Static vulnerability of an existing r.c. structure and seismic retrofitting by CFRP and base-isolation: A case study

The assessment of the static vulnerability under gravity loads of existing reinforced concrete (r.c.) framed buildings is a serious problem that requires the use of reliable methodologies to evaluate ductile and brittle mechanisms. The present work compares alternative formulations of member chord rotation and section and joint shear strength, proposed by Italian and European seismic codes and guidelines and other expressions available in the scientific literature. To this end, a r.c. framed building built sixty years ago with bi-directional (perimeter) and mono-directional (interior) plane frames, originally designed for five storeys then elevated to six during construction, is studied. A full characterization of the structure and its materials is carried out by means of destructive and non-destructive methods. Then, retrofitting based on the use of both innovative material, such as carbon fibre reinforced polymers (CFRP), and technology, such as base-isolation, are adopted to improve the static and seismic performances of the original structure. Finally, nonlinear analyses are carried out on a three-dimensional fibre model of the original and retrofitted structures, where an elastic linear law idealizes the behaviour of the CFRP up to tension failure and viscoelastic linear and bilinear models are used to idealize the behaviour of the elastomeric and sliding bearings, respectively.     

Localized failure of fibre‐reinforced elastic–plastic materials subjected to plane strain loading

We consider discontinuous bifurcations as the indicator of a localized failure for a class of composites that are characterized by elastic fibres reinforcing an elastic–plastic matrix. A macroscopic tangent stiffness tensor for the fibre‐reinforced composite is developed by consistently homogenizing the contribution of fibres in a spherical representative volume element. Analytical solutions are derived for the critical hardening modulus and corresponding bifurcation directions for the case of plane strain loading. Properties of the solutions are further illustrated on the example of the non‐associated Drucker–Prager model at onset of yielding. Results show that presence of fibres decreases the critical hardening modulus, thus inhibiting the onset of strain localization. The rate of decrease in the critical hardening modulus is the highest for pure shear, followed by uniaxial tension, uniaxial compression, biaxial tension and biaxial compression. The main fibre parameters that control the onset of strain localization are their volumetric content and their stiffness modulus whereby very stiff fibres can produce the most significant decrease in the critical hardening modulus, especially for the state of biaxial tension. The critical hardening modulus for the non‐associated Drucker–Prager model exhibits a full range of localization modes including compaction bands, dilation bands, and transition in the form of shear bands regardless of the presence of fibres. Presence of fibres affects bifurcation directions, except in the case when Poisson's ratio of the matrix is equal to 0.25. The results demonstrate stabilizing effects of fibres by which they provide the control against the onset of strain localization. Copyright © 2006 John Wiley & Sons, Ltd.     

多波束测深边缘波束误差的综合校正

边缘波束误差是影响多波束测深数据精度的主要因素,数据精度影响其可信度和使用范围,也是进行相关研究的基础.多波束勘测系统声呐参数的精确校正、勘测区声场模型的建立以及实时勘测海洋噪声的合理剔除是影响边缘波束数据质量的关键因素,严重时甚至导致勘测数据出现沿测线方向的条带状假地形或地形位置偏移.上述因素对多波束勘测数据的影响是一个综合作用的过程,靠单一的校正或编辑方法很难提高采集数据的精度.以多波束勘测原理和声学理论为指导,以多波束实测数据为研究基础和分析对象,运用GIS面向对象方法,全面分析造成多波束勘测大误差边缘波束的原因,并探寻改善已勘测多波束大误差数据、提高数据精度的综合处理方法,最终以可视化的方式实现人机交互处理.该项研究成果已初步应用于海洋项目总图编绘工作,并取得了预期效果.     

Identification of groundwater exfiltration,interflow discharge,and hyporheic exchange flows by fibre optic distributed temperature sensing supported by electromagnetic induction geophysics

Water exchange across the sediment–water interface of streams impresses a characteristic thermal pattern at the interface. The use of fibre optic distributed temperature sensing at the sediment–water interface in a small sand‐bed stream identifies such temperature patterns. Groundwater and interflow can be differentiated based on the temporal evolution of temperature patterns. Additionally, sudden temperature changes at the sediment–water interface observed during the transit of floods enable spatial identification of local up and downwelling. Electromagnetic induction geophysics can detect subsurface texture structures that support groundwater–surface water exchange. Our results show that areas of permanent temperature anomalies observed with fibre optic distributed temperature sensing match areas of comparatively homogeneous electrical conductivity. This indicates groundwater discharge and enables differentiating groundwater discharge from interflow and local downwelling.     

巯基棉富集—大口径毛细管气相色谱法测定水样中痕量烷基汞

介绍了巯基棉富集-大口径毛细管柱气相色谱法测定环境水中痕量烷基汞的分析方法,并与玻璃填充柱色谱分离烷基汞进行了对比。取1000ml环境水样时,本法的检出限为:甲基汞(MMC)0.5pg;乙基汞(EMC)0.7pg。     

Analysis of lateral loading of pile groups using embedded beam elements with interaction surface

The numerical simulation of soil-pile interaction problems, by means of full 3D finite element models, involves a large number of degrees of freedom (DOF) and difficulties during the mesh generation process. In order to reduce the unknowns and simplify and properly analyze such class of geotechnical problems, the so-called embedded beam elements (EBE) have recently been developed. In a preceding contribution of the authors, an improved EBE formulation, which brings into play the soil-pile interaction surface, was proposed with the aim to localize material plasticity in the soil surrounding the pile. This embedded beam model couples two different finite elements, each described by distinct kinematics (ie, solid and beam). The coupling is incorporated in the formulation by means of kinematical constrains established over the solid and beam displacement fields on the interaction surface. One of the main advantages of the embedded elements is that the addition of beams structural members immersed within the 3D soil model does not represent a constraint for the solid mesh, which can be adopted independently from the beam mesh. In this paper, the lateral loading of pile groups is studied by means of the proposed EBE approach with elasto-plastic interfaces. In order to represent a rigid cap, a master node and a special set of kinematical restrictions are incorporated into the formulation. The paper presents results obtained by means of the present formulation compared against other well-established analysis methods and test results published in the literature, for both elastic and elasto-plastic cases.     

Tip-tilt requirements for coupling starlight into single-mode photonic crystal fibres using a lenslet: A first analysis

Single-mode fibres do not suffer from modal noise and are available in polarisation maintaining form. Single-mode photonic crystal fibres (smPCF) have a wavelength insensitive mode field allowing coupling of the telescope exit pupil into the fibre using a lenslet with a flat spectral response. This may allow single-mode Integral Field Spectroscopy (IFS) and fibre fed integral field spectro-polarimetry. Initial results from a theoretical analysis of telescope/smPCF coupling using a lenslet are presented. Coherent imaging theory is used to determine the coupling efficiency into the fibre and thence the fibre numerical aperture is defined and used to compute the sample size on the sky. A higher degree of tip-tilt correction is shown to be required for coupling into single-mode fibres with a lenslet than the multimode fibre alternative, for sparse sampling integral field and multi-object spectroscopy and interferometry, but the magnitude of which is within the scope of current NIR and planned (extreme) VIS AO systems. Extension of the model to contiguous integral field spectroscopy is also considered.     

The physics of E × B-drifting jets

E × B-drifting jets have been generally ignored for the past 25 years even though they may well describe all the astrophysical jet sources, both on galactic and stellar scales. Here we present closed-form solutions for their joint field-and-particle distribution, argue that the observed jets are near equipartition, with extremely relativistic, monoenergetic e^±-pairs of bulk Lorentz factor γ ≲ 10⁴, and are first-order stable. We describe plausible mechanisms for the jets’ (i) formation, (ii) propagation, and (iii) termination. Wherever a beam meets with resistance, its frozen-in Poynting flux transforms the delta-shaped energy distribution of the pairs into an almost white power law,E ² N _E ∼E ^−∫ with ∫ ≳ 0, via single-step falls through the huge convected potential.