249 research outputs found
Influence of Fire on the Shear Capacity of Cold-Formed Steel Framed Shear Walls
This paper presents experimental investigations of the performance of common lateral force-resisting systems used in cold-formed steel construction under sequential thermal (fire) and mechanical (earthquake) loading. Wall specimens with gypsum-sheet steel composite sheathing, Oriented Strand Board (OSB) sheathing, or steel strap bracing were tested. The results demonstrate that the lateral capacity of wall systems can be reduced by exposure to fire. Additionally, fire performance of wall systems can be affected by pre-damage to the fire-resistive components that provide fire protection to these walls. The results are useful for fire compartmentation design when significant lateral deformation of a building is anticipated and post-fire assessment to repair or replace a structure. The study represents a step toward developing fire fragility functions for cold-formed steel framed shear wall systems to enable performance-based fire design
Measuring Three-Dimensional Temperature Distributions in Steel-Concrete Composite Slabs Subjected to Fire using Distributed Fiber Optic Sensors
Detailed information about temperature distribution can be important to understand structural behavior in fire. This study develops a method to image three-dimensional temperature distributions in steel–concrete composite slabs using distributed fiber optic sensors. The feasibility of the method is explored using six 1.2 m × 0.9 m steel–concrete composite slabs instrumented with distributed sensors and thermocouples subjected to fire for over 3 h. Dense point clouds of temperature in the slabs were measured using the distributed sensors. The results show that the distributed sensors operated at material temperatures up to 960◦C with acceptable accuracy for many structural fire applications. The measured non-uniform temperature distributions indicate a spatially distributed thermal response in steel–concrete composite slabs, which can only be adequately captured using approaches that provide a high density of through-depth data points
Extraction of Resonances from Meson-Nucleon Reactions
We present a pedagogical study of the commonly employed Speed-Plot (SP) and
Time-delay (TD) methods for extracting the resonance parameters from the data
of two particle coupled-channels reactions. Within several exactly solvable
models, it is found that these two methods find poles on different Riemann
sheets and are not always valid. We then develop an analytic continuation
method for extracting the nucleon resonances within a dynamical coupled-channel
formulation of the and reactions. The main focus is on
resolving the complications due to the coupling with the unstable channels which decay into states. By using the
results from the considered exactly solvable models, explicit numerical
procedures are presented and verified. As a first application of the developed
analytic continuation method, we present the nucleon resonances in the
and partial waves extracted within a recently developed
coupled-channels model of reactions. The results from this realistic
model, which includes , , , , and
channels, also show that the simple pole parametrization of the
resonant propagator using the poles extracted from SP and TD methods works
poorly.Comment: 32 pages, 14 figure
Significance of anaerobic methane oxidation in methane-rich sediments overlying the Blake Ridge gas hydrates
A unique set of geochemical pore-water data, characterizing the sulfate reduction and uppermost methanogenic zones, has been collected at the Blake Ridge (offshore southeastern North America) from Ocean Drilling Program (ODP) Leg 164 cores and piston cores. The δ13 C values of dissolved CO2(Σ CO2) are as 13 C-depleted as –37.7‰ PDB (Site 995) at the sulfate-methane interface, reflecting a substantial contribution of isotopically light carbon from methane. Although the geochemical system is complex and difficult to fully quantify, we use two methods to constrain and illustrate the intensity of anaerobic methane oxidation in Blake Ridge sediments. An estimate using a two-component mixing model suggests that ~24% of the carbon residing in the Σ CO2 pool is derived from biogenic methane. Independent diagenetic modeling of a methane concentration profile (Site 995) indicates that peak methane oxidation rates approach 0.005 μmol cm–3 yr–1, and that anaerobic methane oxidation is responsible for consuming ~35% of the total sulfate flux into the sediments. Thus, anaerobic methane oxidation is a significant biogeochemical sink for sulfate, and must affect interstitial sulfate concentrations and sulfate gradients. Such high proportions of sulfate depletion because of anaerobic methane oxidation are largely undocumented in continental rise sediments with overlying oxic bottom waters. We infer that the additional amount of sulfate depleted through anaerobic methane oxidation, fueled by methane flux from below, causes steeper sulfate gradients above methane-rich sediments. Similar pore water chemistries should occur at other methane-rich, continental-rise settings associated with gas hydrates
Factors that control the stable carbon isotopic composition of methane produced in an anoxic marine sediment
The carbon isotopic composition of methane produced in anoxic marine sediment is controlled by four factors: (1) the pathway of methane formation, (2) the isotopic composition of the methanogenic precursors, (3) the isotope fractionation factors for methane production, and (4) the isotope fractionation associated with methane oxidation. The importance of each factor was evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Methane did not accumulate during the initial 42-day period when sediment contained sulfate, indicating little methane production from 'noncompetitive' substrates. Following sulfate depletion, methane accumulation proceeded in three distinct phases. First, CO2 reduction was the dominant methanogenic pathway and the isotopic composition of the methane produced ranged from -80 to -94 per thousand. The acetate concentration increased during this phase, suggesting that acetoclastic methanogenic bacteria were unable to keep pace with acetate production. Second, acetate fermentation became the dominant methanogenic pathway as bacteria responded to elevated acetate concentrations. The methane produced during this phase was progressively enriched in C-13, reaching a maximum delta(C-13) value of -42 per thousand. Third, the acetate pool experienced a precipitous decline from greater than 5 mM to less than 20 micro-M and methane production was again dominated by CO2 reduction. The delta(C-13) of methane produced during this final phase ranged from -46 to -58 per thousand. Methane oxidation concurrent with methane production was detected throughout the period of methane accumulation, at rates equivalent to 1 to 8 percent of the gross methane production rate. Thus methane oxidation was too slow to have significantly modified the isotopic signature of methane. A comparison of microcosm and field data suggests that similar microbial interactions may control seasonal variability in the isotopic composition of methane emitted from undisturbed Cape Lookout Bight sediment
Temperature and Strain Measurements with Fiber Optic Sensors for Steel Beams Subjected to Fire
This paper presents measurements of high temperatures using a Brillouin scattering based fiber optic sensor and large strains using an extrinsic Fabry-Perot interferometric sensor for assessing the thermo-mechanical behaviors of simply supported steel beams subjected to combined thermal and mechanical loading. The distributed fiber optic sensor captures detailed, non- uniform temperature distributions that are compared with thermocouple measurements resulting in an average relative difference of less than 5 % at 95 % confidence level. The extrinsic Fabry- Perot interferometric sensor captures large strains at temperatures above 1000 C. The strain results measured from the distributed fiber optic sensors and extrinsic Fabry-Perot interferometric sensors were compared, and the average relative difference was less than 10 % at 95 % confidence level
Investigation of the Neutron Form Factors by Inclusive Quasi-Elastic Scattering of Polarized Electrons off Polarized He: A Theoretical Overview
The theory of quasi-elastic inclusive scattering of polarized leptons off
polarized He is critically reviewed and the origin of different expressions
for the polarized nuclear response function appearing in the literature is
explained. The sensitivity of the longitudinal asymmetry upon the neutron form
factors is thoroughly investigated and the role played by the polarization
angle for minimizing the proton contribution is illustrated.Comment: Phys. Rev C in press; 9 figs. (available upon request
The Decuplet Revisited in PT
The paper deals with two issues. First, we explore the quantitiative
importance of higher multiplets for properties of the decuplet in
chiral perturbation theory. In particular, it is found that the lowest order
one--loop contributions from the Roper octet to the decuplet masses and
magnetic moments are substantial. The relevance of these results to the chiral
expansion in general is discussed. The exact values of the magnetic moments
depend upon delicate cancellations involving ill--determined coupling
constants. Second, we present new relations between the magnetic moments of the
decuplet that are independent of all couplings. They are exact at the
order of the chiral expansion used in this paper.Comment: 7 pages of double column revtex, no figure
Relativistic resonances: Their masses, widths, lifetimes, superposition, and causal evolution
Whether one starts form the analytic S-matrix definition or the requirement
of gauge parameter independence in renormalization theory, a relativistic
resonance is given by a pole at a complex value s of energy squared. The
complex number s does not define the mass and width separately and this
definition does not lead to interfering Breit-Wigner if two or more resonances
are involved. To accomplish both we invoke the decaying particle aspect of a
resonance and associate to each pole a space of relativistic Gamow kets which
transform irreducibly under causal Poincare transformations. A Gamow state has
an exponential time evolution and one can choose of the many possible width
parameters, that parameter as the width of the relativistic resonance which
equals the inverse lifetime. This uniquely defines the mass and width
parameters for a relativistic resonance. Two or more poles in the same partial
wave are given by the sum of Breit-Wigners in the scattering amplitude and by a
superposition of Gamow vectors with each Gamow vector corresponding to one
Breit-Wigner. In addition to the sum of Breit-Wigners the scattering amplitude
contains a background amplitude representing direct production of the final
state (contact terms).This contact amplitude is associated to a background
vector which is a continuous superposition of Lippmann-Schwinger states.
Omitting this continuum gives the Weisskopf-Wigner approximation.Comment: 22 pages, REVTe
A model for two-proton emission induced by electron scattering
A model to study two-proton emission processes induced by electron scattering
is developed. The process is induced by one-body electromagnetic operators
acting together with short-range correlations, and by two-body
currents. The model includes all the diagrams containing a single correlation
function. A test of the sensitivity of the model to the various theoretical
inputs is done. An investigation of the relevance of the currents is
done by changing the final state angular momentum, excitation energy and
momentum transfer. The sensitivity of the cross section to the details of the
correlation function is studied by using realistic and schematic correlations.
Results for C, O and Ca nuclei are presented.Comment: 30 pages, 18 figures, 3 table
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