将会大大促进地震学的发展.     

Stationary phase approximation in the ambient noise method revisited

The method of extracting Green's function between stations from cross correlation has proven to be effective theoretically and experimentally.It has been widely applied to surface wave tomography of the crust and upmost mantle.However,there are still controversies about why this method works.Snieder employed stationary phase approximation in evaluating contribution to cross correlation function from scatterers in the whole space,and concluded that it is the constructive interference of waves emitted by the scatterers near the receiver line that leads to the emergence of Green's function.His derivation demonstrates that cross correlation function is just the convolution of noise power spectrum and the Green's function.However,his derivation ignores influence from the two stationary points at infinities,therefore it may fail when attenuation is absent.In order to obtain accurate noise-correlation function due to scatters over the whole space,we compute the total contribution with numerical integration in polar coordinates.Our numerical computation of cross correlation function indicates that the incomplete stationary phase approximation introduces remarkable errors to the cross correlation function,in both amplitude and phase,when the frequency is low with reasonable quality factor Q.Our results argue that the distance between stations has to be beyond several wavelengths in order to reduce the influence of this inaccuracy on the applications of ambient noise method,and only the station pairs whose distances are above several (5) wavelengths can be used.     

Three dimensional shear wave velocity structure of the crust and upper mantle beneath China from ambient noise surface wave tomography

We determine the three-dimensional shear wave velocity structure of the crust and upper mantle in China using Green's functions obtained from seismic ambient noise cross-correlation.The data we use are from the China National Seismic Network,global and regional networks and PASSCAL stations in the region.We first acquire cross-correlation seismograms between all possible station pairs.We then measure the Rayleigh wave group and phase dispersion curves using a frequency-time analysis method from 8 s to 60 s.After that,Rayleigh wave group and phase velocity dispersion maps on 1° by 1° spatial grids are obtained at different periods.Finally,we invert these maps for the 3-D shear wave velocity structure of the crust and upper mantle beneath China at each grid node.The inversion results show large-scale structures that correlate well with surface geology.Near the surface,velocities in major basins are anomalously slow,consistent with the thick sediments.East-west contrasts are striking in Moho depth.There is also a fast mid-to-lower crust and mantle lithosphere beneath the major basins surrounding the Tibetan plateau (TP) and Tianshan (Junggar,Tarim,Ordos,and Sichuan).These strong blocks,therefore,appear to play an important role in confining the deformation of the TP and constraining its geometry to form its current triangular shape.In northwest TP in Qiangtang,slow anomalies extend from the crust to the mantle lithosphere.Meanwhile,widespread,a prominent low-velocity zone is observed in the middle crust beneath most of the central,eastern and southeastern Tibetan plateau,consistent with a weak (and perhaps mobile) middle crust.     

Love wave tomography in Italy from seismic ambient noise

We estimate Love wave empirical Green's functions from cross-correlations of ambient seismic noise to study the crust and uppermost mantle structure in Italy.Transverse-component ambient noise data from October 2005 through March 2007 recorded at 114 seismic stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) national broadband network,the Mediterranean Very Broadband Seismographic Network (MedNet) and the Austrian Central Institute for Meteorology and Geodynamics (ZAMG) yield more than 2 000 Love wave group velocity measurements using the multiple-filter analysis technique.In the short period band (5-20 s),the cross-correlations show clearly one-sided asymmetric feature due to non-uniform noise distribution and high local activities,and in the long period band (20 s) this feature becomes weak owing to more diffusive noise distribution.Based on these measurements,Love wave group velocity dispersion maps in the 8-34 s period band are constructed,then the SH wave velocity structures from the Love wave dispersions are inverted.The final results obtained from Love wave data are overall in good agreement with those from Rayleigh waves.Both Love and Rayleigh wave inversions all reveal that the Po plain basin is resolved with low velocity at shallow depth,and the Tyrrhenian sea is characterized with higher velocity below 8 km due to its thin oceanic crust.     

Torsional response of a rigid circular foundation on a saturated half-space to SH waves

An analytical approach is used to study the torsional vibrations of a rigid circular foundation resting on saturated soil to obliquely incident SH waves. Biot’s poroelastic dynamic theory is considered to characterize the saturated soil below the foundation, which is solved by Hankel transform later. In order to consider the scattering phenomena caused by the existence of the foundation, the total wave field in soil is classified into free-field, rigid-body scattering field and radiation scattering field. According to the classification of wave field and the mixed boundary-value conditions between the soil and the foundation, torsional vibrations of the foundation are formulated in two sets of dual integral equations. Then, the dual integral equations are reduced to Fredholm integral equation of the second kind to be solved. Combining with the dynamic equilibrium equations of the foundation, the expressions for the torsional vibrations of the foundation are obtained. Numerical results are presented to demonstrate the influence of excitation frequency, incident angle, the torsional inertia moment of the foundation and permeability of the saturated half-space on the torsional vibrations of the foundation.