锈蚀钢筋混凝土框架中节点抗剪强度研究

随着服役时间的增长，侵蚀环境下钢筋混凝土框架节点因钢筋发生不同程度的锈蚀而造成承载性能下降，严重影响建筑结构的安全使用。本文在已有钢筋混凝土框架节点抗剪强度理论模型的基础上，考虑钢筋锈蚀对框架节点受力性能的影响，建立锈蚀钢筋混凝土框架中节点受剪承载力计算公式。通过11组锈蚀钢筋混凝土节点试验数据，对建议理论模型进行验证。研究结果表明，锈蚀钢筋混凝土节点受剪承载力试验值与理论计算值之比的平均值为0.951，方差为0.075，二者吻合较好，本文建议的计算方法可用于锈蚀钢筋混凝土框架中节点承载力分析。     

Investigation of the elevation of saltwater wedge due to subsurface dams

Subsurface dams are rather effective and used for the prevention of saltwater intrusion in coastal regions around the world. We carried out the laboratory experiments to investigate the elevation of saltwater wedge after the construction of subsurface dams. The elevation of saltwater wedge refers to the upward movement of the downstream saltwater wedge because the subsurface dams obstruct the regional groundwater flow and reduce the freshwater discharge. Consequently, the saltwater wedge cannot further extend in the longitudinal direction but rises in the vertical profile resulting in significant downstream aquifer salinization. In order to quantitatively address this issue, field-scale numerical simulations were conducted to explore the influence of various dam heights, distances, and hydraulic gradients on the elevation of saltwater wedge. Our investigation shows that the upward movement of the saltwater wedge and its areal extension in the vertical domain of the downstream aquifer become more severe with a higher dam and performed a great dependence on the freshwater discharge. Furthermore, the increase of the hydraulic gradient and the dam distance from the sea boundary leads to a more pronounced wedge elevation. This phenomenon comes from the variation of the freshwater discharge due to the modification of dam height, location, and hydraulic gradient. Large freshwater discharge can generate greater repulsive force to restrain the elevation of saltwater wedge. These conclusions provide theoretical references for the behaviour of the freshwater–seawater interface after the construction of subsurface dams and help optimize the design strategy to better utilize the coastal groundwater resources.     

Microtremor-based analysis of the dynamic response characteristics of earth-fissured sites in the Datong basin,China

This study conducted microtremor testing along six survey lines that cross three typical earth fissures in the Datong basin to determine the dynamic response characteristics of earth fissure sites with regard to the Fourier amplitude spectrum, response spectrum, and Arias intensity. The results show the following.(1) The predominant frequency of an earth fissure site is mainly affected by the thickness and the shear wave velocity of the soil layer and is minimally effected by the presence of an earth fissure.(2) Earth fissures have a pronounced amplification effect on dynamic response. Fourier amplitude, response acceleration, and Arias intensity are high near an earth fissure and decrease with an increase in distance from the earth fissure, tending toward stability at a distance of 20 m.(3) The area that is seriously affected by this amplification is within 6–8 m of an earth fissure, and the general affected area is farther out than this, to a distance of 25 m.(4) New construction should be avoided in an area affected by the amplification, and existing buildings in general and seriously affected areas need to be reinforced to increase their seismic fortification intensity.     

Basin patterns of upper ocean warming for 1993–2009

A previous study (Lyman et al., Nature 465:334–337, 2010) showed a robust warming signal of the global upper ocean (0–700 m). They examined several sources of uncertainty that contribute to differences among heat content estimations. However, their focus was limited to globally averaged estimation. This study presents the spatial pattern of the global heat content change based on observed gridded datasets (Levitus et al., Geophys Res Lett 36:L07608, 2009). The western Pacific, Atlantic, and Indian Oceans showed significant warming trends, whereas eastern Pacific and some areas of the Gulf Stream experienced negative trends during 1993–2009. Steady warming trend was obtained from the first EOF mode when El Nino and Southern Oscillation (ENSO)-related signals were removed. This result implies that the rapid increase in heat content of the upper ocean around 2000–2005 is not related to a sampling transition from XBT to Argo observations but is associated with a natural variability dominated by strong ENSO-related signals.     

Analytical investigations of seismic responses for reinforced concrete bridge columns subjected to strong near-fault ground motion

Strong near-fault ground motion, usually caused by the fault-rupture and characterized by a pulse-like velocity- wave form, often causes dramatic instantaneous seismic energy (Jadhav and Jangid 2006). Some reinforced concrete (RC) bridge columns, even those built according to ductile design principles, were damaged in the 1999 Chi-Chi earthquake. Thus, it is very important to evaluate the seismic response of a RC bridge column to improve its seismic design and prevent future damage. Nonlinear time history analysis using step-by-step integration is capable of tracing the dynamic response of a structure during the entire vibration period and is able to accommodate the pulsing wave form. However, the accuracy of the numerical results is very sensitive to the modeling of the nonlinear load-deformation relationship of the structural member. FEMA 273 and ATC-40 provide the modeling parameters for structural nonlinear analyses of RC beams and RC columns. They use three parameters to define the plastic rotation angles and a residual strength ratio to describe the nonlinear load- deformation relationship of an RC member. Structural nonlinear analyses are performed based on these parameters. This method provides a convenient way to obtain the nonlinear seismic responses of RC structures. However, the accuracy of the numerical solutions might be further improved. For this purpose, results from a previous study on modeling of the static pushover analyses for RC bridge columns (Sung et al. 2005) is adopted for the nonlinear time history analysis presented herein to evaluate the structural responses excited by a near-fault ground motion. To ensure the reliability of this approach, the numerical results were compared to experimental results. The results confirm that the proposed approach is valid.     

含气介质临界点、流体和骨架弹性参数的数值计算方法

目前临界孔隙度及孔隙介质的研究主要为测试分析的实验方法,如何运用数值计算方法求取孔隙流体介质临界点和组分弹性参数一直是备受关注的课题.本文提出了求取孔隙介质临界点、孔隙内流体和骨架弹性参数的数值计算公式及方法,并结合含气样品测试数据实现了这种数值计算.文章首先给出了3个包含孔隙度的线性方程,通过有机组合每个线性方程中的两个系数得到求取有关弹性参数的数值计算公式.然后,详细阐述了计算的方法和步骤以及需要注意的问题,把室内含气砂岩样品实测的综合介质密度、纵波速度和横波速度作为数值计算的输入数据,求得了临界点和流体及骨架等相关弹性参数的具体数值.通过比较分析本文数学方法求得的计算数据与实验方法测得的实测数据,表明了该数值计算公式的正确性和数值实现方法的有效性.     

Application of bivariate frequency analysis to the derivation of rainfall–frequency curves

Bivariate distributions have been recently employed in hydrologic frequency analysis to analyze the joint probabilistic characteristics of multivariate storm events. This study aims to derive practical solutions of application for the bivariate distribution to estimate design rainfalls corresponding to the desired return periods. Using the Gumbel mixed model, this study constructed rainfall–frequency curves at sample stations in Korea which provide joint relationships between amount, duration, and frequency of storm events. Based on comparisons and analyses of the rainfall–frequency curves derived from univariate and bivariate storm frequency analyses, this study found that conditional frequency analysis provides more appropriate estimates of design rainfalls as it more accurately represents the natural relationship between storm properties than the conventional univariate storm frequency analysis.     

地裂缝场地建筑结构动力响应特征及抗震设防研究

为研究地震作用下地裂缝场地建筑结构的动力响应特征,以西安地裂缝场地为研究背景,分别考虑独立基础、片筏基础、桩基础和桩筏基础上的框架结构,对结构峰值加速度、层间位移等动力响应特征及其影响因素和影响规律进行系统研究。研究结果表明：(1)地裂缝附近场地上的结构加速度响应和层间位移响应具有明显的放大效应,动力响应峰值随着距地裂缝距离的增大逐渐减小,最终趋于稳定,放大效应的影响范围约为24 m,此范围内的结构需提高抗震设防水平;(2)结构的峰值加速度随层高的增大而增大,整体呈现出“S”形,层间位移角随层高的增大先增大后减小,二者均表现出明显的“上盘效应”,即上盘结构响应强于下盘,且随着输入地震动强度的增大,上、下盘结构动力响应差异进一步扩大,上盘效应愈发显著;(3)片筏基础、桩基础和桩筏基础结构的加速度响应接近,而独立基础结构的加速度与前三者差异明显,但随着楼层的增大,基础形式对结构层间位移角的影响逐渐减弱;(4)上、下盘结构的峰值加速度、峰值位移大小及峰值出现的时间等存在一定差异,这是由于地震波经过地裂缝时发生复杂的反射和折射,结构受到非一致性激励造成的。     

Molecular fossil and paleovegetation records of paleosol S4 and adjacent loess layers in the Luochuan loess section, NW China

Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the pa- leoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predomi- nance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data dem- onstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period.