Hydrodynamics in Holocene Lake Mega-Chad

Holocene Lake Mega-Chad (LMC) was the largest late Quaternary water-body in Africa. The development of this giant paleo-lake is related to a northward shift of the isohyetes interpreted as evidence for an enhanced Monsoon (African Humid Period). Numerous preserved coastal features have been described all around the LMC shore. Such features reveal the main paleo-hydrodynamical tendencies. In the context of a closed water-body like LMC, hydrodynamics are forced mainly by winds. We use a three-dimensional numerical model (SYMPHONIE) to simulate the mean hydrodynamics in LMC under both Harmattan-like (northeasterly trade winds) and Monsoon-like (southwesterly winds) forcings. The northern part of LMC displays coastal features, such as sand spits, that are consistent with the simulations forced by Harmattan-like winds. Geomorphic features related to Monsoon-driven hydrodynamics are not clearly expressed. They could have developed during the early stage of LMC but subsequently reworked. At the time of sand-spit building, Harmattan-like driven hydrodynamics prevailed and related coastal features were preferentially preserved in the sedimentary record.     

中国探空气球水平漂移总体特征分析

利用2003年4月至2008年8月全国119个探空台站采集的气球漂移信息,分析了中国探空资料水平漂移距离、漂移距离标准差以及漂移方向的分布特征。研究表明:各层探空气球的漂移距离冬季明显大于夏季;一般探空气球的漂移距离随探空高度的增加而增大,但在夏季南方地区由于风向随高度的反转会出现探空气球水平漂移距离随高度减小的状况。探空气球水平漂移距离的标准差随高度增加而增大,且其大值区分布基本上与水平漂移距离的大值区分布一致。从气球漂移方向上看,冬季的情况相对简单,全国各探空台站各层的漂移方向基本以向东为主;但夏季探空气球漂移的方向比较复杂,在对流层低层,全国各台站漂移方向基本以向东为主,而在对流层高层和平流层,北方地区漂移方向基本向东,而南方地区以向西为主。     

顾及星间单差残差的GPS斜路径水汽含量估计

基于精密单点定位(precise point positioning,PPP)技术,对海上动态GPS斜路径水汽含量(SWV)估计进行研究。在现有计算方法的基础上,提出了顾及星间单差残差的SWV提取方法。利用渤海湾船载动态GPS测量数据和同步气象观测数据,对新方法的有效性进行了验证。结果表明,相对于MM5模式积分水汽结果,采用新方法计算的SWV值的RMS为1.2mm,比传统"非差残差法"减小了1mm。     

钢筋混凝土框架结构造价与失效概率之间的近似关系研究

本文以层间位移角作为结构性能参数,分析了钢筋混凝土框架结构层间变形能力,以及在水平地震作用下层间位移反应。考虑钢筋混凝土框架结构材料强度和几何尺寸以及地震作用的不确定性,得出了在设计基准期内结构的失效概率。同时分析了不同设计参数下结构的最小造价,在此基础上,确定了结构最小造价和失效概率之间的近似关系。目的是为采用“投资—效益“准则确定该类型结构目标性能水平提供分析依据,从而为采用基于性能抗震设计理念制定建筑结构抗震设计规范提供基础研究。本文中,结构失效概率指结构最终极限状态的失效概率。     

A static predictor of seismic demand on frames based on a post‐elastic deflected shape

Predictors of seismic structural demands (such as inter‐storey drift angles) that are less time‐consuming than nonlinear dynamic analysis have proven useful for structural performance assessment and for design. Luco and Cornell previously proposed a simple predictor that extends the idea of modal superposition (of the first two modes) with the square‐root‐of‐sum‐of‐squares (SRSS) rule by taking a first‐mode inelastic spectral displacement into account. This predictor achieved a significant improvement over simply using the response of an elastic oscillator; however, it cannot capture well large displacements caused by local yielding. A possible improvement of Luco's predictor is discussed in this paper, where it is proposed to consider three enhancements: (i) a post‐elastic first‐mode shape approximated by the deflected shape from a nonlinear static pushover analysis (NSPA) at the step corresponding to the maximum drift of an equivalent inelastic single‐degree‐of‐freedom (SDOF) system, (ii) a trilinear backbone curve for the SDOF system, and (iii) the elastic third‐mode response for long‐period buildings. Numerical examples demonstrate that the proposed predictor is less biased and results in less dispersion than Luco's original predictor. Copyright © 2006 John Wiley & Sons, Ltd.     

Simulation Studies on the Settlement of Composite Foundation with Contact Element of Zero Thickness

INTRODUCTIONCurrently, the contact element is a problematicpoint problem in the mechanics of rock and soil engi neering. In the tenth international conference onComputer Methods and Advances in Geomechanics,held in America in January, 2001, the …     

Trend Extraction Using Average Interpolating Subdivision

Spatial averages are robust components of data sets, and therefore well suited as starting points for trend analysis. A natural way of constructing a global trend surface from average values, is to use a subdivision algorithm. This procedure will in some cases reproduce exactly the drift of the data set. Numerical experiments on multibeam echo sounder data show that residual data sets contain no long range correlations. In addition, a variogram analysis clearly identifies the amount of noise in the original data.     

Estimation of seismic drift and ductility demands in planar regular X‐braced steel frames

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.     

统计同步算法原理及在局域网中的应用

针对局域网中计算机间的时间同步问题,分析了统计同步算法PCS的基本原理,介绍了局域网中网络延迟精确测定的方法,提出了利用一台时统设备实现局域网中PCS算法精度测试的方法,给出了频率漂移的修正公式,并在局域网中对算法性能进行了测试。测试结果表明,计算机时钟的漂移是影响算法精度的主要因素。经过频漂修正,PCS算法性能得到了很大的改善,同步周期较设计周期大大增加,算法同步精度在20min内优于1 ms。     

Deep water bottom current deposition in the northern South China Sea

There are some active bottom currents on the northern continental slope of the South China Sea (SCS). Reflection seismic profiles show that the bottom current channels occur in the water depth range of 1000 to 2700 m,extending from the NE to the SW,leading to accumulation of discontinuous drifts with higher sedimentation rates on the eastern side of the channel. The stacking pattern of the layers sug-gests that these drifts propagated southwestward,following the direction of the bottom currents. One sedimentary drift to the southeast of the Dongsha Islands has the highest sedimentation rate of 97cm/ka in the last 12 ka. The sedimentary characteristics of the sediment layers indicate that these bottom currents are most likley caused by the water movement of a branch of the West Pacific Ocean Current,which enters the northern SCS via the Bashi Strait. Once formed,the bottom currents trans-port sediments along the northern slope of SCS southwestward and finally disappear into the central basin of the SCS. Due to the bottom current activity,the deep-sea sedimentary process in the northern SCS is complex.