Higher-mode force response in multi-story strongback-braced frames     

Barbara G. Simpson Ph.D. 《地震工程与结构动力学》2020,49(14):1406-1427

Strongback-braced frames employ an essentially elastic steel truss, or strongback, that distributes demands more uniformly to delay or prevent story mechanisms. Because inertial forces are no longer limited by the formation of a story mechanism, strongback-braced frames can exhibit large elastic force demands, particularly in the higher modes. This paper characterizes the higher-mode force response of strongback-braced frames. Four-story archetypes were designed using nonlinear dynamic analyses to incorporate higher-mode force demands into the design process. The response of the archetypes was compared with that of reference buckling-restrained braced frames that were allowed to form story mechanisms. The force demands in the strongback were then described using equivalent-static forces to represent the inertial forces induced by the higher modes. Force demands in the strongback arise from a yielding first-mode ‘pivoting’ and elastic higher-mode ‘bending’ response. These higher-mode force demands are elastic, ill-constrained by the strength of the yield mechanism, and depend significantly on the choice of ground motion record used for the analysis. In remaining elastic in the higher modes, the strongback distributes demands more uniformly and mitigates the formation of story mechanisms. Consequently, design and analysis methods for strongback-braced frames need to include estimates for these near-elastic higher-mode force demands.  相似文献   

Ecohydrologic feedbacks controlling sizes of cypress wetlands in a patterned karst landscape     

Xiaoli Dong  A. Brad Murray  James B. Heffernan 《地球表面变化过程与地形》2019,44(5):1178-1191

Many landforms on Earth are profoundly influenced by biota. In particular, biota play a significant role in creating karst biogeomorphology, through biogenic CO₂ accelerating calcite weathering. In this study, we explore the ecohydrologic feedback mechanisms that have created isolated depressional wetlands on exposed limestone bedrock in South Florida – Big Cypress National Preserve –as a case study for karst biogeomorphic processes giving rise to regularly patterned landscapes. Specifically, we are interested in: (1) whether cypress depressions on the landscape have reached (or will reach) equilibrium size; (2) if so, what feedback mechanisms stabilize the size of depressions; and (3) what distal interactions among depressions give rise to the even distribution of depressions in the landscape. We hypothesize three feedback mechanisms controlling the evolution of depressions and build a numerical model to evaluate the relative importance of each mechanism. We show that a soil cover feedback (i.e. a smaller fraction of CO₂ reaches the bedrock surface for weathering as soil cover thickens) is the major feedback stabilizing depressions, followed by a biomass feedback (i.e. inhibited biomass growth with deepening standing water and extended inundation period as depressions expand in volume). Strong local positive feedback between the volume of depressions and rate of volume expansion and distal negative feedback between depressions competing for water likely lead to the regular patterning at the landscape scale. The individual depressions, however, are not yet in steady state but would be in ~0.2–0.4 million years. This represents the first study to demonstrate the decoupling of landscape-scale self-organization and the self-organization of its constituent agents. © 2018 John Wiley & Sons, Ltd.  相似文献   

Effectiveness of Monitored Natural Attenuation (MNA) as a Groundwater Remedy for Arsenic in Phosphatic Wastes     

Brad A. Bessinger  Remy J.-C. Hennet 《Ground Water Monitoring & Remediation》2019,39(4):52-68

A study was conducted to evaluate monitored natural attenuation (MNA) as a remedy for arsenic in groundwater at a former phosphate mining and manufacturing facility. The mineralogy, speciation, and lability of arsenic in phosphatic wastes present in soils were characterized using sequential extraction procedures, leaching experiments, batch adsorption tests, and microchemical speciation analysis. A PHREEQC-based reactive transport model was also parameterized using these laboratory results, and it was used to evaluate the importance of identified attenuation mechanisms on arsenic concentrations along a vertical flow path from a shallow, alluvial aquifer to the underlying Floridan aquifer. Arsenic was found to occur in several chemical forms in phosphatic wastes, including unstable sulfide minerals, adsorbed surface complexes, and relatively insoluble phosphate and oxide minerals. Most arsenic was associated with stable minerals. The reactive transport model predicted that historical leaching of solid-phase waste materials in soils would not have generated enough arsenic to explain the concentrations observed in downgradient groundwater; instead, the source of arsenic to groundwater was likely acidic and saline process water that infiltrated though unlined ponds and ditches during historical manufacturing operations. A key factor affecting the long-term effectiveness of natural attenuation of arsenic in groundwater is the occurrence and stability of iron oxyhydroxides in aquifer sediments. According to laboratory and reactive transport model results, sufficient levels were found to be present at the site to effectively limit arsenic migration at concentrations exceeding drinking water standards in the future in the Floridan aquifer. This study presents the geochemical evaluations that are needed to satisfy EPA guidelines on determining whether or not MNA is an acceptable remedy for a site. It specifically details the characterization and modeling that were used to demonstrate effectiveness at a site where MNA was ultimately selected as the remedy for arsenic in groundwater.  相似文献   

Mineral chemical provenance of Neolithic pitchstone artefacts from Ballygalley,County Antrim,Northern Ireland     

Jeremy Preston  Ian Meighan  Derek Simpson  Malcolm Hole 《Geoarchaeology》2002,17(3):219-236

A large number of pitchstone fragments and artefacts have recently been discovered at a Neolithic settlement site in Ballygalley, Co. Antrim, Northern Ireland. They consist predominantly of flakes and un‐reworked lumps and cores, with only one complete tool being found. Since no sources of workable pitchstone exist in Ireland, the source must have been the abundant pitchstone volcanic rocks found on the Hebridean Islands of northwest Scotland. The composition of the glass from a number of artefacts is highly siliceous, indicating that they were derived from pitchstones on the Island of Arran; pitchstones from all other Scottish locations are less silica‐rich. In addition, analysis of pyroxene and amphibole microcrystallites within the pitchstone suggests that the Corriegills area of Arran is the most likely source of the Ballygalley artefacts, although the precise outcrop has proved elusive. These finds, and others across Ireland, show that raw materials were being transported and probably traded over considerable distances despite there being suitable alternative sources of material for making tools (flint, etc.) available in the local area. This suggests that the pitchstone had a very specialist use. © 2002 Wiley Periodicals, Inc.  相似文献   

不同类型草地挥发性有机物排放特征的研究   总被引：2，自引：1，他引：1  

白建辉  Brad Baker 《大气科学》2006,30(1):119-130

2003年9月,对内蒙古草原不同类型草地挥发性有机物(VOC)的排放、太阳辐射、气象参数等进行观测,结果表明,固定和新的羊草样地、封育样地、不同程度放牧样地异戊二烯、α蒎烯、β蒎烯、柠檬烯、蒈烯的排放均有明显的日变化规律,并与可见光辐射以及温度有较好的一致性.羊草样地、封育样地、过度放牧、适度放牧样地异戊二烯排放的最大值分别为139.5、25.9、132.3、107.1(单位:μg·m-2·h-1,以碳计).多数情况下,异戊二烯排放通量的测定结果为羊草样地高于封育样地、过度放牧样地高于适度放牧样地.不同程度放牧样地α蒎烯、β蒎烯、柠檬烯、蒈烯等的排放具有明显差别,一般是过度放牧草地高于适度放牧草地.剪草增大了绝大部分VOC组分的排放、以及过度和适度放牧草地VOC的排放,剪草可以造成过度和适度放牧草地异戊二烯排放在中午前后的增加.剪草后,过度放牧草地异戊二烯、α蒎烯、β蒎烯、柠檬烯、蒈烯等的排放通量均远大于适度放牧草地的相应值.土壤异戊二烯、α蒎烯、β蒎烯等的排放很小,分别占相近可见光辐射和温度条件下固定羊草样地排放的0.2%、0、3.0%.  相似文献   

Recent shifts in coastline change and shoreline stabilization linked to storm climate change   总被引：1，自引：0，他引：1       下载免费PDF全文

Jennifer M. Johnson  Laura J. Moore  Kenneth Ells  A. Brad Murray  Peter N. Adams  Richard A. MacKenzie III  John M. Jaeger 《地球表面变化过程与地形》2015,40(5):569-585

Since cuspate coastlines are especially sensitive to changes in wave climate, they serve as potential indicators of initial responses to changing wave conditions. Previous work demonstrates that Cape Hatteras and Cape Lookout, North Carolina, which are largely unaffected by shoreline stabilization efforts, have become increasingly asymmetric over the past 30 years, consistent with model predictions for coastline response to increases in Atlantic Ocean summer wave heights and resulting changes in the distribution of wave‐approach angles. Historic and recent shoreline change observations for Cape Fear, North Carolina, and model simulations of coastline response to an increasingly asymmetric wave climate in the presence of beach nourishment, produce comparable differences in shoreline change rates in response to changes in wave climate. Results suggest that the effect of beach nourishment is to compensate for – and therefore to mask – natural responses to wave climate change that might otherwise be discernible in patterns of shoreline change alone. Therefore, this case study suggests that the effects of wave climate change on human‐modified coastlines may be detectable in the spatial and temporal patterns of shoreline stabilization activities. Similar analyses of cuspate features in areas where the change in wave climate is less pronounced (i.e. Fishing Point, Maryland/Virginia) and where local geology appears to exert control on coastline shape (i.e. Cape Canaveral, Florida), suggest that changes in shoreline configuration that may be arising from shifting wave climate are currently limited to sandy wave‐dominated coastlines where the change in wave climate has been most pronounced. However, if hurricane‐generated wave heights continue to increase, large‐scale shifts in patterns of erosion and accretion will likely extend beyond sensitive cuspate features as the larger‐scale coastline shape comes into equilibrium with changing wave conditions. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Quantifying the rate and depth dependence of bioturbation based on optically‐stimulated luminescence (OSL) dates and meteoric 10Be          下载免费PDF全文

Michelle O. Johnson  Simon M. Mudd  Brad Pillans  Nigel A. Spooner  L. Keith Fifield  Mike J. Kirkby  Manuel Gloor 《地球表面变化过程与地形》2014,39(9):1188-1196

Both the rate and the vertical distribution of soil disturbance modify soil properties such as porosity, particle size, chemical composition and age structure; all of which play an important role in a soil's biogeochemical functioning. Whereas rates of mixing have been previously quantified, the nature of bioturbation's depth dependence remains poorly constrained. Here we constrain, for the first time, the relationship between mixing rate and depth in a bioturbated soil in northeast Queensland, Australia using a novel method combining OSL (optically‐stimulated luminescence) ages and meteoric beryllium‐10 (¹⁰Be) inventories. We find that the best fit mixing rate decreases non‐linearly with increasing soil depth in this soil and the characteristic length scale of 0.28 m over which the mixing coefficient decays is comparable to reported rooting depth coefficients. In addition we show that estimates of surface mixing rates from OSL data are highly dependent on erosion rate and that erosion rate must be constrained if accurate mixing rates are to be quantified. We calculate surface diffusion‐like mixing coefficients of 1.8 × 10^?4 and 2.1 × 10^?4 m² yr^?1 for the studied soil for two different estimates of soil erosion. © 2014 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.  相似文献   

Copper,chromium, and lead in Manukau Harbour sediments     

J. Aggett  J. D. Simpson 《新西兰海洋与淡水研究杂志》2013,47(4):661-663

A limited survey was undertaken to determine the concentrations of copper, lead, and chromium in sediments of the upper Manukau Harbour, New Zealand. The concentrations of copper and chromium (33–70 and 70–107 μg g^‐1 respectively) were of the same order of magnitude as those reported for other New Zealand harbours. However, lead concentrations (98–247 μg g^‐1) were higher than those found for the other harbours. This may be a consequence of higher traffic densities in the surrounding area.  相似文献   

A Wet/Wet Differential Pressure Sensor for Measuring Vertical Hydraulic Gradient     

by Brad G. Fritz  Rob D. Mackley 《Ground water》2010,48(1):117-121

Vertical hydraulic gradient is commonly measured in rivers, lakes, and streams for studies of groundwater–surface water interaction. While a number of methods with subtle differences have been applied, these methods can generally be separated into two categories; measuring surface water elevation and pressure in the subsurface separately or making direct measurements of the head difference with a manometer. Making separate head measurements allows for the use of electronic pressure sensors, providing large datasets that are particularly useful when the vertical hydraulic gradient fluctuates over time. On the other hand, using a manometer-based method provides an easier and more rapid measurement with a simpler computation to calculate the vertical hydraulic gradient. In this study, we evaluated a wet/wet differential pressure sensor for use in measuring vertical hydraulic gradient. This approach combines the advantage of high-temporal frequency measurements obtained with instrumented piezometers with the simplicity and reduced potential for human-induced error obtained with a manometer board method. Our results showed that the wet/wet differential pressure sensor provided results comparable to more traditional methods, making it an acceptable method for future use.  相似文献   

Reconciling discrepant chronologies for the geomagnetic excursion in the Mono Basin,California: Insights from new 40Ar/39Ar dating experiments and a revised relative paleointensity correlation     

William S. Cassata  Brad S. Singer  Joseph C. Liddicoat  Robert S. Coe 《Quaternary Geochronology》2010,5(5):533-543

Lacustrine sediments of the Wilson Creek Formation in the Mono Basin, California, record a paleomagnetic field excursion constrained by ¹⁴C and ⁴⁰Ar/³⁹Ar geochronology to have occurred within the last 50 ka. However, ¹⁴C and ⁴⁰Ar/³⁹Ar ages are discordant, making it difficult to distinguish which of two possible excursions during this period, the Mono Lake or Laschamp, is recorded in the Mono Basin. New ⁴⁰Ar/³⁹Ar age determinations from sanidine, as well as the first biotite and obsidian ages, for three of the nineteen rhyolitic ashes intercalated with these sediments are presented and compared to previous ¹⁴C and ⁴⁰Ar/³⁹Ar data sets. Although the sanidine ages of the three ashes are stratigraphically consistent with each other and previously determined ⁴⁰Ar/³⁹Ar ages for other ashes in the Wilson Creek Formation, each is significantly older than ¹⁴C ages obtained from stratigraphically equivalent beds, relative paleointensity field correlations, oxygen isotope records, and glacial histories. These data indicate an absence of juvenile, eruptive crystals and most likely reflect the incorporation of crystals from older volcanic centers or underlying sediment. We examine the strengths and weaknesses of all available geochronologic data for the section exposed at Wilson Creek to arrive at an internally consistent set of age constraints. Using these constraints we propose two new relative paleointensity correlations for the section, both of which indicate that the excursion recorded in the Mono Basin occurred at ～30–34 ka on the Greenland Ice Sheet Project 2 (GISP2) ice core time scale.  相似文献