随机地震动场多点激励下大跨度连续刚构桥的地震反应分析

本文基于随机地震动场的功率谱模型和多点地震激励，建立了大跨度桥梁在随机地震动场多点激励的地震反应分析方法，并数值模拟了某四跨预应力混凝土连续刚构桥的地震反应，考虑了行波效应、部分相干效应和局部场地效应等因素的影响，并与确定性地震一致激励下的计算结果进行了比较。对工程建设具有参考意义。     

考虑场地效应的非一致激励下桥梁地震响应特点分析

本研究拟从常规桥梁（跨径不超过150m且桥长不超过600m）出发,考虑局部场地效应,对某工程场地的地震反应进行三维动力有限元分析。将计算得到的地表地震动作为桥梁桥墩处的非一致输入,然后再通过有限元时程分析计算得到桥梁的地震反应。通过与一致激励及考虑行波效应激励的地震反应计算结果进行比较,得出以下结果:由于局部场地条件对地震动的频谱、峰值加速度都有影响,与一致激励相比,考虑局部场地的非一致激励对于桥梁的下部结构反应影响较小,而对于上部结构响应影响明显;考虑行波效应的非一致激励对于桥梁地震响应有减弱效果。研究结果表明,仅考虑行波效应引起的地震动非一致性开展桥梁地震响应分析并不具备保守性。     

协同式空腹夹层板动力特性分析及舒适度研究

为了解贵州省博物馆新馆协同式空腹夹层板的动力特性及人行舒适度性能,采用有限元法计算楼盖结构模态及加速度响应,并进行现场实测。还推导了基频的简化计算公式。其中有限元分析时,考虑了不同边界条件、不同单元类型等因素对分析结果的影响。分析结果表明:楼盖自振频率及加速度峰值计算值、实测值均满足规范。实体-壳单元模型、双层梁-壳单元模型均能较为准确反映协同式空腹夹层板动力特性,不同频率下定点激励及行走路线激励下的楼盖加速度峰值满足规范要求,基频简化计算方法能够满足工程精度要求。     

基于自平衡力的弹塑性动力反应分析方法

本文给出了一个基于自平衡力的弹塑性结构动力反应分析的新方法，详细叙述了方法的原理和过程，并对用改进的Ｔａｋｅｄａ滞变模型描述的梁单元框架结构给出了具体的分析方法。这个方法与传统的方法不同，在分析中不必修正结构的刚度矩阵，而代之以计算代表塑性铰变形引起的应力重分布的动力自平衡力，因而大大地节省了计算机时，最后通过算例对本文方法进行了验证。     

行波效应对连续曲线箱梁桥地震反应的影响

对于目前在城市立交和高架桥建设中应用广泛的多跨连续曲线箱梁桥,不考虑地震动行波效应的影响已经不能满足工程抗震的需要。针对一座七跨连续曲线箱梁桥考虑行波效应时的地震反应,采用大型有限元分析程序ANSYS,选取空间梁单元建立动力计算模型,研究了连续曲线箱梁桥的动力特性;并选取了E1 Centro地震动,考虑三种工况组合进行了多点激励与一致激励下的地震时程反应分析,给出了曲线箱梁和桥墩的内力、位移时程反应。结果表明:桥墩的刚度对连续曲线箱梁桥的动力特性影响较大,低阶频率和振型对桥墩变形的影响明显大于曲线箱梁;连续曲线箱梁桥在多点激励与一致激励下的地震反应差别较大。     

多维地震动各方向相关性对拱坝动应力的影响

本文将随机振动的虚拟激励法与拱坝-地基动力相互作用FE-BE-IBE时域模型结合，发展了一个可以考虑多维随机地震动作用下的拱坝动力响应计算模型，并用Monte Garlo方法对模型进行了验证，计算结果表明，地震动分量的相关性对结构的动力响应存在一定影响，合理考虑地震动各方向分量的相关性可以更好地计算实际地震作用下的拱坝动力响应。     

地震动的空间变异性对多支承结构的影响

导致地震动空间变异性的原因主要在于：几何不相关性效应、行波效应、局部场地条件效应。利用时程分析的方法分析了单跨框架结构和美国Las Vegas市区内某24跨立交桥的简化模型，对地震动空间变异性三个主要因素的单独影响、综合影响分别进行了系统分析。结果表明地震动的空间变异性改变了一致激励下结构的动力反应，并且引入了一致激励情况所不存在的拟静力反应，对结构的总反应具有很大的影响。与较激励的情况相比，结构的反应可能增大，也可能减小，这依赖于结构上截面位置、场地条件和所采用的地震动时程样本。     

埋地管线数值模拟中土体计算区域的选取问题研究

详细论述了动力无限元基本理论，提出了一种在非一致激励时无限元边界面上地震动的输入方法。在此基础上，考虑管线-土体之间的黏结滑移效应，采用动力无限元边界，建立了埋地管线的三维有限元-无限元耦合的精细化模型，对非一致地震波激励下埋地管线的动力响应进行了数值分析，对管线周围的土体计算范围的取值进行了系统研究，给出了管线周围土体的有效计算宽度W、有效计算长度L和有效计算深度H的取值范围。结果表明：W的取值与管径D紧密相关，H可取30～60 m,L应至少包含一个地震动的卓越周期段的波长，此分析结果可满足大多数工程需求。     

端承桩在分层地基中的横向地震反应分析

本文对分层弹性地基中端承桩基础按Ｗｉｎｋｌｅｒ（温克尔）地基土模型，通过特性分析，建立了合理的力学模型，经过动力分析，给出了端承桩基础横向自由振动特性及在横向动力载荷与地震载荷作用下强迫反应的解析解。文中的解析公式为分层弹性地基中端承桩基础在横向动力载荷与地震载荷作用下的动力反应分析，提供了一种新的解析方法。     

液化场地桩-土-桥梁结构地震相互作用振动台试验数值模拟方法研究

直接针对大型振动台模型试验,建立液化场地桩-土-桥梁结构地震相互作用数值模拟的二维分析模型和计算方法。根据桩基平面应变假定,将空间桩体转换成平面板桩,并考虑桩的尺寸效应;基于桩截面节点位移协调条件和平衡力系等效原理,建立四结点梁单元刚度矩阵且对Timoshenko梁杆单元刚度矩阵进行增广修正,以考虑桩的横向尺寸影响桩周土位移场分布的尺寸效应。根据有效应力原理进行土动反应分析,采用满足M asing准则的修正双曲线模型描述土动力变形的本构关系,同时考虑因孔压上升造成土体软化而对土动力性能的影响,由迭代法处理土的动力非线性。采用并联弹簧-阻尼器模拟计算域人工边界,以考虑边界波的反射作用对体系动力反应的干扰和土粘滞阻尼的影响。采用W ilson-θ逐步积分法计算体系的地震反应。通过与试验结果的对比分析,评估数值模拟的建模途径和计算方法的可靠性。     

SCIENCE CHINA Earth Sciences SUBJECT INDEX TO VOLUME 57(2014)

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Distribution of throughfall and stemflow in multi‐strata agroforestry,perennial monoculture,fallow and primary forest in central Amazonia,Brazil

The partitioning of rain water into throughfall, stemflow and interception loss when passing through plant canopies depends on properties of the respective plant species, such as leaf area and branch angles. In heterogeneous vegetation, such as tropical forest or polycultural systems, the presence of different plant species may consequently result in a mosaic of situations with respect to quantity and quality of water inputs into the soil. As these processes influence not only the water availability for the plants, but also water infiltration and nutrient leaching, the understanding of plant effects on the repartitioning of rain water may help in the optimization of land use systems and management practices. We measured throughfall and stemflow in a perennial polyculture (multi‐strata agroforestry), monocultures of peach palm (Bactris gasipaes) for fruit and for palmito, a monoculture of cupuaçu (Theobroma grandiflorum), spontaneous fallow and primary forest during one year in central Amazonia, Brazil. The effect on rain water partitioning was measured separately for four useful tree species in the polyculture and for two tree species in the primary forest. Throughfall at two stem distances, and stemflow, differed significantly between tree species, resulting in pronounced spatial patterns of water input into the soil in the polyculture system. For two tree species, peach palm for fruit (Bactris gasipaes) and Brazil nut trees (Bertholletia excelsa), the water input into the soil near the stem was significantly higher than the open‐area rainfall. This could lead to increased nutrient leaching when fertilizer is applied close to the stem of these trees. In the primary forest, such spatial patterns could also be detected, with significantly higher water input near a palm (Oenocarpus bacaba) than near a dicotyledonous tree species (Eschweilera sp.). Interception losses were 6·4% in the polyculture, 13·9 and 12·3% in the peach palm monocultures for fruit and for palmito, respectively, 0·5% in the cupuaçu monoculture and 3·1% in the fallow. With more than 20% of the open‐area rainfall, the highest stemflow contributions to the water input into the soil were measured in the palm monocultures and in the fallow. Copyright © 1999 John Wiley & Sons, Ltd.     

A Monte Carlo study of rainfall forecasting with a stochastic model

A procedure for short-term rainfall forecasting in real-time is developed and a study of the role of sampling on forecast ability is conducted. Ground level rainfall fields are forecasted using a stochastic space-time rainfall model in state-space form. Updating of the rainfall field in real-time is accomplished using a distributed parameter Kalman filter to optimally combine measurement information and forecast model estimates. The influence of sampling density on forecast accuracy is evaluated using a series of a simulated rainfall events generated with the same stochastic rainfall model. Sampling was conducted at five different network spatial densities. The results quantify the influence of sampling network density on real-time rainfall field forecasting. Statistical analyses of the rainfall field residuals illustrate improvement in one hour lead time forecasts at higher measurement densities.     

Anthropogenic Eutrophication and Red Tide Outbreak in Lacustrine Systems of the Kashmir Himalaya

Red tide, a recurrent phenomenon has become conspicuous in several Kashmir lake ecosystems since 1991. The responsible organism (Euglena pedunculata), a rare flagellate rediscovered in the Kashmir Himalaya (Khan 1993) caused first and unprecedented red tide outbreak, constituting a maximum of 96% of resident numerical phytoplankton density in Dal Lake. At present, conflicting hypotheses exist on the generation of causal assemblage(s) imparting redness to waters: Jeeji Bai (1991) linked its origin to acid precipitation – a fallout of burning oil‐fields during the Gulf War – whilst Khan (1993) holds local factor(s) responsible. Field/experimental studies support the latter contention that the influx of untreated sewage, in unison with warm temperatures, high levels of PhAR, iron and interruption to hydrological flow‐pattern together with absence/or reduction in grazing activity created conducive environmental milieu for red tide outbreak. Dal Lake “red tide” drifted the bloom‐inoculum to other waters, including Lake Wular, where additional ecological niches were carved out, threatening the aesthetic value and biological diversity of Kashmir lakes. Ecological monitoring indicates frequent seasonal red tide occurrence in Dal Lake (including summer‐autumn event of 1998) which testifies its unabated eutrophication status. Further studies are needed on ecological adaptability and biogeographic distribution of this rare and unique red tide‐causing flagellate.