基于神经网络的钢筋混凝土异型节点抗裂能力预测

利用神经网络方法研究了低周反复荷载作用下钢筋混凝土异型节点抗裂承载力与各主要影响因素之间复杂的非线性关系,建立了承载力的BP神经网络预测模型,预测结果与试验结果吻合较好。分析结果表明神经网络计算是钢筋混凝土构件力学性能研究中的一种很有潜力的新方法。     

Surficial fractures in the Navajo sandstone,south-western USA: the roles of thermal cycles,rainstorms, granular disintegration,and iterative cracking

Deep (> 5 m) sheeting fractures in the Navajo sandstone are evident at numerous sites in southern Utah and derive from tectonic stresses. Strong diurnal thermal cycles are, however, the likely triggers for shallow (< 0.3 m) sheeting fractures. Data from subsurface thermal sensors reveal that large temperature differences between sensors at 2 and 15 cm depth on clear summer afternoons are as great as those that trigger sheeting fractures in exposed California granite. Extensive polygonal patterns in the Navajo sandstone are composed of surface-perpendicular fractures and were produced by contractile stresses. Numerous studies have shown that porewater diminishes the tensile strength of sandstone. Based on our thermal records, we propose that cooling during monsoonal rainstorms triggers polygonal fracturing of temporarily weakened rock. On steep outcrops, polygonal patterns are rectilinear and orthogonal, with T-vertices. Lower-angle slopes host hexagonal patterns (defined by the dominance of Y-vertices). Intermediate patterns with rectangles and hexagons of similar scale are common. We posit that outcropping fractures are advancing downward by iterative steps, and that hexagons on sandstone surfaces (like prismatic columns of basalt) have evolved from ancestral orthogonal polygons of similar scale. In lava flows, fractures elongate intermittently as they follow a steep thermal gradient (the source of stress) as it rapidly moves through the rock mass. In our model, a steep, surficial thermal gradient descends through unfractured sandstone, but at the slow pace of granular disintegration. Through time, as the friable rock on stable slopes erodes, iterative cracking advances into new space. Hexagonal patterns form as new fractures, imperfectly guided by the older ones, propagate in new directions, and vertices drift into a configuration that minimizes the ratio of fracture length to polygon area. © 2020 John Wiley & Sons, Ltd.     

新型装配式混凝土框架连接节点抗震性能试验

为推广装配式混凝土框架结构的应用,提出3种不同的新型装配式钢筋混凝土框架中节点连接形式,进行低周往复荷载试验。对比各试件的破坏形态、滞回性能、刚度退化、累积耗能和节点剪切变形等抗震指标。研究结果表明:采用方钢管连接的装配式混凝土节点呈现梁端弯曲破坏,采用工字钢连接或对拉螺栓连接的节点呈现节点核心区剪切破坏。采用方钢管的连接形式既能改善节点核心区的破坏形态,又能提高其承载能力、变形能力、耗能能力和梁端转动能力,同时显著改善节点的滞回特性,减小核心区的剪切变形。在弹塑性和塑性变形阶段,采用方钢管连接形式的装配式混凝土节点的抗震性能优于工字钢连接和对拉螺栓连接的节点。此外,采用工字钢连接形式比对拉螺栓连接形式的节点具有更高的承载能力、耗能能力和较小的核心区剪切变形。     

不同减隔震措施下平板支座抗震性能试验研究

为探究不同减隔震措施对短边距混凝土网架平板支座的抗震性能和破坏模式的影响,分别针对不同混凝土边缘距离以及采用不同减隔震措施的四个平板支座缩尺试件进行拟静力加载试验,对平板支座破坏模式、滞回曲线、箍筋应变进行分析。试验结果表明:平板支座的运动伴随着平动和转动。当混凝土边距不足时,边缘混凝土在压剪作用下,会发生冲切破坏。开长孔支座能较好的释放水平位移,并且对锚栓约束较弱,使其受力较小,从而减轻了边缘混凝土的受力。对采用橡胶垫板的支座,混凝土边缘未发生破坏,原因是橡胶垫具有良好的变形能力,可以释放支座的位移和转角,使得边缘混凝土受力较小。因此,针对混凝土边缘距离较短的平板支座节点,为防止混凝土发生边缘破坏,可采用开长孔支座,但需要对孔长进行合理的设计。采用橡胶垫支座也可避免混凝土发生边缘破坏,橡胶垫板可按照我国相关设计规范进行设计。     

RESEARCH ON THE CHARACTERISTICS OF QUATERNARY ACTIVITIES OF SU-XI-CHANG FAULT

Running across the urban areas of Changzhou, Wuxi and Suzhou, the NW-trending Su-Xi-Chang Fault is an important buried fault in Yangtze River Delta. In the respect of structural geomorphology, hilly landform is developed along the southwest side of the Su-Xi-Chang Fault, and a series of lakes and relatively low-lying depressions are developed on its northeast side, which is an important landform and neotectonic boundary line. The fault controlled the Jurassic and Cretaceous stratigraphic sedimentary and Cenozoic volcanic activities, and also has obvious control effects on the modern geomorphology and Quaternary stratigraphic distribution. Su-Xi-Chang Fault is one of the target faults of the project "Urban active fault exploration and seismic risk assessment in Changzhou City" and "Urban active fault exploration and seismic risk assessment in Suzhou City". Hidden in the ground with thick cover layer, few researches have been done on this fault in the past. The study on the activity characteristics and the latest activity era of the Su-Xi-Chang Fault is of great significance for the prevention and reduction of earthquake disaster losses caused by the destructive earthquakes to the cities of Changzhou, Wuxi and Suzhou. Based on shallow seismic exploration and drilling joint profiling method, Quaternary activities and distribution characteristics of the Su-Xi-Chang Fault are analyzed systematically. Shallow seismic exploration results show that the south branch of the Su-Xi-Chang Fault in Suzhou area is dominated by normal faulting, dipping to the north-east, with a dip angle of about 60° and a displacement of 3~5m on the bedrock surface. The north branch of the Su-Xi-Chang Fault in Changzhou area is dominated by normal faulting, dipping to the south, with a dip angle of about 55°~70° and a displacement of 4~12m on the bedrock surface. All breakpoints of Su-Xi-Chang Fault on the seismic exploration profiles show that only the bedrock surface was dislocated, not the interior strata of the Quaternary. On the drilling joint profile in the Dongqiao site of Suzhou, the latest activity of the south branch of Su-Xi-Chang Fault is manifested as reverse faulting, with maximum displacement of 2.9m in the upper part of Lower Pleistocene, and the Middle Pleistocene has not been dislocated by the fault. The fault acts as normal fault in the Pre-Quaternary strata, with a displacement of 3.7m in the Neogene stratum. On the drilling joint profile in the Chaoyang Road site of Changzhou, the latest activity of the north branch of Su-Xi-Chang Fault is manifested as reverse faulting too, with maximum displacement of 2.8m in the bottom layer of the Middle Pleistocene. The fault acts as normal fault in the Pre-Quaternary strata, with a displacement of 10.2m in the bedrock surface. Combining the above results, we conclude that the latest activity era of Su-Xi-Chang Fault is early Middle Pleistocene. The Su-Xi-Chang Fault was dominated by the sinistral normal faulting in the pre-Quaternary period, and turned into sinistral reverse faulting after the early Pleistocene, with displacement of about 3m in the Quaternary strata. The maximum magnitude of potential earthquake on the Su-Xi-Chang Fault is estimated to be 6.0.     

Effective elastic thickness of the lithosphere from joint inversion in western China and its implications*

The western China lies in the convergence zone between Eurasian and Indian plates. It is an ideal place to study the lithosphere dynamics and tectonic evolutions on the continental Earth. The lithospheric strength is a key factor in controlling the lithosphere dynamics and deformations. The effective elastic thickness (T_e) of the lithosphere can be used to address the lithospheric strength. Previous researchers only used one of the admittance or coherence methods to investigate the T_e in the western China. Moreover, most of them ignored the internal loads of the lithosphere during the T_e calculation, which can produce large biases in the T_e estimations. To provide more reliable T_e estimations, we used a new joint inversion method that integrated both admittance and coherence techniques to compute the T_e in this study, with the WGM2012 gravity data, the ETOPO1 topographic data, and the Moho depths from the CRUST1.0 model. The internal loads are considered and investigated using the load ratio (F). Our results show that the joint inversion method can yield reliable T_e and F values. Based on the analysis of T_e and F distributions, we suggest (1) the northern Tibetan Plateau could be the front edge of the plate collision of Eurasian and Indian plates; (2) the southern and part of central Tibetan Plateau have a strong lithospheric mantle related to the rigid underthrusting Indian plate; (3) the southeastern Tibetan Plateau may be experiencing the delamination of lithosphere and upwelling of asthenosphere.     

USE OF SEISMIC WAVEFORMS AND INSAR DATA FOR DETERMINATION OF THE SEISMOTECTONICS OF THE MAINLING MS6.9 EARTHQUAKE ON NOV.18, 2017

On November 18, 2017, a M_S6.9 earthquake struck Mainling County, Tibet, with a depth of 10km. The earthquake occurred at the eastern Himalaya syntaxis. The Namche Barwan moved northward relative to the Himalayan terrane and was subducted deeply beneath the Lhasa terrane, forming the eastern syntaxis after the collision of the Indian plate and Asian plates. Firstly, this paper uses the far and near field broadband seismic waveform for joint inversion (CAPJoint method)of the earthquake focal mechanism. Two groups of nodal planes are obtained after 1000 times Bootstrap test. The strike, dip and rake of the best solution are calculated to be 302°, 76° and 84° (the nodal plane Ⅰ)and 138°, 27° and 104° (the nodal plane Ⅱ), respectively. This event was captured by interferometric synthetic aperture radar (InSAR)measurements from the Sentinel-1A radar satellite, which provide the opportunity to determine the fault plane, as well as the co-seismic slip distribution, and assess the seismic hazards. The overall trend of the deformation field revealed by InSAR is consistent with the GPS displacement field released by the Gan Wei-Jun's team. Geodesy (InSAR and GPS)observation of the earthquake deformation field shows the northeastern side of the epicenter uplifting and the southwestern side sinking. According to geodetic measurements and the thrust characteristics of fault deformation field, we speculate that the nodal plane Ⅰ is the true rupture plane. Secondly, based on the focal mechanism, we use InSAR data as the constraint to invert for the fine slip distribution on the fault plane. Our best model suggests that the seismogenic fault is a NW-SE striking thrust fault with a high angle. Combined with the slip distribution and aftershocks, we suggest that the earthquake is a high-angle thrust event, which is caused by the NE-dipping thrust beneath the Namche Barwa syntaxis subducted deeply beneath the Lhasa terrane.     

应力波斜入射对岩体节理端部的影响

岩体节理在受到应力波扰动时端部受力会发生不同程度的连续性变化,为具体分析这种动态变化与应力波入射角度之间的关系,引用岩石非线性法向本构关系与线性切向本构关系的组合模型以及相应的在P波斜入射节理的应力波传播方程,结合岩石断裂力学中Ⅰ、Ⅱ型裂纹尖端应力和位移场的相关理论,得出组合形式下的节理端部应力场和位移场随质点振速变化的计算公式。通过不同入射角的模拟脉冲信号作用对端部应力位移的变化分析：应力波斜入射节理导致节理端部上、下两侧应力与位移场非对称分布,随着入射波质点振速的增减变化,应力集中位置会出现变化;计算并整理模型中节理端部上、下两侧0.005 m位置的数据,节理法向刚度由入射波质点振速带来的变化直接影响到应力波的透射与反射,进而会导致端部的应力与位移出现滞后效应;节理端部横向位移值与入射角度并非单调变化,而竖向位移会随着入射角的增大呈现下降趋势。     

运营期水下盾构隧道管片接缝张开度变化规律

水下盾构隧道管片间接缝张开度对隧道防水及运营期结构安全有重要影响。以南京扬子江隧道结构健康监测系统中两个典型监测断面的接缝监测数据为基础,通过多元线性回归分析方法分析在两种不同地层组合下接缝张开度随水位、温度变化的规律;并对接缝张开度随水位变化的规律进行了有限元数值模拟,数值模拟结果与回归分析结果吻合较好。上述分析结果表明,纵缝张开度和横缝张开度随水位、温度的变化均呈现较好的线性关系,具体变化趋势如下：当盾构隧道穿越上砂下岩的上透下不透地层时,环内上半部分管片纵缝张开度随水位的增大而增大,受温度影响则相对较小;当盾构隧道穿越砂性全透水地层时,环内管片纵缝张开度随水位的增大而减小,受温度影响也相对较小;环间横缝张开度在上述两种地层下随水位、温度的变化趋势相同,均随水位增大而增大,随温度的升高而减小。