基于压磁效应的锈蚀钢筋应力状态检测试验研究

钢筋锈蚀是严重威胁结构安全的耐久性问题,基于压磁效应,研究均匀锈蚀和坑蚀两种锈蚀钢筋的应力状态与压磁场变化的关系。首先,采用通电加速锈蚀方法进行钢筋均匀锈蚀和点蚀试验;然后,通过轴向拉伸静载试验和疲劳加载试验,分析锈蚀钢筋的压磁信号特征。试验结果显示,磁感强度与钢筋应力状态之间具有较好的对应关系,屈服阶段不同锈蚀率下的钢筋磁感强度曲线有较明显区别,疲劳荷载作用下锈蚀钢筋的法向残余磁感强度和磁滞回环面积均呈现疲劳三阶段变化规律,可进一步运用于钢筋应力状态的检测中。     

反复荷载作用下矩形钢管混凝土树状节点的非线性分析

本文在确定了往复应力作用下钢管混凝土的钢材和核心混凝土的应力-应变关系的基础上,利用纤维杆元模型,有限元模型对钢管混凝土Y形柱和十字形钢梁连接的节点的荷载-位移滞回关系曲线及其骨架曲线进行了计算,并与试验结果进行了比较。结果表明：由纤维杆元模型与试验所得的低周反复荷载作用下的骨架曲线极为相似,但在峰值荷载后差异较大;由纤维杆元模型和有限元所得的在低周反复荷载作用下滞回曲线也与在反复荷载作用下试验所得的结果相一致。纤维杆元模型能准确地预测节点的弹塑性行为和整体抗震性能,可用于节点滞回性能的非线性参数分析研究。在此基础上研究了方钢管混凝土柱的轴压比,宽厚比及核心混凝土强度对节点受力性能的影响。     

新型钢管混凝土柱框架节点低周反复荷载试验研究

本文介绍了新型钢管混凝土柱框架节 低以复荷载作用下的试验结果,对其抗震性能进行了探讨,并提出了改进设计的建议。     

Calibration of magnetic granulometric trends in oceanic basalts

The validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains. For example, populations with dominantly superparamagnetic (SP) or multidomain (MD) grains both are characterized by low remanence and coercivity, and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain (SD) size range. The natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size, from SP–SD mixtures in submarine basaltic glass to SD–MD mixtures in flow interiors. Based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the Mid-Atlantic Ridge (ages <1 Ma to 70 Ma), a model of preferential dissolution with time of the finest-grained titanomagnetites has recently been suggested as the major process contributing to long-term temporal changes in remanent intensity of mid-ocean ridge basalts. We evaluated the local and long-term temporal trends in effective magnetic grain size predicted by this model using hysteresis data from a large number of submarine basalt samples which span a range of ages from 0 to 122 Ma. Specimens were systematically taken along transects perpendicular to the chilled margin of each sample. The large number of data (750 loops) and the inferred progressive change in grain size approaching the chilled margin allow recognition of mixing trends between MD and SD grains and between SD and SP grains on a Day-plot. These trends in hysteresis parameters are crucial to resolving the inherent, but frequently overlooked, ambiguity in inferring grain size from hysteresis parameters. We illustrate that two additional rock magnetic tests (warming of a low-temperature isothermal remanence and hysteresis loop shapes) often used to address these ambiguities are inconclusive, requiring some independent knowledge of whether SP or MD grains are likely to be present. Even with a considerably larger data set the substantial intrasample variability in oceanic basalts precludes recognition of any systematic trend in magnetic grain size with age.     

基于非协调参数Drucker-Prager模型的预应力节段拼装混凝土桥墩滞回性能研究

混凝土的数值计算模型在滞回性能分析中往往比较难收敛,利用有限元软件ADINA提供Drucker-Prager模型的Cap修正屈服准则确立的非协调参数Drucker-Prager模型对预应力节段拼装混凝土桥墩进行数值分析,对预应力节段拼装桥墩在低周往复荷载作用下的滞回性能进行分析。通过比较采用非协调参数Drucker-Prager模型建立的数值模型计算得到的滞回曲线与试验得到的结果,两者吻合程度较高。基于这一成果可以通过数值模拟的方式获得预应力节段拼装桥墩的滞回曲线与相关数据,可应用于预应力节段拼装桥墩抗震计算和设计。     

Exploring suspended sediment delivery dynamics of two Mediterranean nested catchments

This paper investigates suspended sediment transport and dynamics of two nested agricultural lowland Mediterranean catchments with a difference of two orders of magnitude in the surface area (i.e., 1 and 264 km²). The effects of the drainage catchment area over the specific suspended sediment yield are assessed by using the nested approach over various timeframes. A detailed analysis of the rainfall–runoff–sediment transport relationships during the 2‐year study period shows that the hydrological and sedimentological responses were extremely variable for both catchments. Very low or no correlations were observed between the rainfall intensity and the selected hydrological variables and sediment loads. However, remarkable or high correlations were obtained between the rainfall intensity and the maximum and average suspended sediment concentrations, indicating that rainfall per unit time has little control on the hydrological response, but that, simultaneously, its high‐erosive power triggers sediment production, increasing the sedimentary response of the catchments. This study also illustrates how sediment is mainly transported during floods, producing predominantly clockwise hysteretic loops. Moreover, the small headwater catchment exerts a reduced (or even negligible) effect over the hydro‐sedimentary response of the larger downstream catchment, caused by the reduced sediment availability in a landscape with an inherent disconnection of the sediment pathways.     

Width control on event-scale deposition and evacuation of sediment in bedrock-confined channels

In mixed bedrock–alluvial rivers, the response of the system to a flood event can be affected by a number of factors, including coarse sediment availability in the channel, sediment supply from the hillslopes and upstream, flood sequencing and coarse sediment grain size distribution. However, the impact of along-stream changes in channel width on bedload transport dynamics remains largely unexplored. We combine field data, theory and numerical modelling to address this gap. First, we present observations from the Daan River gorge in western Taiwan, where the river flows through a 1 km long 20–50 m wide bedrock gorge bounded upstream and downstream by wide braidplains. We documented two flood events during which coarse sediment evacuation and redeposition appear to cause changes of up to several metres in channel bed elevation. Motivated by this case study, we examined the relationships between discharge, channel width and bedload transport capacity, and show that for a given slope narrow channels transport bedload more efficiently than wide ones at low discharges, whereas wider channels are more efficient at high discharges. We used the model sedFlow to explore this effect, running a random sequence of floods through a channel with a narrow gorge section bounded upstream and downstream by wider reaches. Channel response to imposed floods is complex, as high and low discharges drive different spatial patterns of erosion and deposition, and the channel may experience both of these regimes during the peak and recession periods of each flood. Our modelling suggests that width differences alone can drive substantial variations in sediment flux and bed response, without the need for variations in sediment supply or mobility. The fluctuations in sediment transport rates that result from width variations can lead to intermittent bed exposure, driving incision in different segments of the channel during different portions of the hydrograph. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Maximum seismic displacement of inelastic systems based on energy concept

The energy balance and energy input of Single-Degree-of-Freedom (SDF) systems under earthquake motion is studied for elastic and inelastic systems. The maximum displacement of an inelastic system is related to that of an elastic system having the same initial stiffness and mass by considering the earthquake energy input per cycle of oscillation. With an assumption that the cyclic energy input is equal for both elastic and inelastic system for intermediate- and long-period systems, a simplified relation is suggested. Newmark's equal energy rule is shown to be the upper bound of the simplified relation; the lower bound of maximum response displacement is also derived in this paper. The numerical analysis results were mostly shown to fall between the proposed upper and lower bounds. A separate approximate relation is proposed for short-period systems. The reason for divergence from the suggested relations is discussed for short-period systems. Copyright © 1999 John Wiley & Sons Ltd.     

不同土壤冻融特征曲线的试验研究

土壤冻融特征曲线是反映冻土强度、冻土的热性质和土壤冻融过程中的水分和盐分迁移规律的重要参数.为了研究不同植物养分对不同土壤冻融过程的影响,采用NMR法对加入5种不同溶质的4种土壤的冻融特征曲线进行了测定与分析.结果表明:混合Ⅱ5%溶液均对这4种土壤的冻融特征曲线影响最明显,Ca(NO₃)₂ 5%溶液次之,再次为加入K₂S0₄和混合Ⅰ5%溶液的,P₂O₅ 5%溶液最小.这主要是由于在负温下不同溶质的溶解度和土壤对不同溶质的固持作用等不同所致.在加入相同溶液的情况下,康平下层土壤的冻融特征曲线明显低于其它3种土壤,而这3种土壤之间未冻水含量高低次序随所加入的溶质不同而不同.各试样的冻融特征曲线均出现了滞后现象.