Stability of composite river banks

The stability of a river bank depends on the balance of forces, motive and resistive, associated with the most critical mechanism of failure. Many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size, geometry and structure of the bank, the engineering properties of the bank material, the hydraulics of flow in the adjacent channel and climatic conditions. Rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt/clay. Bank erosion occurs by fluvial entrainment of material from the lower, cohesionless bank at a much higher rate than material from the upper, cohesive bank. This leads to undermining that produces cantilevers of cohesive material. Upper bank retreat takes place predominantly by the failure of these cantilevers. Three mechanisms of failure have been identified: shear, beam and tensile failure. The stability of a cantilever may be analysed using static equilibrium and beam theory, and dimensionless charts for cantilever stability constructed. Application of the charts requires only a few simple measurements of cantilever geometry and soil properties. In this analysis the effects of cracks and fissures in the soil must be taken into account. These cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface. Following mechanical failure, blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue. Hence, the rate of bank retreat is fluvially controlled, even though the mechanism of failure of the upper bank is not directly fluvial in nature. This cycle of bank erosion: undermining, cantilever failure and fluvial scour of the toe, operates over several flood events and has important implications for river engineering, channel changes, and the movement of sediment through fluvial systems.     

Accounting for site effect in probabilistic assessment of seismic hazard for Romania and Bucharest: a case of deep seismicity in Vrancea zone

The paper presents recent achievements in evaluations of site-dependent seismic hazard in Romania and the capital city of Bucharest caused by the Vrancea focal zone (SE-Carpathians). The zone is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume at depths 60–170 km. The database that was used for the hazard evaluation includes parameters of seismicity, ground-motion source scaling and attenuation models (Fourier amplitude spectra), and site-dependent spectral amplification functions. Ground-motion characteristics were evaluated on the basis of several hundred records from more than 120 small magnitude (M 3.5–5) earthquakes occurred in 1996–2001 and a few tens of acceleration records obtained during four large (M 7.4, 7.2, 6.9 and 6.3) earthquakes. The data provide a basis for probabilistic seismic hazard assessment in terms of peak ground acceleration, peak spectral acceleration and MSK intensity using Fourier amplitude spectra for various exceedance probabilities or average return periods. It has been shown that the influence of geological factors plays very important role in distribution of earthquake ground-motion parameters along the territory of Romania.     

汶川地震农居震害特征及其成因分析与恢复重建建议

本文总结了汶川地震中农居倒塌、严重破坏与中等破坏的特点,并分析了破坏程度与地震动、地震地表破裂、滑坡、滚石和房屋质量等因素的关系。为了减轻未来地震造成的震害,城镇选址应避开危险地段和抗震不利地段,且农居设计与施工应严格遵循抗震设计规范中的相关条款。同时,本文也强调了当地政府在灾区重建中的重要作用。     

关于抗震设计规范与地震动区划图的有关探讨

本文总结了国内外桥梁抗震设计规范和建筑结构抗震设计规范的设计思想和设计方法,介绍了我国和美国地震动参数区划图的发展和现状,对比分析了抗震规范使用地震区划图的情况。针对我国抗震设计规范和地震动参数区划图的现状,提出了两条建议：（1）地震动参数区划图编制部门应直接提供多水准的地震动参数区划图;（2）抗震设计规范应彻底抛弃设防烈度概念,以地震重现期取代原地震危险程度标示方法。     

Selection of earthquake ground motion records: A state-of-the-art review from a structural engineering perspective

This paper reviews alternative selection procedures based on established methods for incorporating strong ground motion records within the framework of seismic design of structures. Given the fact that time history signals recorded at a given site constitute a random process which is practically impossible to reproduce, considerable effort has been expended in recent years on processing actual records so as to become ‘representative’ of future input histories to existing as well as planned construction in earthquake-prone regions. Moreover, considerable effort has been expended to ensure that dispersion in the structural response due to usage of different earthquake records is minimized. Along these lines, the aim of this paper is to present the most recent methods developed for selecting an ‘appropriate’ set of records that can be used for dynamic analysis of structural systems in the context of performance-based design. A comparative evaluation of the various alternatives available indicates that the current seismic code framework is rather simplified compared to what has actually been observed, thus highlighting both the uncertainties and challenges related to the selection of earthquake records.     

Design energy input spectra for moderate-to-high seismicity regions based on Colombian earthquakes

Energy input spectra applicable to the seismic design of structures in moderate-to-high seismicity regions are proposed. Such design inputs are derived from the bilinear envelope of individual spectra obtained for 144 ground motions recorded in Colombia. In the short period region the spectra account for the increase of input energy, due to plastification of the structure, through a new formula derived from extensive nonlinear analyses. The proposed energy input design spectra are compared with the provisions of the Colombian seismic code, and with those proposed for Japan, Spain, Iran and Greece. It is found that the proposed spectra are more demanding than the current Colombian seismic code, and that they agree with those developed recently for six cities in Greece, yet applying a different approach. An empirical equation for estimating the portion of the total seismic input energy that contributes to structural damage is also developed.     

双线偏振雷达的设计问题

双线偏振雷达作为一种实用的特种天气雷达正日益受到人们的关注，对双线偏振雷达设计中特有的天线系统，发射接收系统，数据处理系统等应注意的一些问题及设计指标进行了讨论，其结果对双线偏振雷达的研制有一定的参考价值。     

电视气象节目三维虚拟布景的设计制作

概述了三维虚拟布景的基本特征及三维虚拟布景与虚拟演播室之间的比较,通过实例对电视气象节目三维虚拟布景设计方案中的气象属性、审美要求、透视关系、视点、细节和可行性等6个问题进行了分析,并从应用软件、建模、质感、灯光、色彩和安全范围等方面对三维虚拟布景制作进行了探讨。     

基于WebGIS的江西省山洪灾害预报系统设计与应用

针对南方丘陵多雨地区山洪灾害防御需求,采用B/S三层体系结构设计了基于WebGIS技术和水文预报模型的江西省山洪灾害预报系统。系统提供山洪灾害实时预报、临近预报和短期预报山洪灾害风险产品,发布山洪灾害风险等级、基础地理信息等数据,实现了地图浏览、GIS基本操作、图层管理、数据查询等功能。预报时效检验表明系统预报结果较山洪灾害实际发生时间提前3 h以上,并在2009年7月2—3日赣州地区一次山洪过程进行模拟,能够准确预报出山洪风险等级,应用效果良好。     

提升铁路勘察设计企业绩效管理能力的思考

多数铁路勘察设计企业绩效管理存在着观念落后、配套措施不到位、未充分发挥促进员工发展的作用等问题。为进一步提升铁路勘察设计企业绩效管理能力，本文提出了树立以企业战略为导向的绩效管理观念、坚持“以人为本”的绩效管理理念、建立科学的绩效管理体系、差异化设定考核指标、兼顾财务指标和非财务指标等对策建议。