基于损伤塑性模型的地下洞室结构地震作用分析

基于损伤力学原理,通过引入损伤变量的方法,详细推导了ABAQUS中的损伤塑性本构模型,描述混凝土后继屈服阶段及其卸荷后的损伤机制和力学行为。此本构模型可考虑循环荷载条件下材料刚度退化和应变率的影响,适用于类似混凝土材料的脆性材料,如岩石等。以某水电站地下厂房洞室结构为例,进行二维动力时程非线性分析,考虑不同的刚度恢复权重因子和应变率相关性对结构损伤的影响。多个数值算例表明：洞室混凝土结构和其周围一定深度的围岩的损伤是一个渐进积累的过程;材料的率相关性效应并不是在地震动作用开始时就立即产生,而是在一段时间后,结构经历一定程度的损伤后才慢慢体现出来;此损伤塑性模型适合地震荷载等循环荷载作用下地下洞室结构的动力时程非线性分析,尤其是强震下采用此损伤塑性模型是合理和必要的。     

刚性单桩复合地基载荷试验数值仿真

针对载荷试验结果不能反映地基基础的真实工作性状和荷载传递机理,同时,试验条件与地基基础的实际工作条件通常存在差异等问题,分析了载荷试验涉及的关键力学问题,进一步建立三维数值仿真模型,并与实际工程条件下的现场载荷试验结果比较。结果表明,数值仿真与载荷试验反映的荷载-沉降关系基本一致,验证了仿真模型的适用性。     

复合加载模式下单桩复合筒型基础地基承载力包络线研究

海上风力发电机属于高耸结构物,其地基基础不仅要承受上部结构传递的竖向荷载V、水平荷载H,更要抵抗巨大的弯矩荷载M。单桩复合筒型基础是新型的海上风力发电机基础型式,建立复合加载模式下单桩复合筒型基础地基的破坏包络线,并据此评价其稳定性是地基基础设计的重要手段。在Swipe加载方法和固定位移比加载方法的基础上,通过数值分析研究了不排水饱和软黏土中单桩复合筒型基础在V-H、V-M、H-M和V-H-M加载模式下的地基承载力包络线,开展了室内模型试验,验证了数值分析结果的可靠性。研究表明,V-H和V-M加载模式下地基承载力包络线具有对称性,而H-M加载模式下呈非对称性;V-H-M加载模式下地基承载力包络线的形状受弯矩荷载M的影响显著,表现为随着弯矩荷载的增加,V-H平面内的破坏包络线逐渐缩小。实际工程中,可根据受力状态与破坏包络面的相对关系,评价单桩复合筒型基础的稳定性。     

孢粉记录的青藏高原东北部更尕海地区中晚全新世植被和气候变化

青藏高原现代季风边缘区中晚全新世气候重建存在较大的差异,对位于青藏高原东北部共和盆地更尕海的GGHA孔岩芯进行了孢粉分析(平均分辨率为80年,最高分辨率约10年),重建了过去6.3ka以来区域植被和气候的演化历史。6.3cal.ka B.P.以来盆区广泛发育以蒿属、藜科和禾本科为主的荒漠植被,以及一定面积的以禾本科和蒿属为主的草原植被,其中:1)6.3~5.6cal.ka B.P.,盆区周围山地发育有少量以松属为主的针叶林,此时夏季风较强,低A/C值指示湖区相对湿度偏低,但湖区周围仍有小面积沼泽草甸发育;2)5.6~4.1cal.ka B.P.,乔木花粉含量显著降低表明周围山地森林面积逐渐减小,至4.1cal.ka B.P.山地森林已衰退消失,盆区草原和荒漠面积有所扩大,莎草科花粉含量低指示湖泊周围沼泽草甸面积缩小,A/C值指示总体湿度有增加趋势;3)4.1~3.1cal.ka B.P.,周围山地森林不再发育,湖区湿度剧烈波动,湖泊周围沼泽草甸发育停滞;4)3.1cal.ka B.P.以来,蒿属和莎草科含量呈下降趋势,A/C值逐渐降低指示干旱化程度加深,以藜科为主荒漠草原面积增加并伴随以禾本科和蒿属为主的草原面积减小,麻黄和白刺含量有所增加,说明研究区气候较前一阶段更加干旱。中晚全新世以来盆区气候和盆区周围山地森林的衰退可能与夏季太阳辐射的降低以及与之相关的亚洲夏季风减弱有关。     

江淮丘陵区秋季开花植物的花粉形态与生态因子

应用光学显微镜对江淮丘陵区合肥秋季开花的24种植物花粉形态进行了观察和研究,结果表明:花粉粒呈超扁球形至长球形,P/E值最大和最小的花粉分别为丝兰(Yucca smalliana Fern.)和桂花(Osmanthus fragrans(Thunb.)Lour.);体积最大的花粉为丝瓜(Luffa cylindrica (L.) Roem.),最小的为香槐(Cladrastis wilsonii Takeda.)。花粉的萌发孔主要有单沟、3～6沟、不规则散沟、三孔沟、2～4孔及散孔等6种类型,其外壁纹饰主要包括刺状、细网状、粗网状、颗粒状及条纹穿孔等5种类型。该研究为花粉分布区提供了地理位置(31°31′～32°37′N,116°40′～117°52′E)、海拔高度(70～20 m)、年降水量(900～1 000 mm)和≥10℃的年积温(4 500～5 300℃)等主要生态因子。     

Design and full‐scale experimental evaluation of a seismically endurant steel buckling‐restrained brace system

This paper presents the results of 12 full‐scale tests on buckling‐restrained brace (BRB) specimens. A simple‐to‐fabricate all‐steel encasing joined by high‐strength bolts was used as the buckling‐restrainer mechanism. Steel BRBs offer significant energy dissipation capability through nondeteriorating inelastic response of an internal ductile core. However, seismic performance of BRBs is characterized by interaction between several factors. In this experimental study, the effects of core‐restrainer interfacial condition, gap size, loading history, bolt spacing, and restraining capacity are evaluated. A simple hinge detail is introduced at the brace ends to reduce the flexural demand on the framing components. Tested specimens with bare steel contact surfaces exhibited satisfactory performance under the American Institute of Steel Construction qualification test protocol. The BRBs with friction‐control self‐adhesive polymer liners and a graphite‐based dry lubricant displayed larger cumulative inelastic ductility under large‐amplitude cyclic loading, exceeding current code minimum requirements. The BRB system is also examined under repeated fast‐rate seismic deformation history. This system showed significant ductility capacity and remarkable endurance under dynamic loading. Furthermore, performance is qualified under long‐duration loading history from subduction zone's megathrust type of earthquake. Predictable and stable performance of the proposed hinge detail was confirmed by the test results. Internally imposed normal thrust on the restrainer is measured using series of instrumented bolts. Weak‐ and strong‐axis buckling responses of the core are examined. Higher post‐yield stiffness was achieved when the latter governed, which could be advantageous to the overall seismic response of braced frames incorporating BRBs.     

动载下无筋砌体墙的应力场及裂隙分布

为探索动载下建筑物墙体失稳的力学机制,基于弹性力学理论,分析一端刚性固定无筋砌体墙受垂直地震波扰动时的应力场分布规律,得到墙体的应力解析表达式,结果显示应力特征由墙体质点振速、墙体恒载、墙体密度及其几何尺寸等因素共同决定。结合具体算例发现:墙体水平应力、垂直应力与剪应力具有不同的分布特征。水平应力值分别是剪切和垂直应力值的5倍和25倍,为墙体应力场的主控应力,其应力集中区位于刚性固定侧墙体的顶、底角,应力变化梯度为180MPa/m,且随地震强度增加,其应力场进一步向墙体刚性固定侧集中。在此基础上,基于主应力主导墙体裂隙的原则,预测了墙体张开型裂隙的分布位置、长度及角度,获得了墙体最易出现破坏失稳的部位。通过模型计算可为掌握动载下墙体力学行为提供参考。     

Stable oxygen isotope and pollen records from eastern Scotland and a consideration of Late-glacial and early Holocene climate change for Europe

The presence of marl deposits belonging to the Lateglacial period in a former lake basin at Lundin Tower in Fife, Scotland has allowed palaeoenvironmental investigations by means of carbonate δ¹³C and δ¹⁸O, and organic matter δ¹³C, in addition to palynology. The variations that emerge reveal strong similarities between the pollen and isotope records and these are interpreted as reflecting climatic shifts. The classic Late-glacial pattern of Oldest Dryas–Bølling–Older Dryas–Allerød–Younger Dryas may be evident and other climatic oscillations are shown to have occurred not only during the Allerød but also in the Preboreal. The problem of the time discordance between isotopic change and pollen representation is addressed through explanations involving lags in plant colonization. A comparison of the δ¹⁸O records from 43 sites across Europe reveals two different regional patterns, which raises fundamental questions over the nature of Late-glacial palaeoclimates.     

神农架巴山冷杉林花粉与植物关系及其森林植被之演替

在神衣架巴山冷杉林中,冷杉花粉数量与其植物盖度接近。在林外,冷杉花粉数量迅速下降,至10km以外则仅有个别出现。用在表土中所取R值对化石花粉组合谱进行了校正。说明现代的巴山冷杉林乃是由栎—桦林演变而来,时间约在300—400BP。     

A whole-basin stratigraphic record of sediment and phosphorus loading to the St. Croix River,USA

Lake St. Croix is a natural impoundment of the lowermost 37 km of the St. Croix River in Minnesota and Wisconsin, making this one of a few large river systems in the world possessing a long-term depositional basin at its terminus. The river’s relatively pristine condition led to its designation as a National Scenic Riverway in 1968, but increasing urbanization in its lower reaches has raised concerns about impacts on water quality. This study was initiated to reconstruct historical loadings of suspended sediment and phosphorus (P) from the sediment record in Lake St. Croix. Twenty-four piston cores, with an average length of 2 m, were collected along eight transects of the lake. Dated chronologies from ²¹⁰Pb, ¹³⁷Cs and ¹⁴C were used to calculate the rate of sediment accumulation in the lake over the past 100+ years. Diatom microfossil analysis was used to reconstruct historical lakewater P concentrations over the same time period, and sediment P analysis quantified the amount of P trapped in lake sediments. Using a whole-lake mass balance approach, the loading of sediment and P to Lake St. Croix over the last 100+ years was calculated. Beginning in 1850, sediment accumulation increased dramatically to a peak in 1950–1960 of eight times background rates prior to European settlement. The peak is driven largely by sediment contributions from small side-valley catchment tributaries to the downstream half of the lake. The total P load to the lake increased sharply after 1940 and remains high, at around four times the level of pre-European settlement conditions. The timing of peak sediment and P loading to the lake shows that early settlement activities, such as logging and the conversion of forest and prairie to agricultural land between 1850 and 1890, had only modest impacts on the lake. By contrast, the mid-1900s brought major increases in sediment and P loading to the lake, suggesting that relatively recent activities on the landscape and changes to nutrient balances in the watershed have caused the current eutrophic condition of this important recreational and natural resource. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.