Showing papers on "Slab published in 1997"

Evidence for deep mantle circulation from global tomography

Abstract: Seismic tomography based on P-wave travel times and improved earthquake locations provides further evidence for mantle-wide convective flow. The use of body waves makes it possible to resolve long, narrow structures in the lower mantle some of which can be followed to sites of present-day plate convergence at the Earth's surface. The transition from subduction-related linear structures in the mid-mantle to long-wavelength heterogeneity near the core-mantle boundary remains enigmatic, but at least some slab segments seem to sink to the bottom of the mantle.

Element transport from slab to volcanic front at the Mariana arc

Abstract: We present a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab. The lavas display many features typical of island arc volcanics, with all samples showing large negative niobium anomalies and enrichments in alkaline earth elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many of these key ratios correlate with a large range in 238U excesses, (238U/230Th) = 0.97–1.56. Geochemical features show island to island variations; lavas from Guguan have the largest 238U-excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small 238U excesses, the least radiogenic 143Nd/144Nd, and the largest negative cerium and niobium anomalies. These highly systematic variations enable two discrete slab additions to the subarc mantle to be identified. The geochemical features of the Agrigan lavas are most consistent with a dominant subducted sediment contribution. The added sedimentary component is not identical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fractionation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible elements, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab-derived aqueous fluid phase, and Pb and Sr isotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary component, but near-constant aqueous fluid flux along arc strike, can create the compositional trends observed in the Mariana lavas. High field strength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan lavas are higher than those of most mid-oceanic ridge basalts and suggest a highly depleted subarc mantle prior to any slab additions. The 238U-230Th systematics indicate >350 kyr between sediment and mantle melting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the aqueous fluid itself may trigger major mantle melting.

Thermal and dynamical evolution of the upper mantle in subduction zones

Abstract: We present results from two-dimensional (2-D) numerical experiments on the thermal and dynamical evolution of the subducting slab and of the overlying mantle wedge for a range in subduction parameters. These include subduction rate and the age and rheology of both subducting and overriding plates. Experiments also consider the influence of slab forcing conditions (from purely kinematic to purely dynamic) on the evolution of both the slab and mantle wedge. One goal is to determine how different parameters control thermal evolution of the slab-wedge interface, from just after subduction initiation up through roughly 500–600 km of subduction, where temperatures are approaching steady state. An additional goal is to define optimal conditions for the melting of slab sediments and crust. Results show slab surface temperatures (SSTs) depend strongly on subduction velocity, plate thermal structure, and upper mantle (or wedge) viscosity structure. Fast subduction beneath a thick (>70 km) overriding plate results in the coolest SSTs. Maximum SSTs are recorded as an early transient event for cases of slow subduction ( 100 km) which deflects the zone of maximum shear away from slab-wedge interface.

Mantle structure beneath Indonesia inferred from high-resolution tomographic imaging

Abstract: We investigated mantle structure beneath the Indonesian region by means of tomographic inversions of traveltime residuals of direct P and the surface-reflected depth phases pP and pwP. The hypocentres and phase data used in the inversions were derived from the reprocessing of data reported to international data centres, which include data from the Australian SKIPPY project. We used more than 6.0 × 10⁶ data from more than 50 000 globally distributed earthquakes recorded at a subset of nearly 3000 seismological stations worldwide. The use of depth phases improved the sampling of the upper-mantle structure, in particular beneath the back-arc regions. The mapping of distant aspherical mantle structure into the study region was minimized by combining a high-resolution regional with a low-resolution global inversion. The inversion was linearized around the ak135 reference velocity model, and a combination of minimum norm and gradient damping was applied to constrain the solution. Inversion of different subsets of the data and test inversions of synthetic data indicate that resolution is sufficient to substantiate the conclusions pertinent to the complex large-scale structure of the subducted slab beneath the Indonesian archipelago. The tomographic images confirm previous evidence for regional slab penetration into the upper part of the lower mantle but suggest a more complex trajectory of mantle flow beneath Indonesia. The implied lateral variation in the shape of the upper-mantle slab is generally in good agreement with inferences from seismicity. The data reveal a seismic anomaly beneath the Sunda arc, which is detected to at least 1500 km in depth and forms the eastern end of a deep anomaly associated with the past subduction of lithosphere of the Mesozoic Tethys ocean. Below Java the lithospheric slab is probably continuous from the Earth's surface to the lower mantle, but beneath Sumatra the deep slab seems to be detached from the seismogenic slab. The high-resolution images suggest that the subducted slab is deflected in the transition zone beneath the Banda arc and that the descending slabs form a spoon-shaped structure. North of the Banda Sea we detect the two opposing subducted slabs of the Molucca Sea plate; this is consistent with previous studies, but our images suggest that the westward subduction has a significantly steeper dip than the eastward subduction. Slab structure beneath the Banda and Molucca seas is explained in the context of the continent-arc collision between Australia and the southeast Indonesian Island arc, and the arc-arc collision between Sangihe and Halmahera. Inversion of traveltime residuals of direct S phases, a novelty for this region, largely confirms the inferences from the P-wave data, even though less detail is resolved.

Seismicity and structure of the Kamchatka subduction zone

Abstract: The configuration of the Pacific plate subducted beneath the Kamchatka peninsula and the stress distribution in the Kamchatka subduction zone (KSZ) were studied using the catalog of the Kamchatka regional seismic network, focal mechanism solutions estimated from P wave first motions, the formal inversion of long-period waveforms, and centroid moment tensor solutions. To the south of ∼55°N, the slab shows an approximately constant dip angle of ∼55°. To the north of ∼55°N, the dip of the slab becomes shallower reaching ∼35°. The maximum depth of seismicity, Dm, varies from ∼500 km depth near 50°N to ∼300 km depth at ∼55°N. The volcanic front is almost linear along the main part of the KSZ whereas it is sharply shifted landward to the north of ∼55°N. The variation of Dm is apparently consistent with the standard empirical relation Dm=ƒ(ϕ), where ϕ is the thermal parameter of the subducted slab. To the north of ∼55°N, the slab is offset toward the northwest, and it is sharply deformed in a narrow contorted zone which is ∼30 km wide (∼56°N, ∼161°E). To the north of this contortion, Dm decreases to ∼100 km. The landward shift of the northern part of the slab is reflected by a sharp deviation of the volcanic front to the northwest which follows the ∼90–160 km isodepth range of the subducted slab. The observed value of Dm in the northern segment significantly diverges from the global relation Dm=ƒ(ϕ). We interpret this as an effective decrease of the thermal thickness of the subducted lithosphere.

P and S deep velocity structure of the Hellenic area obtained by robust nonlinear inversion of travel times

Abstract: We derived a new model for the P and S velocity structure of the lithosphere in the subduction area of the Hellenic arc and the Aegean Sea from the inversion of travel times of local events The inversion technique applied is nonlinear, since three-dimensional ray tracing is incorporated At the same time, an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results The P-S coherency is controlled by an additional damping of the VP/VS ratio The study focuses mainly on the structure of the Hellenic subduction in the southern Aegean region Interesting features and details of the subducted slab can be recognized in the final tomographic images On the western part, the shallower section of the slab is dipping at a very small angle (≃10°) This changes to approximately 25° for the deeper part of the subduction, resulting in a prominent kink at a depth of about 70 km, which is in accordance with the general characteristics of the associated Benioff zone The smaller eastern part of the slab is not resolved with the same detail, but a clearly steeper subduction zone can be recognized Moreover, detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments Finally, the results confirm the recent suggestion concerning the existence of a low-velocity crustal layer at shallow depths (≃10–15 km) in the accretionary prism of the Alpine belt

Observation of leaky slab modes in an air-bridged semiconductor waveguide with a two-dimensional photonic lattice

Abstract: An air-bridged, 120-nm-thick semiconductor slab with a two-dimensional (2D) square array of through holes on a 480 nm pitch (Λ) was fabricated using selective wet etching techniques. The second order photonic resonances of the structure were studied by comparing broadband optical scattering data with numerical solutions of Maxwell’s equations. Features observed in these spectra over a 1200 cm−1 range, near 9500 cm−1, indicate that the 2D texture splits the energy degeneracy of slab modes with propagation constants {±2π/Λ,0} and {0,±2π/Λ} by as much as 14%.

Photon migration through a turbid slab described by a model based on diffusion approximation. II. Comparison with Monte Carlo results

Abstract: In our companion paper we presented a model to describe photon migration through a diffusing slab. The model, developed for a homogeneous slab, is based on the diffusion approximation and is able to take into account reflection at the boundaries resulting from the refractive index mismatch. In this paper the predictions of the model are compared with solutions of the radiative transfer equation obtained by Monte Carlo simulations in order to determine the applicability limits of the approximated theory in different physical conditions. A fitting procedure, carried out with the optical properties as fitting parameters, is used to check the application of the model to the inverse problem. The results show that significant errors can be made if the effect of the refractive index mismatch is not properly taken into account. Errors are more important when measurements of transmittance are used. The effects of using a receiver with a limited angular field of view and the angular distribution of the radiation that emerges from the slab have also been investigated.

Density gradient effects on aluminium foam compression behaviour

Abstract: The compression properties of an aluminium foam containing a nonuniform density gradient have been examined. Specimens were taken from various locations within the foam slab, and were tested in two directions. Measured foam properties were compared to calculated values using models derived by Ashby and Gibson [1]. The effect of the density gradient on the compression properties and also the energy absorption characteristics of the foam was found to be significant.

Seismic image of the subducted trailing fragments of the Farallon plate

Abstract: The Farallon plate was an enormous oceanic plate located west of the Americas during the Cenozoic and Mesozoic eras. This plate has now been almost completely subducted beneath the American plates. In the Northern Hemisphere, the Farallon plate broke up to form a number of independent smaller plates1–3 when its western edge approached the North American plate. Here we present a tomographic image of the subducted trailing fragments of the Farallon plate in the upper mantle beneath the western margin of North America. The relatively cold, subducted fragments appear as an intricate region of high seismic S-wave velocity. Comparison of the structure of this high-velocity region with tectonic plate reconstructions and volcanic records enables us to identify individual fragments of the subducted Farallon plate and thus reconstruct qualitatively the kinematic evolution of the Farallon slab in the upper mantle beneath North America.

Across-arc variations of isotope and trace element compositions from Quaternary basaltic volcanic rocks in northeastern Japan: Implications for interaction between subducted oceanic slab and mantle wedge

Abstract: Isotopic compositions of Pb, Sr, and Nd and concentrations of trace elements were determined for Quaternary island arc basaltic rocks from northeastern Japan. Sr and Pb isotopic ratios decrease, and Nd isotopic ratios increase from the volcanic front toward the back arc. The isotopic compositions nearest the back arc side are nearly identical to those of mid-ocean ridge basalt (MORB). The high field strength elements and heavy rare earth elements show homogeneous and MORB-like characteristics. These observations indicate that the mantle wedge beneath northeastern Japan originally had a MORB-type mantle composition that was homogeneous across the arc. Pb isotope compositions show a mixing relationship between mantle wedge and oceanic sediments reflecting the introduction of subduction component into the mantle wedge, Across-arc isotopic variations were caused by interaction between MORB-type mantle wedge and the subducting slab, and the amount of subduction component correlates with the depth to the slab. The isotopic compositions of subduction component are expressed by bulk mixing of 15 wt % of oceanic sediment and 85 wt % of altered MORB. Inversion analyses of isotopic compositions using two-component mixing relationships show that the Sr/Nd and Pb/Nd ratios in subduction component decrease with increasing depth to the slab, while the Sr/Pb ratio is nearly constant. These changes can be explained only by a preferential discharge of the elements into the wedge mantle associated with continuous dehydration of the subducting slab. The present study further demonstrates that a very wide range of isotopic and elemental compositions in island arc magmas is a consequence of the interaction between subducting slab and mantle wedge without the involvement of an oceanic island basalts component, and the slab can carry water and supply a subduction component as a fluid to the overlying mantle wedge to depths exceeding 150 km.

Slab detachment in continental collision zones : An analysis of controlling parameters

Abstract: During the early stages of continental collision when continental lithosphere is subducted after a prolonged period of normal oceanic subduction, changes in the temperature distribution and stress field of the subducting lithosphere can lead to failure within the lithosphere, and result in detachment of the deeper portion of subducted lithosphere (slab detachment or slab breakoff). We use a 2D time-dependent numerical model to identify the parameters controlling slab detachment and to estimate breakoff depths and time lengths. Temperature and rheology of subducting continental lithosphere are found to control slab detachment.

Arching action strength enhancement in laterally-restrained slab strips

Abstract: The behaviour and ultimate load capacity of laterally-restrained reinforced concrete slabs can be considerably enhanced by the development of arching or compressive membrane action. This paper presents a simple method for predicting the enhanced ultimate load capacity of laterally-restrained slab strips. The method is based on deformation theory and utilizes an elastic-plastic stress-strain criterion for concrete. The loads carried by bending and arching action are calculated separately and then added to give the total ultimate load capacity. A simple equivalent strip approach, based on a three-hinged arch analogy, allows for the degree of lateral restraint. The method of prediction has been validated by correlation with a wide range of test results from various sources. (A)

Topography of the “410” and “660” km seismic discontinuities in the Izu-Bonin Subduction Zone

Abstract: We study the effects of a subducting lithospheric slab on the “410” and “660” km mantle seismic discontinuities in the Izu-Bonin region using data collected from networks of vertical component seismometers in the United Kingdom and northwestern United States. The consequences of a phase change origin for these discontinuities is tested by examining the effect of laterally changing temperature in and near the subducting slab. We analyse the data collected from 21 earthquakes with depths between 40 and 548 km. Based on arrival time and slowness we identify reflections and mode conversions in slant stacks with the aim of mapping the “410” and “660” km discontinuities. The combination of UK and US array data permits interrogation of the discontinuity properties on both sides of and also inside the slab. Results indicate a “410” minimum depth of 350 km in the interior of the slab and a “660” maximum depth of 700 km. A phase change origin for the discontinuities is consistent with the effects of lateral temperature changes on discontinuities with Clapeyron slopes of opposite signs. Here we see more topography on the “410” than the “660” consistent with thermodynamic Clapeyron slope estimates. “410” elevation suggests temperatures in the slab of 600±75°C at a depth of 350 km. We see weak evidence for a broader 410 near the slab interior, consistent with expected thermochemical effects.

Effect of nonlinear temperature gradient on curling stress in concrete pavements

Abstract: Temperature and moisture gradients can lead to significant tensile stresses at the slab top and bottom. Current techniques for assessing the internal stresses due to such gradients are based on the assumption that temperature and moisture distributions through the slab thickness are linear. However, the actual distributions of such gradients have been found to be highly nonlinear. A new closed form solution technique for calculating the stresses in a pavement slab due to nonlinear gradients is introduced. The analysis is separated into two parts. In the first, an expression is presented for calculating the self-equilibrated stresses within a cross section due to internal restraint (i.e., satisfying equilibrium conditions and continuity of the strain field within the cross section). These stresses are independent of slab dimensions and boundary conditions. In the second, the stresses due to external restraint (i.e., self-weight and subgrade reaction) are calculated using an equivalent linear temperature gradient obtained from the first part and existing closed form solutions by Westergaard or Bradbury. The solution to this step includes slab length and boundary conditions. Total internal stresses due to nonlinear gradients are obtained by using the superposition principle. The methodology has been applied to field data from two studies in which the temperature profiles were recorded throughout a 24-hr period. Linear gradient solution methods cannot accurately predict the curling stresses in concrete pavements. This is especially pronounced during nighttime and early morning hours, during which nonlinear analysis predicts tensile stress in both the slab bottom and top before the application of any traffic loading.

Simulation of f- and p-Mode Interactions with a Stratified Magnetic Field Concentration

Abstract: The interaction of f- and p-modes with a slab of vertical magnetic field of sunspot strength is simulated numerically in two spatial dimensions. Both f-modes and p-modes are partially converted to slow magnetoatmospheric gravity (MAG) waves within the magnetic slab because of the strong gravitational stratification of the plasma along the magnetic lines of force. The slow MAG waves propagate away from the conversion layer guided by the magnetic field lines, and the energy they extract from the incident f- and p-modes results in a reduced amplitude for these modes as they exit from the back side of the slab. In addition, the incident p-modes are partially mixed into f-modes of comparable frequency, and therefore larger spherical harmonic degree, when they exit the magnetic flux concentration. These findings have important implications for the interpretation of observations of p-mode absorption by sunspots, both in terms of the successes and failures of this simple numerical simulation viewed in the sunspot seismology context.

Deformation of subducted oceanic lithosphere

Abstract: SUMMARY The deformation of subducted oceanic lithosphere is revealed by detailed studies of the distribution of seismicity in Benioff zones and by the increasingly well-resolved images of the upper mantle obtained by seismic P-wave tomography. By using numerical experiments based on a simplified dynamical model of a subducted lithospheric slab, we test the idea that the observed deformation of the subducted slab results from a balance between internal buoyancy forces and the viscous stresses associated with deformation. The simplified model assumes that the lithosphere has uniform physical properties, it is denser and much more viscous than the upper mantle, and its penetration below 670 km depth is resisted by a step-like density increase at that level. The primary determinant of the style of deformation is the buoyancy number, F, a ratio of buoyancy stress generated by the mass anomaly in the slab to the stress associated with viscous deformation at a strain rate U,/L, defined in terms of subduction rate U, and slab thickness L. For a small buoyancy number, deformation of the slab is dominated by viscous flexure. For large F, down-dip extension of the slab appears to dominate, and a buckling instability of the slab is observed if there is resistance to penetration of the 670 km level. The transition value of F at which this change in behaviour is observed is about 0.05 for constant viscosity, and about 0.2 for stressdependent viscosity with a stress versus strain-rate exponent of n = 3. With n = 3, a boudinage-type instability of the slab is observed for large F. Penetration of the slab below the 670 km level depends on the density contrast between slab and lower mantle. Resisted only by the density difference, the slab may initially penetrate several hundred kilometres below the 670 km level (for plausible density parameters) before a buckling instability causes this part of the slab to rotate and ascend. Comparison of the deforming slab geometry and stress field with seismicity distribution and tomographic images from the Tonga subduction zone suggests that the effective buoyancy number F for this slab is approximately equal to the transition value that we determined experimentally. We use this constraint to estimate average rheological parameters for the Tonga slab which, when compared with published creep deformation laws for olivine, are consistent with the average temperature of the slab being about 0.4 to 0.45 times the melting temperature, based on constitutive laws for olivine. At shallow depths (< 100 km) these temperatures correspond to around 590°C for wet olivine or around 700°C for dry olivine.

Deep-footing and wide-slab effects in reinforced sandy ground

Abstract: Quantitative evaluations are performed on two failure mechanisms, namely, deep-footing and wide-slab mechanisms, that dominate the bearing-capacity characteristics of sandy ground reinforced with horizontal reinforcing layers. The improvement contributed by reinforcement, creating a quasi-rigid, wide earth slab immediately under the footing to the bearing capacity is analyzed. The maximization of the bearing-capacity increase of a footing is a matter of optimizing the depth and width of the quasi-rigid earth slab. The results of a total of 105 model tests, including reduced-scale model tests and some centrifuge tests under 15 g, are analyzed using calibrated internal friction angles of sand and an experimentally verified failure mechanism in reinforced sandy ground, namely, the deep-footing mechanism. Back-calculations are performed on the load-spreading angle α, which represents the total effect of the deep footing and wide slab for all the tests. Both linear and nonlinear multiple-variable data regressions are performed to find the relationships between the effect of reinforcing and the factors that control the scheme of reinforcement. The analytical procedure and equation derived in the present study substantiate the prediction of the ultimate bearing capacity of sandy ground, reinforced with horizontal reinforcement.

Optical device having low insertion loss

Abstract: An optical device 201, having one or more input waveguides 25 coupled to one side of a slab waveguide 20 and an array of output waveguides 26 coupled to an other side of the slab waveguide, is improved by including a transition region 22 immediately adjacent to the slab that operates to reduce insertion loss between the waveguide array 26 and the slab waveguide. The transition region includes a number of silica paths (a I . . . a n ) that intersect the waveguide array. The silica paths have widths W(a n ) that progressively decrease as they become further away from the slab waveguide. The silica paths are parallel to each other and perpendicular to the waveguide array. Disclosed embodiments include an M×N star coupler, a Dense Waveguide Division Multiplexer, and a 1×N power splitter. In each of these embodiments, insertion loss is substantially reduced over similar devices not employing such a transition region.

Green's functions for planar soft and hard surfaces derived by asymptotic boundary conditions

Abstract: The Green's functions for different realisations of planar soft and hard surfaces are developed by using the asymptotic boundary conditions and the spectral-domain approach. The geometries considered are the ideal perfect electric conductor/perfect magnetic conductor (PEC/PMC) strip surface, the strip-loaded grounded dielectric slab and the corrugated surface. In all cases the strips and corrugations are straight. The asymptotic boundary conditions are valid in the limiting sense, when the period of the strips or corrugations approaches zero. The Green's functions developed have poles corresponding to surface waves. These are of three types: an ordinary surface wave in the grounded dielectric slab propagating radially out from the source; a strip wave propagating along the strips of the strip-loaded dielectric slab and also along the strips of the PEC/PMC strip surface; and surface waves occurring due to the corrugated surface. Fulfilment of both the soft and hard boundary conditions is discussed in both the near- and far-field regions. The strip wave of the strip-loaded dielectric slab prevents the boundary conditions in the near-field from being fulfilled, and is thus undesired. The other two surface waves are needed to realise the hard boundary condition.

Plate coupling in the northern South Island and southernmost North Island, New Zealand, as illuminated by earthquake focal mechanisms

Abstract: Subduction of the Pacific plate in the northern South Island and southernmost North Island of New Zealand is transitional, insofar as the crustal thickness of the Pacific plate increases significantly along strike in the northern South Island. Focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes. The stress regime in the subducted plate appears to be dominated by slab pull. T axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone, and the average azimuth of these T axes is rotated some 25° clockwise out of the direction of dip of the subducted plate. This can be related to the asymmetrical shape of the subducted slab. In contrast, the stress regime in the overlying plate appears to be dominated by subhorizontal compression. Low-angle thrust events near the plate interface in Cook Strait and the southernmost North Island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from Global Positioning System (GPS) observations. An absence of low-angle thrust events near the plate interface in the northern South Island and the tendency of P axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost North Island. Differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern South Island.

Dynamic properties of polypropylene fiber-reinforced concrete slabs

Abstract: A series of experiments were performed on both simply supported and on-grade circular slab specimens, reinforced with different volumes of fibrillated polypropylene fibers, to gauge the fibers' influence on the slab's impact resistance and natural frequency. Numerical simulations were then used to reproduce the natural frequencies. In conjunction with these experiments, standard compression and flexural tests were run to determine the material strength of the concrete. It was found that the inclusion of fibers produced an improvement in impact resistance proportional to the fiber volume contained in the concrete and had no affect on the natural frequency. An inverse relationship between fiber volume and material strength was also found for both compression and flexure.

The influence of semi-rigid connections on the performance of steel framed structures in fire.

Abstract: Observation of fire damaged structures and recent fire tests at the Cardington LBTF have suggested that even nominally `simple' connections are capable of providing significant restraint at elevated-temperatures. As most frames are designed assuming pinned response at ambient-temperature, with no account being taken of the reduction in mid-span moments, this is an aspect of connectivity which may be utilised in the assessment of the fire resistance of steel framed buildings, without necessitating changes in the approach adopted in ambient-temperature design or construction. To date the assessment of the influence of connection response on frame behaviour has been limited by the quantity of available test data, although initial studies based on postulated moment-rotation- temperature characteristics concluded that the failure temperatures for beams are increased due to the rigidity of `simple' connections. Moment-rotation relationships have been measured for a flush end-plate connection, both as bare-steel and as composite with a concrete slab across a range of temperatures. To define accurately the full moment-rotation-temperature response a series of tests have been conducted for each arrangement, where specimens were subject to varying constant levels of load and increasing temperatures. Observed failure mechanisms have been compared with those for a nominally identical specimen tested at ambient-temperature, and initial recommendations presented for the degradation of ambient-temperature connection characteristics. A mathematical expression is proposed in order to represent the test data at a number of temperatures. It is clearly unrealistic to expect that many such tests can be anticipated in the future, and as such a spring-stiffness model has been presented for both bare-steel and composite flush end-plate connections. The use of a spring-stiffness model compares favourably with other forms of modelling due to the combination of efficient solution and the ability to follow accurately the full non-linear range of connection response, based on an understanding of the response of the component parts. A multi-linear representation of response has been adopted, where the stiffness of the connection is revised as elements enter the plastic range of response. Comparison has been made between the response predicted and that recorded experimentally. Experimentally derived connection characteristics have been incorporated within analysis of typical sub-frames, with parameters including connection stiffness, capacity and temperature being varied. Further studies are presented considering the sensitivity of overall frame behaviour to inaccuracies in the representation of connection response and the use of simplified models to generate elevated-temperature connection characteristics. Based on postulated elevated-temperature moment-rotation characteristics for the connections contained within the Cardington test frame, predictions have been presented for the response of the structure subject to a series of full scale fire tests, with semi-rigid behaviour being compared with the common assumptions of pinned and rigid characteristics.

Computer controlled method and apparatus for meat slabbing

Abstract: A computer controlled method and apparatus for meat slabbing according to updated information from the meat product and/or the slab cut from the meat product. Such information may include the weight and fat content of the cut slab. This information may be used to reoptimize the slabbing of the meat product and also may be used to further process the cut slab. Such processing might include marking the location of fat to be trimmed, trimming the fat from the slab, portioning the slab and sorting the slab in accordance with preselected parameters.