Erosion and Nutrient Loss on Sloping Land under Intense Cultivation in Southern Vietnam

To help improve the well‐being of the local people, a joint Vietnamese‐UK team set out to establish a way of estimating soil and nutrient losses under different land management scenarios, using field data extrapolated through remote sensing and GIS, to obtain catchment‐wide estimates of the impact of land cover change. Immigration from remote provinces to the Dong Phu District of Binh Phuóc Province, about 120 km north of Ho Chi Minh City, has led to disruption of soil surface stability on easily eroded clayey sandstones, creating rapid nutrient depletion that affects crop yields and siltation in the channel of the Rach Rat river downstream. The poor farmers of the areas see crop yields drop dramatically after two or three years of cultivation due to the fertility decline. Soil loss varies dramatically between wet season and dry season and with ground cover. Erosion bridge measurements showed a mean loss of 85.2 t ha^?1 y^?1 under cassava saplings with cashew nuts, 43.3 t ha^?1 y^?1 on uncultivated land and 41.7 t ha^?1 y^?1 under mature cassava. The rates of erosion were higher than those reported in many other parts of Vietnam, reflecting the high erodibility of the friable sandy soils on the steep side‐slopes of the Rach Rat catchment. However, although the actual measurements provide better soil loss data than estimates based on the parameters of soil loss equations, a large number of measurement sites is needed to provide adequate coverage of the crop and slope combinations in this dissected terrain for good prediction using GIS and remote sensing.     

近代皖北交通中心变迁再探--以津浦铁路淮河大桥选址变更为中心

临淮关凭借传统交通与商业中心的地位,成为津浦铁路淮河大桥桥址的首选之地,但由于该处地势低洼,不易建桥,桥址最终选定在蚌埠。蚌埠得津浦铁路和淮河水运之利,迅速崛起,成为皖北地区新的交通与商业中心,而临淮关则渐趋衰落,最终成为淮河岸边一个普通小城镇。究其原因,主要是由于桥址变更,引起交通区位优势的转换,蚌埠的交通优势转化为产业优势;铁路与淮河水运在蚌埠的有机结合以及政治因素的助推。     

边界:建立睦邻友好国家关系的纽带和桥梁

本文就边界研究中的一系列重大问题,诸如边界概念、边界研究的内容和特点、边界研究中必须注意区别的几种关系、边界变化的规律、边界研究的条件和意义,以及我国过去边界问题研究的教训等,逐一进行了理性的思考,并在这种理性思考的过程中,根据新时期的新形势和我国政府提出的新安全观、国际观与睦邻友好的外交政策,提出边界是建立睦邻友好国家关系的纽带和桥梁的论点,进而提出边界学概念。期望能对我国边界理论的建设和边界问题研究事业的进展有所助益。     

Trajan's Bridge over the Danube

For the final conquest of Dacia, Apollodorus of Damascus, Trajan's principal engineer, built a monumental bridge over the Danube in AD 105. This paper aims at understanding the historical circumstances and environmental conditions during the building of Trajan's bridge, as well as its construction technology, by analysing the limited written and iconographic ancient sources. It also offers an overview of archaeological research on the structure to date, with special attention to the 2003 underwater archaeological survey, which established the state of preservation of the remaining piers, emphasizing the need for their long-term protection.
© 2009 The Author     

SEISMIC RESPONSE CONTROL OF A CABLE-STAYED BRIDGE BY VARIABLE DAMPERS

Cable-stayed bridges exhibit unique responses under a strong motion. It is partly due to the complexity in their damping mechanism. Recently, the benchmark problem of a cable-stayed bridge was developed to clarify the effectiveness of various seismic control strategies. Due to the new development of magnetorheological dampers, the application of variable dampers in bridges becomes possible. In this study, the effectiveness of the nonlinear viscous damping force scheme and the two-step friction damping force scheme are investigated. It is found that the nonlinear viscous damping force scheme is effective to control the response of the cable-stayed bridge with less demand for the damping force capacity of a damper. In addition, the two-step friction damping force scheme shows the improvement over conventional friction damping because the energy dissipation of a damper can be increased.     

SEISMIC RESPONSE CONTROL OF CABLE-STAYED BRIDGE USING DIFFERENT CONTROL STRATEGIES

The goal of this research is to study the effect of cable vibration through a number of control cases of a cable-stayed bridge. In order to consider the complicated dynamic behaviour of the full-scale bridge, a three-dimensional numerical model of the MATLAB-based analysis tool has been developed by the complete simulation of the Gi-Lu bridge. The dynamic characteristics of cables in the cable-stayed bridge are verified between the field experiment and the result from numerical simulation using geometrically nonlinear beam elements in MATLAB program. Three types of control devices are selected to reduce the response of the bridge deck which includes: actuators, viscous-elastic dampers with large capacity, and base isolations. Moreover, two types of control devices, MR dampers and viscous dampers, are installed either between the deck and cables and/or between two neighbouring cables for controlling the cable vibration. A modified bi-viscous model combined with convergent rules is used to describe the behaviour of MR dampers. Finally, through evaluation criteria the control effectiveness on the cable-stayed bridge using different control strategies is examined.     

SEISMIC BEHAVIOUR OF A R/C WALL: NUMERICAL SIMULATION AND EXPERIMENTAL VALIDATION

This paper describes the numerical simulation of the seismic behaviour of a mock-up of a six-floor building, constituted by two parallel R/C walls and experimentally tested on a shaking table. Within the scope of an international benchmark the mock-up was submitted to three earthquakes with intensities up to 0.71 g, which induced nonlinear behaviour in the concrete and reinforcement. For the numerical simulations concrete is discretised with 2D finite elements, and its behaviour reproduced via a constitutive model with two scalar damage variables. Steel rebars are discretised with 2-noded truss elements, and their constitutive behaviour under cyclic conditions reproduced by the Menegottb-Pinto model. Specific attention is devoted to Rayleigh damping, focusing on two different strategies: (i) disregarding the damping contribution, or (ii) adopting a damping matrix that takes into account the stiffness changes during the nonlinear analyses. Main results and strategies for simulating the benchmark axe presented, with emphasis on the comparison between the numerical and the experimental results, which show good agreement when the damping contribution is neglected.     

明代石拱桥永昌桥的加固修缮技术研究

为明晰明代石拱桥的结构性能,并提出适宜的加固修缮方法,以典型的明代石拱桥永昌桥为例进行研究。首先对其进行精确测绘,获得其准确可靠的几何信息;然后对其建筑形制、构造工艺及残损状况进行研究,为结构性能分析提供依据;接着利用ANSYS软件建立永昌桥主体结构的有限元模型,进行包括荷载影响分析、重要度分析和裂缝分析的结构性能研究;最后基于现状分析和结构性能研究,通过对比拱背套钢筋混凝土拱加固法和拱背粘贴碳纤维布加固法对石拱桥的结构性能的影响,提出了明代石拱桥永昌桥的加固修缮方法。研究成果可为同类型石拱桥的加固修缮提供参考。     

Experimental Studies and Vulnerability Assessment of Timber-Arched Lounge Bridges

ABSTRACT

There are many preserved timber-arched lounge bridges in China and some of these bridges are hundreds of years old. The woven arch is the main bearing structure of a timber-arched lounge bridge. Few studies still exist about woven arch mechanical performance, a fact that contributes to the improper repair. This article concerns the structural performance of the woven arch. The results of experimental studies on deformation, stress distribution, force-transferring mechanism, and failure modes were presented. Two woven arch models with and without П-shape braces under symmetrical load and asymmetrical load were investigated. The results indicated that the woven arch had high bearing capacity, especially under symmetrical load. The comparison of experimental results obtained for models with and without brace showed that braces modified the failure mechanism and increased the load carrying capacity and integrity. Based on the failure conditions observed experimentally, limit criteria were defined in order to facilitate vulnerability assessment of timber lounge bridges.