A methodological approach for finite element modeling of pretensioned concrete members at the release of pretensioning

This paper presents a methodological approach for finite element simulation of pretensioned concrete members. The three-dimensional analysis presented in this paper involves a rectangular [150 mm (6 in.) tall × 150 mm (6 in.) wide × 2440 mm (96 in.) long] concrete member hosting one 15-mm (0.6 in.) diameter 7-wire low-relaxation Grade 1860 MPa (270 ksi) prestressing strand. The finite element models are divided into two general classifications: (i) concentrically pretensioned, and (ii) eccentrically pretensioned. The finite element models are analyzed based on elastoplastic material behaviors as well as mesh sensitivity. Two approaches are examined for finite element modeling of the pretensioned concrete specimens: (i) the extrusion technique utilizing friction-based contact simulations, and (ii) the embedment technique simulating equivalent responses while being a computationally less expensive solution. A comparative analysis is presented to measure the validity as well as accuracy of the findings by the finite element techniques against the commonly used closed-form solutions based on elastic beam theory. The validity of the finite element approach is further verified by comparative analysis of the analytical data against the experimental findings. The paper concludes that the embedment technique provides an accurate and numerically efficient alternative in comparison with the extrusion method for the simulation of the pretensioned concrete members. While the extrusion technique provides more detailed information corresponding to the regions located immediately around the prestressing strands, including the interface overstresses and bond slippage, the embedment technique appears to have the ability to simulate the overall response of the concrete members with comparable accuracy.     

Electrocatalytic hydrogen evolution on iron-cobalt nanoparticles encapsulated in nitrogenated carbon nanotube

Using low-cost nonprecious metals to replace Pt as hydrogen evolution reaction (HER) electrocatalyst is promising, but still limited by their efficiency and stability. Herein, with low-cost dicyandiamide and metal salts as precursor, FeCo alloy nanoparticles encapsulated in nitrogenated carbon nanotubes (FeCo-NCNTs) were facially synthesized as efficient HER electrocatalyst. Addition of iron as second element, though not facilitating the formation of carbon nanotube, was utilized to improve the physicochemical properties of metals. Via optimizing the atomic molar ratios of Fe/Co nanoparticles, the optimal Fe_0.4Co_0.6-NCNTs with thin carbon shell (c.a. 5–10 layer) and equally distribution of embedded alloy nanoparticles was found with outstanding HER activity. To achieve a current density of 10 mA cm⁻², only overpotential of 50 mV, 157 mV and 202 mV were needed in acidic (0.5 M H₂SO₄), alkaline (1 M KOH), and neutral solutions. Its higher electrochemically active surface areas and lower electron transfer resistance may contribute to the excellent electrocatalytic HER. Furthermore, the illustrated current density slightly changed over 20 h, suggesting excellent stability. Hence, the present method provides cost-effective, high efficiency, and stable materials in developing the sustainable energy conversion systems.     

太原一电厂六期锅炉柱脚固定新方案的确立和实施

太原一电厂１３号锅炉柱脚施工中,采用了取消埋设地脚螺栓的定位方法,缩短了工期,提高了效益,取得良好效果。     

Preparation of Bio-beads and Their Atrazine Degradation Characteristics

Screened atrazine-mineralizing bacterium-Pseudomonas W4 was embedded inside an improved PVA- H<,3>BO<,3> embedment matrix to make bio-beads to degrade atrazine. The atrazine degradation characteristics were stu- died. The preparation procedure of bio-beads was as follows: (1) preparing a mixture of 100, 12.5, 10, 1.5 and 1 g/L PVA, bentonite(Ca), activated carbon powder, sodium alginate and centrifuged Pseudomonas W4 bacterium, respec- tively; (2) the mixture was dropped into a gently stirred cross linker solution(pH=6.7) and cured at 10 ℃ for 24 h. The optimal atrazine degradation conditions by bio-beads were as follows: pH=7, the auxiliary carbon source was glucose, and the concentration of glucose was greater than 325 mg/L. The bio-beads demonstrated stronger tolerance ability than the free microorganism to the increase of PCBs, hydrogen ion and hydroxide ion. SEM images show the uniform distribution of the microorganism inside bio-beads and the porous cross-linked structure of bio-beads which provides excellent mass transfer capacity.     

浅谈输电线钢管杆的设计

本文对输电线钢管杆设计中的材料选择、杆段连接形式、横担连接形式和地脚螺栓形式及锚固长度等各方面进行了介绍,以期指出输电线路钢管杆设计中的关键点,为工程建设提供参考。     

Lateral and Upward Soil-Pipeline Interactions in Sand for Deep Embedment Conditions

The soil–pipeline interactions in sand under lateral and upward movements are investigated with particular attention to the peak forces exerted on the pipe. The analytical solutions for estimating the peak forces are summarized and it is shown that, for deep embedment condition, there is large uncertainty in the true values since the bounds established by the analytical solutions are large. In order to find the solution for the peak force and to investigate its transition from shallow to deep failure mechanism, finite element analyses of lateral and upward pipe movements are performed for different embedment conditions. Two different soil models (Mohr–Coulomb and Nor–Sand models) are used for the simulations. The accuracy of the analysis is first examined by simulating experimental tank tests. The analysis is further extended to deeper embedment ratios of as large as 100. The obtained finite element results are used to construct a design chart for deep embedded pipelines.     

高层建筑基础埋置深度不足时的结构设计

本文结合工程实例,针对高层建筑不满足基础埋置深度的情况,对小,中,大震工况下桩基的抗压、抗拉和水平承载力进行验算,确认主体结构在地震作用和水平风荷载作用下满足整体倾覆验算需采取的措施。     

Scanning electron microscopic observations on deembedded biological tissue sections: Comparison of different fixatives and embedding materials

Conventional plant histological specimens fixed in formalin-acetic acid-alcohol, chromic acid-acetic acid-formaldehyde, or glutaraldehyde-osmium and embedded in either paraffin or plastic are examined as possible rapid methods for providing an alternative image of cellular structure by using scanning electron microscopy. Using the mitotic figures of actively growing onion root tips as a study specimen, the organization of the nucleus and spindle apparatus is reasonably well preserved as compared with isolated mitotic spindles and studies of mitosis in endosperm tissue. Relief of internal structure in this technique is obtained through the coagulant nature of the fixative. Used judiciously, this technique can reveal aspects of the three-dimensional nature of internal tissue structure that may otherwise be difficult to discern.     

Closed-Form Model and Parametric Study on Connection of Concrete-Filled FRP Tubes to Concrete Footings by Direct Embedment

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been introduced as a new system for piles, columns, and poles. A simple moment connection based on direct embedment of the CFFT into concrete footings or pile caps, without using dowel-bar reinforcement, has been proposed by the authors. Robust analytical models to predict the critical embedment length (Xcr) were also developed and experimentally validated. In this paper, a comprehensive parametric study is carried out using the models developed earlier along with a newly developed closed-form model for the general case of axial loading, bending, and shear applied to the CFFT member. The parameters studied are the diameter (D), thickness (t), length outside the footing (L), and laminate structure of the FRP tube, as well as the tube-concrete interface bond strength (τmax?), concrete compressive strength in the CFFT (fct′) and footing (fc′), and the magnitude and eccentricity of axial compressive or tensile loads. It was shown that increasing D, L/D, τmax?, and fc′ of the footing, or the axial compression load, reduces (X/D)cr, whereas increasing t and fct′ of the CFFT, the fraction of longitudinal fibers in the tube, or the axial tension load, increases Xcr. As the axial load eccentricity increases, Xcr reduces for tension loads and increases for compression loads until both cases converge asymptotically to the same Xcr value, essentially that of pure bending.     

Einbindetiefen und Schnittgrößen bei gestützten Wänden mit beliebiger Fußauflagerung

Dieser Beitrag beinhaltet die Ermittlung von Schnittgrößen und Einbindetiefen bei gestützten Wänden. Zunächst wird ein Modell der Fußauflagerung vorgestellt, das sowohl eine freie Auflagerung, eine Teil einspannung als auch eine volle Einspannung umfasst. Die erforderliche Einbindetiefe lässt sich durch Ansatz von Bemessungsgrößen wesentlich einfacher berechnen als am System mit charakteristischen Größen. Bei einer teilweisen Einspannung im Boden können durch Vorgabe eines Drehwinkels im theoretischen Auflagerpunkt die charakteristischen Bodenreaktionen eindeutig festgelegt werden. Für den Fall, dass nur ständige Einwirkungen auftreten, können charakteristische Größen und Bemessungsgrößen durch einen Faktor ineinander umgerechnet werden. Eine weitere Vereinfachung ergibt sich, wenn man, den Empfehlungen des Arbeitskreises “Baugruben” folgend, die veränderlichen Einwirkungen mit einem Erhöhungsfaktor vergrößert und wie für den Lastfall mit ständigen Einwirkungen behandelt. Embedment depths and sectional forces for propped walls with different foot supports. In the following contribution the calculation of the embedment depth and the sectional forces is treated for the case of propped walls. A model for the earth support at the foot of the wall is presented including a free, a partly fixed and a fixed earth support. The determination of the embedment depth at the system with design loads is essentially easier compared to a calculation with characteristic quantities. Assuming a value for the rotation angle at the theoretical foot point of the wall permits to calculate the magnitude of the characteristic soil reactions. In the case that only permanent actions react on the wall, characteristic and design quantities are related by a simple factor. According to the recommendations of excavations a further simplification is possible, if the transient actions are increased by a factor and are treated as in the case of permanent situations.