基于纤维模型的受火后混凝土框架的有限元分析

为了分析受火后混凝土框架结构的残余力学性能,基于纤维模型的思路,将梁、柱构件截面划分成多个纤维,可以考虑构件截面的不均匀温度场分布以及受火损伤后材料力学性能的变化。以ABAQUS通用有限元软件为开发平台,利用Python编程语言对其进行二次开发,将SAP2000建立的模型文件导入到ABAQUS中,对基于纤维模型的混凝土框架进行受火后的温度场分析和非线性力学分析。通过一个多层多跨三维混凝土框架火灾反应的实例,对该框架在受火后的力学反应进行了分析,可为评估混凝土框架结构受火后的残余力学性能提供参考。     

玄武岩纤维片材和碳纤维片材高温后拉伸性能比较

对未经环氧树脂浸渍以及经环氧树脂充分浸渍的单向玄武岩纤维增强复合材料（BFRP）片材和碳纤维增强复合材料（CFRP）片材进行了高温后拉伸性能试验。试验结果表明,常温下和高温后BFRP片材的拉伸强度和拉伸模量均远远小于CFRP片材,但伸长率一般高于CFRP片材;常温下,环氧树脂的存在能够显著提高FRP片材的拉伸强度,但高温处理后,环氧树脂的作用不明显,尤其对于CFRP片材;浸渍环氧树脂的CFRP片材和BFRP片材的拉伸强度随温度的变化规律基本一致,均呈现出先降后升再降的趋势,并在150℃左右时取得最大;浸渍和未浸渍环氧树脂的FRP片材在经历300～400℃高温处理后强度均出现急剧下降;当处理温度大于200℃时,处理时间越长,FRP片材的拉伸强度和拉伸模量下降越多。     

Advanced frame element for seismic analysis of masonry structures: model formulation and validation

This paper presents a masonry panel model for the nonlinear static and dynamic analysis of masonry buildings suitable for the seismic assessment of new and existing structures. The model is based on an equivalent frame idealization of the structure and stems from previous research on force‐based frame elements. The element formulation considers axial, bending, and shear deformations within the framework of the Timoshenko beam theory. A phenomenological cyclic section law that accounts for the shear panel response is coupled, through equilibrium between shear and bending forces along the element, with a fiber‐section model that accounts for the axial and bending responses. The proposed panel model traces with a low computational burden and numerical stability the main aspects of the structural behavior of masonry panels and is suitable for analyses of multi‐floor buildings with a relatively regular distribution of openings and with walls and floors organized to grant a box‐like behavior under seismic loads. The model capabilities are validated though analyses of simple unreinforced masonry panels and comparisons with published experimental results. The model accuracy is strongly dependent on the fiber and shear constitutive laws used. However, the formulation is general, and laws different from those employed in this study are easily introduced without affecting the model formulation. Copyright © 2015 John Wiley & Sons, Ltd.     

青藏冻结粉土与玻璃钢接触面本构模型研究

冻土与构筑物基础接触面的本构模型是分析冻土区构筑物与冻土相互作用、评价工程安全稳定性的基础和关键.为了降低切向冻胀力对基础的上拔作用,防止发生冻拔失稳破坏,青海-西藏±400 kV直流联网工程基础广泛采用玻璃钢覆盖基础表面,以消减冻胀力.为了合理描述玻璃钢与冻土接触面力学特性,采用应变直剪仪开展了青藏冻结粉土与玻璃钢基础接触面直剪试验,获取了不同冻结温度条件下接触面剪应力-位移曲线,分析总结了接触面的强度变化规律和受力变形特性,基于试验得到的接触面本构规律建立了耦合温度效应的冻结粉土与玻璃钢基础接触面剪切应力-位移关系.模型预测结果与试验结果的比较表明,预测结果与试验结果吻合良好,该模型能够较好的描述接触面的力学特性.     

Structural and nonstructural performance of a seismically isolated building using stable unbonded fiber‐reinforced elastomeric isolators

Stable unbonded fiber‐reinforced elastomeric isolators (SU‐FREIs) exhibit a characteristic horizontal softening and stiffening response, similar to other adaptive devices such as the triple friction pendulum and sliding systems with variable curvature. The transition between the softening and stiffening occurs at a displacement corresponding to a unique deformation known as full rollover. In this paper, the full rollover displacement of SU‐FREIs is altered by using modified support geometry (MSG), a geometric modification of the upper and lower supports applied to tailor the hysteresis loops of the isolator. Experimental results are used to calibrate a numerical model of a base‐isolated structure. The model demonstrates that the stiffening regime provides minimal restraint against displacements during events that meet or exceed the maximum considered earthquake. A parametric study revealed that the level of stiffening required to restrain displacements during large events is significant. This increase in stiffness is reflected in an increase in the response of the structure and light nonstructural components. Full rollover and MSG is considered advantageous to maintain horizontal stability and provide control over the stiffening of SU‐FREIs. Copyright © 2015 John Wiley & Sons, Ltd.     

应用于火山活动监测的分布式光纤光栅传感技术

光纤光栅传感器作为一种新型的光纤传感器,具有体积小、可靠性高、耐腐蚀、抗电磁干扰能力强、易于组建传感网络等优点,现已广泛应用于航空航天、海洋探测、地质勘探等领域.已有学者将光纤传感器技术应用于地震监测,并取得较理想的效果.文中介绍了光纤布拉格光栅传感器( Fiber Bragg Grating,FBG)基本原理、火山活...     

Experimental study on slender reinforced concrete columns wrapped with CFRP

By axial compression tests on 6 reinforced concrete slender columns wrapped with carbon fiberreinforced plastic （ CFRP）, with slenderness ratio（SR） from 4. 5 to 17.5, the results show that when SR increases the retrofitting effect declines. In the case of same SR, the stability coefficient （ SC ） for the reinforced concrete（RC） columns with CFRP is much less than that without CFRP. There is 20% increase of stable bearing capacity to the former as compared with the latter when the SR in less than 17.5. The study summarized the simplified formula for SC, which provides a reference for engineering designers.     

Effect of Long-term Experimental Warming on the Nutritional Quality of Alpine Meadows in the Northern Tibet

The nutritional quality of grasslands is closely related to recruitment of young and population dynamics of livestock and wild herbivores. However, the response of nutritional quality to climate warming has not been fully understood in the alpine meadow on the Tibetan Plateau, especially in the Northern Tibet. Here, we investigated the effect of experimental warming ( beginning in 2008) on nutritional quality in three alpine meadows (site A: 4313 m, B: 4513 m and C: 4693 m) in the Northern Tibet. Crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), crude ash (Ash), ether extract (EE) and water-soluble carbohydrate (WSC) were examined in 2018-2019. Experimental warming only increased the content of CP by 27.25%, ADF by 89.93% and NDF by 41.20%, but it decreased the content of Ash by 57.76% in 2019 at site B. The contents of CP and WSC both increased with soil moisture (SM). The content of CP decreased with vapor pressure deficit (VPD). The combined effect of SM and VPD was greater than air temperature (T_a) in controlling the variations of the CP content, ADF content and nutritional quality. Compared to T_a, VPD explained more of the variation in NDF and Ash content. All of these findings suggest that warming effects on nutritional quality may vary with site and year, and water availability may have a stronger effect on the nutritional quality than temperature in the alpine meadow of the Northern Tibet.     

Uncertainties in the preparation of Ra Mn fiber standards

Delayed coincidence counters (RaDeCC), used for measuring ²²³Ra and ²²⁴Ra preconcentrated from water onto MnO₂-impregnated acrylic fiber (“Mn-fiber”), require a standard Mn-fiber column that has a precisely known activity of ²²⁴Ra for calibration. This may be done by adding an aged ²²⁸Th standard solution to adsorb both ²²⁸Th and its daughter ²²⁴Ra quantitatively onto a Mn fiber. We used both seawater and deionized water (DIW) for testing the adsorption efficiency of Th and Ra onto Mn fibers. Our experimental results show that more than 50% of thorium (²³²Th and ²²⁸Th) breaks through the Mn-fiber column when DIW is used as a medium. However, near quantitative recoveries are obtained if filtered (0.45 μm) seawater is used to prepare the standard. In the case of pure DIW, the pH (initial pH  5.3) rises to > 10 after passing through the column while seawater (initial pH  7.8) changes to  7.2. Thus, the lack of thorium adsorption in DIW may be attributed to this huge increase of pH and the consequent formation of Th(OH)₄ and polyhydroxyl colloids. Based on these observations, we recommend that one should use either artificial seawater or natural seawater (which has negligible ²²⁴Ra and ²²⁸Th) as a loading solution after 0.45 μm filtration. In addition, the thorium adsorption efficiency should be confirmed either by thorium analysis of the effluent solution or long-term monitoring of the supported ²²⁴Ra on the Mn fiber using the RaDeCC. Similar cautions are likely necessary for making ²²³Ra standards by adsorption of ²²⁷Ac onto Mn fibers.