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Shape is a fundamental property of all objects, including sedimentary particles, but it remains one of the most difficult to characterize and quantify for all but the simplest of shapes. Despite a large literature on the subject, there remains widespread confusion regarding the meaning and relative value of different measures of particle shape. This paper re‐examines the basic concepts of particle shape and suggests a number of new and modified methods which are widely applicable to a range of sedimentological problems; it is shown that the most important aspects of particle form are represented by the I/L ratio (elongation ratio) and S/I ratio (flatness ratio). A combination of these two ratios can be used to classify particles in terms of 25 form classes. A method of obtaining a quantitative measure of particle roundness using simple image analysis software is described, and a new visual roundness comparator is presented. It is recommended that measurements of both roundness and circularity (a proxy measure of sphericity) are made on grain images in three orthogonal orientations and average values calculated for each particle. A further shape property, irregularity, is defined and a classification scheme proposed for use in describing and comparing irregular or branching sedimentary particles such as chert and coral. 相似文献
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基于三亚(109.6°E,18.4°N)VHF电离层相干散射雷达观测,分析了我国低纬电离层E区场向不规则结构连续性回波的发生特征.研究结果表明:白天,E区连续性回波的多普勒速度范围为-50至25m/s,多普勒宽度主要分布在20至70m/s;连续性回波的高度大约以1km/h的速度缓慢下降,与偶发E层(Es)底部所在高度(hbEs)有很好的相关性,表明在背景电场影响下,Es经梯度漂移不稳定性产生场向不规则结构,引起E区连续性回波.夜间,E区连续性回波的多普勒速度范围为-50至50m/s,多普勒宽度为20至110m/s,回波在时间-高度-强度图上常呈现多层结构,可能与潮汐引起的多个离子层相关;而E区连续性回波的短暂中断,以及120km以上高E区连续性回波的发生,则可能归因于赤道扩展F极化电场的影响.此外,对E区连续性回波多普勒速度与全天空流星雷达风场观测的比较发现,在100km以下,多普勒速度与子午风场有很好的相关性. 相似文献
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带耗能腋撑竖向不规则短肢剪力墙结构减震性能分析 总被引:2,自引:0,他引:2
在不影响建筑使用空间前提下,提出在抗侧构件不连续处设置耗能腋撑以改善竖向不规则结构抗震性能。以底部大空间短肢剪力墙结构为研究对象,利用大型通用有限元程序ETABS研究耗能器类型与场地土对耗能腋撑工作性能和竖向不规则结构受力性能的影响。研究表明,黏滞型耗能腋撑对文中分析模型各楼层地震反应有较好的控制效果,对转换层处层间位移角与层剪力最大值减幅最大,分别为40.14%和15.66%,对顶层加速度与基底剪力峰值的最大减幅分别为16.06%和23.57%,黏滞型耗能腋撑最大能耗散输入结构能量的42%,而黏弹型耗能腋撑对结构的控制效果不理想;当地震震级较大、震中距较小时,耗能腋撑对坚硬与软弱场地土的模型结构控制作用相差不大,减震位移比在转换层处达到最小值0.76;随着震级减小或震中距增大,耗能腋撑对该模型结构的控制作用随场地土变硬而逐渐增强,其减震位移比介于0.68~0.74之间。 相似文献
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Boby Mathew B. M. Pathan K. N. Iyer D. R. K. Rao 《Journal of Earth System Science》1991,100(4):331-340
Using simultaneous long-term observations of ionospheric scintillation at equator and anomaly crest region in the same longitude
(Indian) zone comparative features of scintillation occurrence are brought out. The salient features are: (a) predominantly
pre-midnight occurrence of scintillation at equator during winter and equinox seasons, (b) increase of pre-midnight scintillation
occurrence with solar activity (c) shifting of occurrence peak during summer from post-midnight in low to pre-midnight in
high solar activity periods (d) similarity of scintillation behaviour at these locations during winter and equinoxes but dissimilarity
during summer. The solar activity response and magnetic effects indicate that the scintillations at the anomaly crest region
in winter and equinox, particularly during high solar activity periods, are of equatorial origin while the summer events may
be of local or mid-latitude origin. 相似文献
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Masonry buildings are often characterized by geometric irregularities. In many cases, such buildings meet global regularity requirements provided by seismic codes, but they are composed by irregular walls with openings. The latter are masonry walls characterized by (i) openings of different sizes, (ii) openings misaligned in the horizontal and/or vertical direction, or (iii) a variable number of openings per story. An irregular layout of openings can induce not only a nonuniform distribution of gravity loads among masonry piers but also unfavorable damage localizations resulting in a premature collapse of the wall and hence a higher seismic vulnerability. This paper is aimed at providing a simplified methodology to assess the effects of irregularities on the in‐plane seismic capacity of unreinforced masonry (URM) walls with openings. To this end, a macroelement method was developed and validated through experimental results available in the literature. The proposed methodology was based on the quantification of wall irregularities by means of geometric indices and their effects on seismic capacity of URM walls with openings through both sensitivity and regression analyses. Sensitivity analysis was based on a high number of static pushover analyses and allowed to assess variations in key seismic capacity parameters. Regression analysis let to describe each capacity parameter under varying irregularity index, providing empirical models for seismic assessment of irregular URM walls with openings. The in‐plane seismic capacity was found to be significantly affected by wall irregularities, especially in the case of openings with different heights. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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Swaminathan Krishnan 《地震工程与结构动力学》2007,36(7):861-885
This paper describes the three‐dimensional nonlinear analysis of six 19‐storey steel moment‐frame buildings, designed per the 1997 Uniform Building Code, under strong ground motion records from near‐source earthquakes with magnitudes in the range of 6.7–7.3. Three of these buildings possess a reentrant corner irregularity, while the remaining three possess a torsional plan irregularity. The records create drift demands of the order of 0.05 and plastic rotation demands of the order of 4–5% of a radian in the buildings with reentrant corners. These values point to performance at or near ‘Collapse Prevention’. Twisting in the torsionally sensitive buildings causes the plastic rotations on the moment frame on one face of the building (4–5% of a radian) to be as high as twice of that on the opposite face (2–3% of a radian). The asymmetric yield pattern implies a lower redundancy in the lateral force‐resisting system as the failure of the heavily loaded frame could result in a total loss of resistance to torsion. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献