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71.
Near‐fault ground motions are characterized by long‐period horizontal pulses and high values of the ratio between the peak value of the vertical acceleration, PGAV, and the analogous value of the horizontal acceleration, PGAH, which can become critical for base‐isolated (BI) structures. The objective of the present work is to check the effectiveness of the base isolation of framed buildings when using High‐Damping‐Rubber Bearings (HDRBs), taking into consideration the combined effects of the horizontal and vertical components of near‐fault ground motions. To this end, a numerical investigation is carried out with reference to BI reinforced concrete buildings designed according to the European seismic code (Eurocode 8). The design of the test structures is carried out in a high‐risk region considering (besides the gravity loads) the horizontal seismic loads acting alone or in combination with the vertical ones and assuming different values of the ratio between the vertical and horizontal stiffnesses of the HDRBs. The nonlinear seismic analysis is performed using a step‐by‐step procedure based on a two‐parameter implicit integration scheme and an initial‐stress‐like iterative procedure. At each step of the analysis, plastic conditions are checked at the potential critical sections of the girders (i.e. end sections of the sub‐elements in which a girder is discretized) and columns (i.e. end sections), where a bilinear moment–curvature law is adopted; the effect of the axial load on the ultimate bending moment (M‐N interaction) of the columns is also taken into account. The response of an HDRB is simulated by a model with variable stiffness properties in the horizontal and vertical directions, depending on the axial force and lateral deformation, and linear viscous damping. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
72.
Isabella Bordi Klaus Fraedrich Alfonso Sutera Xiuhua Zhu 《Theoretical and Applied Climatology》2012,109(1-2):245-252
A change in CO2 concentration induces a direct radiative forcing that modifies the planetary thermodynamic state, and hence the surface temperature. The infrared cooling, by assuming a constant temperature lapse-rate during the process, will be related to the surface temperature through the Stefan–Boltzmann law in a ratio proportional to the new infrared opacity. Other indirect effects, such as the water vapor and ice-albedo feedbacks, may amplify the system response. In the present paper, we address the question of how a global climate model with a mixed layer ocean responds to different rates of change of a well-mixed greenhouse gas such as CO2. We provide evidence that different rates of CO2 variation may lead to similar transient climates characterized by the same global mean surface temperature but different values of CO2 concentration. Moreover, it is shown that, far from the bifurcation points, the model’s climate depends on the history of the radiative forcing displaying a hysteresis cycle that is neither static nor dynamical, but is related to the memory response of the model. Results are supported by the solutions of a zero-dimensional energy balance model. 相似文献
73.
Octavio Artieda Alfonso Davila Jacek Wierzchos Peter Buhler Rafael Rodríguez‐Ochoa Juan Pueyo Carmen Ascaso 《地球表面变化过程与地形》2015,40(14):1939-1950
Miocene continental saltpans are scattered in the Central Valley of the Atacama Desert, one of the driest regions on Earth. These evaporitic deposits are hydrologically inactive, and are detached from groundwater brines or aquifers. The surface of the saltpans, also known as salars, comprises desiccation polygons, commonly with nodular salt structures along their sides. The morphology and bulk mineralogy of salt polygons differs between and within salars, and the shape and internal structure of salt nodules varies between different polygon types. Based on field observation, and mineralogy and crystallography data, we generated a conceptual model for the genesis and evolution of these surface features, whereby rare rainfall events are responsible for the transformation of desiccation salt polygons and the initial formation of salt nodules along polygon borders. In addition, frequent, but less intense, deliquescence events further drive the evolution of salt nodules, resulting in a characteristic internal structure that includes laminations, and changes in porosity and crystal morphologies. As a result, and despite the extreme dryness, the surfaces of fossil salars are dynamic on timescales of several years to decades, in response to daily cycles in atmospheric moisture, and also to rare and meager rainfall events. We propose that fossil salars in the Atacama Desert represent an end stage in the evolution of evaporitic deposits under extreme and prolonged dryness. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
74.
Nonlinear dynamic response of r.c. framed structures subjected to near-fault ground motions 总被引:2,自引:1,他引:1
The nonlinear dynamic response of reinforced concrete (r.c.) framed buildings subjected to near-fault ground motions is studied
to check the effectiveness of current code provisions with reference to study cases. Three-, six- and twelve-storey r.c. plane
frames, representative of symmetric framed buildings, are designed according to the European seismic code (EC8), assuming
medium and high ductility classes and stratigraphic profiles A (rock) and D (soft soil) in a high-risk seismic region. The
nonlinear seismic analysis is performed using a step-by-step procedure; a bilinear model idealizes the behaviour of the r.c.
frame members. Artificially generated motions (matching EC8 response spectra for subsoil classes A and D) and horizontal motions
(recorded on rock- and soft soil-site at near-fault areas) are considered. The results indicate that near-fault ground motions
may require a special consideration in the code, in particular when designing r.c. framed structures placed on a soft soil-site;
particular attention should be paid to the design of the frame members of the lower storeys. 相似文献
75.
Thébault H Rodriguez Y Baena AM Andral B Barisic D Albaladejo JB Bologa AS Boudjenoun R Delfanti R Egorov VN El Khoukhi T Florou H Kniewald G Noureddine A Patrascu V Pham MK Scarpato A Stokozov NA Topcuoglu S Warnau M 《Marine pollution bulletin》2008,57(6-12):801-806
The common mussel Mytilus galloprovincialis was selected as unique biomonitor species to implement a regional monitoring programme, the CIESM Mediterranean Mussel Watch (MMW), in the Mediterranean and Black Seas. As of today, and upon standardization of the methodological approach, the MMW Network has been able to quantify (137)Cs levels in mussels from 60 coastal stations and to produce the first distribution map of this artificial radionuclide at the scale of the entire Mediterranean and Black Seas. While measured (137)Cs levels were found to be very low (usually < 1 Bq kg(-1) wet wt) (137)Cs activity concentrations in the Black Sea and North Aegean Sea were up to two orders of magnitude higher than those in the western Mediterranean Basin. Such effects, far from representing a threat to human populations or the environment, reflect a persistent signature of the Chernobyl fallout in this area. 相似文献
76.
77.
Alfonso Mucci Louis-Filip Richard Marc Lucotte Constance Guignard 《Aquatic Geochemistry》2000,6(3):293-324
The distribution and partitioning of dissolved andparticulate arsenic and phosphorus in the water columnand sediments of the Saguenay Fjord in Quebec, Canada,are compared. In addition, selective and/or sequentialextractions were carried out on the suspendedparticulate matter (SPM) and solid sediments tocontrast their geochemical behaviors in this naturalaquatic system.Results of our analyses show that both arsenic andsoluble reactive phosphate are actively scavenged fromthe water column by settling particles. Upon theiraccumulation at the sediment-water interface some Asand P may be released to porewaters following thedegradation of organic matter to which they areassociated. The porewater concentrations are, however,limited by their strong affinity for authigenic,amorphous iron oxyhydroxides which accumulate in theoxic sediments near the sediment-water interface.The geochemical behavior of arsenic and phosphorusdiverge most strikingly upon the development of anoxicconditions in the sediments. Following their burial inthe anoxic zone, amorphous iron oxyhydroxides arereduced and dissolved, releasing phosphate and arsenicto the porewaters. We observed, however, thatporewater arsenic concentrations increase at shallowerdepths than phosphate in the sediments. The reductionof arsenate, As(V), to arsenite, As(III), and itsdesorption prior to the reductive dissolution of thecarrier phase(s) may explain this observation.Driven by the strong concentration gradientestablished in the suboxic zone, phosphate diffuses uptowards the oxic layer where it is readsorbed byauthigenic iron oxyhydroxides. In the organic-rich andrapidly accumulating sediments at the head of theFjord, porewater sulfate depletion and the resultingabsence of a sulfide sink for Fe(II), may lead to theformation of vivianite in the fermentation zone, apotential sink for phosphate. Arsenite released to theporewaters in the suboxic and anoxic zones of thesediments diffuses either down, where it is adsorbedto or incorporated with authigenic iron sulfides, orup towards the oxic boundary. Arsenite appears tomigrate well into the oxic zone where it may beoxidized by authigenic manganese oxides before beingadsorbed by iron oxyhydroxides present at the samedepth. Whereas, in the absence of authigenic carbonatefluorapatite precipitation, the ability of oxicsediments to retain mineralized phosphate is afunction of their amorphous iron oxyhydroxide content,arsenic retention may depend on the availability ofmanganese oxides, the thickness of the oxic layer and,its co-precipitation with iron sulfides at depth. 相似文献
78.
Tourmaline-rich rocks associated with clastic metasedimentary rocks of Carboniferous age occur in the Cinco Villas massif,
western Pyrenees. Three types of tourmaline-rich rocks were distinguished: (1) Fine-grained stratiform tourmaline-rich rocks,
which are associated with carbonaceous metapelites (TR1); (2) stratabound tourmaline-rich rocks, associated with metapelites
in the contact aureole of the Aya granitoid pluton (TR2); (3) stratabound to massive tourmaline-rich rocks, associated with
psammopelites in contact with granites and pegmatites (TR3). Tourmalines belong to the schorl–dravite solid solution series
and have a wide compositional range, from nearly end-member dravite for TR1 tourmalines to schorl for TR3 tourmalines; TR2
tourmalines have intermediate compositions. The Fe/(Fe+Mg) typically varies between 0.02 and ≈0.55, increasing from TR1 to
TR3. The TR1 tourmalines commonly display a discontinuous chemical zoning with Fe-rich green cores (8–8.5% FeO) and Mg-rich
colorless rims (10–11% MgO). In contrast, crystals that exhibit fine growth lamellae appear to lack significant chemical zoning.
Oxygen and hydrogen isotope compositions also reveal major differences between TR1 and TR3 tourmalines, the former displaying
heavier δ18O values (17.7–19‰) and δD values (−35 to −42‰) than TR3 tourmalines 11 to 13‰ and −47 to −76‰, respectively. The TR2 tourmalines
show intermediate values of 11.3 to 14.6‰ for δ18O and −40 to −55‰ for δD. Linear and continuous chemical variations obtained for major and trace elements of the whole rocks
reflect mixing between clay-rich and quartz-rich end-members, indicative that some tourmaline-rich rocks contain a significant
detrital component. Chondrite normalized REE (rare earth element) patterns of tourmaline-rich rocks are similar to those of
surrounding unaltered clastic metasediments, except for some TR1 rocks which are characterized by low contents of ΣREE. Mass-balance
calculations show that tourmaline-forming processes plus metamorphism led to mass and volume changes at mesoscopic scales
(≈10% for the TR1 tourmalinites). Silicon, Fe, Mn, and REE elements were partially lost from sedimentary rocks, whereas Mg
and particularly B were added to pelitic sediments. Available data, nevertheless, do not allow an assessment of the boron
source. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation
from B-rich hydrothermal fluids or colloids, early diagenetic reactions of carbonaceous pelitic sediments with these fluids,
and subsequent recrystallization during regional metamorphism. The TR2 tourmaline-rich rocks mainly developed by metamorphic
recrystallization of TR1. Tourmaline-rich rocks and veins adjacent to pegmatites and granitic rocks (TR3) are the result of
boron metasomatism; the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and
magmatism.
Received: 18 November 1996 / Accepted: 25 April 1997 相似文献
79.
Tao An Xiao-Yu Hong Wen-Xia Feng Jun-Hui Zhao Shanghai Astronomical Observatory Chinese Academy of Sciences Shanghai National Astronomical Observatories Chinese Academy of Sciences Beijing Harvard-Smithsonian Center for Astrophysics Garden Street MS Cambridge MA 《中国天文和天体物理学报》2004,4(6):527-532
We observed 1938-155, a broad line radio galaxy (BLRG), with the Very Large Array (VLA) at 90cm with an angular resolution of 6.8" × 4.0". The source consists of two components separated by 4 arcsec (- 20kpc, for H0 = 65km s-1 Mpc-1, q0 = 0.5) along the SE-NW direction. Both components show steep-spectra with a similar spectral index α - 0.83 ± 0.07 (Sv ∝ v-α). The bright double components are surrounded by a low-brightness cocoon. The radio properties of the two bright components are consistent with the hot spots produced by twin jets. An upper limit of - 0.0008 for the core dominance parameter (R) is inferred, suggesting there is no prominent radio core in the source. Assuming a modest viewing angle 30f77, a jet velocity is estimated - 0.07 c, based on the jet to counter-jet brightness ratio (J). The lower limit in jet speed inferred is consistent with no Doppler beaming effect on the jet. The radio galaxy 1938-155 could be an exceptional BLRG with no prominent radio core or jet. 相似文献
80.