Showing papers on "Slab published in 1981"

Reinforced Concrete Slabs

Abstract: Provides a thorough review of most of the possible approaches to reinforced concrete slab analysis and design. Includes determination of the distribution of moments and shears using elastic theory; the direct design'' and equivalent frame'' methods of the American Concrete Institute's building code; limit procedures for ultimate load analysis and slab design using general lower bound theory, the strip method, and yield line theory; behavior of slabs at the service load, deflection, and crack control; shear strength; and the effect of membrane action on slab strength. Provides an introduction to prestressed slabs.

Wave propagation in a magnetically structured atmosphere: II: Waves in a magnetic slab

Abstract: Magnetic fields may introduce structure (inhomogeneity) into an otherwise uniform medium and thus change the nature of wave propagation in that medium. As an example of such structuring, wave propagation in an isolated magnetic slab is considered. It is supposed that disturbances outside the slab are laterally non-propagating. The effect of gravity is ignored.The field can support the propagation of both body and surface waves. The existence and nature of these waves depends upon the relative magnitudes of the sound speed co and Alfvén speed ςA inside the slab, and the sound speed ce in the field-free environment.In general terms the slow mode can always propagate, and does so both as a surface wave and as a body wave. On the other hand, the fast mode may propagate only in slabs that are not hotter than their surroundings (ce ≥ c0), and then it is a body wave or a surface wave accordingly as ce is greater than or less than ςA. For example, if ce > co > ςA then a fast body wave propagates with phase-speed between ce and co, a slow body wave between ςA and cT = coςA/(co2 + ςA2)1/2, and a slow surface wave with phase-speed below cT. There are no modes between co and ςA. As a second illustration, if ςA > ce > co, then in addition to the slow body and slow surface waves, as before, there is a fast surface wave with phase-speed between co and ce. There is no fast body wave.The special case of a slender field is also investigated and it is shown how the slender flux tube approximation relates to the more general results described above. In particular, the tube wave with phase-speed cT studied by Defouw (1976) and Roberts and Webb (1978) is shown to be a slow surface wave (sausage mode). Finally, we discuss briefly the generation of resonant modes in a slender slab.

Double seismic zones and stresses of intermediate depth earthquakes

Abstract: Data from Japanese local seismograph networks suggest that the stresses in double seismic zones are in-plate compression for the upper zone and in-plate tension for the lower zone; the stresses do not necessarily appear to be down-dip. It may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in-plate stresses occur in a given segment of slab. A global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled, at least in part, by the age of the slab and the rate of convergence. Old and slow slabs are under in-plate tensile stresses and the amount of in-plate compression in the slab increases with increasing convergence rate or decreasing slab age. Young and fast slabs are an exception to this trend; all such slabs are down-dip tensile. Since these slabs all subduct under continents, they may be bent by continental loading. Double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses. Unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones, although they may contribute to producing some of their characteristics. Sagging or thermal effects, possibly aided by asthenospheric relative motion, may produce the local deviatoric stresses that cause double zones.

The rates of plate creation and consumption

Abstract: Summary. An alternative vector derivation of the expression for the rates of plate creation and consumption is given, from which it can be seen that the rates are proportional to the scalar product of the edges of the geohedron and plate polyhedron corresponding to the same boundary. The plate polyhedron is defined by the set of triple junction vectors. This is used, together with a recent determination of the angular velocity vectors and a number of auxiliary plate boundary vectors, to revise the rates of plate creation and consumption along sections of plate boundaries. All plates which have a substantial proportion of their boundary connected to a downgoing slab are found to be decreasing in area. The total rate of plate generation is used in estimates of the area of the ocean floor and the total heat loss from the oceans. The rate of consumption within a relatively small area around New Guinea implies a large mass flux away from this region. Knowledge of the rates of plate creation and consumption is used in a number of different areas: calculating the contribution of plate creation to the total heat loss of the Earth; calculating geochemical fluxes out of the mantle, such as for the rare gases and other volatiles, and return fluxes back into the mantle resulting from reactions between sea-water and the ocean crust during hydrothermal circulation; calculating the area of the ocean floor as a function of age. Mass fluxes related to the distribution of the rates of creation and destruction along the plate boundaries are important boundary conditions on the flow pattern in the mantle. Previous estimates of these rates (Deffeyes 1970; Chase 1972; Garfunkel 1975) have applied a simple expression derived for spreading ridges (Deffeyes 1970), using angular distances from the poles of rotation measured graphically on a globe. An alternative derivation of the expression for rates of plate creation and consumption, which is independent of assumptions about the nature of the boundary, is given below in terms of the framework used to describe plate motions by McKenzie & Parker (1967, 1974). This derivation emphasizes that the rates depend on two sets of vectors - one the instantaneous rotation vectors defining the relative plate motions, the other a set of triple junction and auxiliary vectors defining the plate boundaries.

Fracture mechanical models of dry slab avalanche release

Abstract: Experimental evidence shows that snow is a pressure sensitive, dilatant, strain-softening material in slow, constant rate, shear deformation. When strain softening initiates in a weak layer underneath a snow slab, avalanche release is hypothesized to be possible with or without additional loading. Specifically, two cases are discussed: (1) a shear-crack-like disturbance can initiate after formation of a slip surface in the weak layer and traverse the layer by a self-propagating progressive failure with or even without loading, and (2) a self-propagating shear instability can develop when a region of the weak layer is driven past peak shear strength by loading. For these cases the implied fracture sequence, fracture geometry, time scale of release, and temperature effects are discussed and shown to be consistent with the known facts of dry slab avalanche release.

Iterative determination of permittivity and conductivity profiles of a dielectric slab in the time domain

Abstract: A numerical method is presented to compute one unknown constitutive parameter of an inhomogeoeous lossy dielectric slab from the reflected field in the time domain. The method is based upon a space-time discretization of the integral equation for the reflected field. In the inversion, especially those space-time points where the numerical computation of the electric-field strength in the slab is most accurate are taken into account. This is achieved by computing the unknown parameter iteratively. Alternately solving equations for an approximate direct-scattering problem and an approximate inverse-scattering problem yields successive approximations for the electric field in the slab and the unknown constitutive coefficient. Both problems lead to an infinite system of linear equations from which a finite subsystem is selected. General criteria for this selection are presented. Various profiles have been reconstructed numerically from the reflected field due to a sine-squared incident pulse.

Steady propagation of delamination events

Abstract: Delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation, accelerating as the sinking slab of detached lithosphere grows longer. This propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings. The form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material. Without coupling, the tip of the intrusion cools and stiffens to form a wedge dividing the crust (cold mode). With coupling, the intrusion is forced to convect and remains ductile (hot mode). The cold mode can propagate at all velocities; the hot mode has a lower limiting velocity of 1–2 cm/year but offers less resistance at higher speeds. Resistance to delamination includes a constant term from the buoyant crustal downwarp, plus a velocity-proportional term representing viscous deformation. However, the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities. Matching this resistance to loading lines of 100- to 800-km slabs sinking in a mantle of 1022 P, velocities of 0.3–8.0 cm/year are obtained. Changes in viscosity affect this rate, but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab. The surface expression of delamination is a leading ‘outer rise’ followed by a submarine trough with a large negative free-air anomaly, which finally evolves into a 1-km plateau. If crustal viscosity and velocity are both low, however, there is a montonic crustal uplift with no trough. Thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm/year driven by slabs up to 400 km in length.

Rigorous Analysis of the Scattering of Surface Waves in an Abruptly Ended Slab Dielectric Waveguide

Abstract: The reflection and the scattering properties of even TE and TM surface waves incident in an abruptly ended dielectric slab waveguide are analyzed. The discontinuity is regarded as a junction between two open waveguides namely the dielectric slab waveguide and the free space waveguide. The boundary conditions acting together with the orthogonality provide singular coupled integral equations on the discrete and the continuous wave amplitudes at the discontinuity. These singular coupled intergral equations with Cauchy kernels and infinite limits of integration are solved by iteration via the Neuman series. Numerical results are presented for the reflectivity of the even TE/sub 0/ and TM/sub 0/ fundamental modes, together with their mode conversion on even TE/sub 2/ and TM/sub 2/ in a slab where two guided modes can propagate. Reflectivity and mode conversion of higher order excitations are also investigated

Simple methods to analyze buckling of rock slopes

Abstract: Simple Methods to Analyze Buckling of Rock Slopes Buckling modes of slope failure are a possibility whenever a throughgoing discontinuity, approximately parallel to the slope, separates a thin slab of rock. Failure by buckling may be initiated by forces external to the slab, by groundwater pressures, by applied forces or by the weight of the slab itself, especially if the slab is curved convex upward. Simple formulae are developed for three possible cases of buckling: flexural buckling of plane slopes, three hinge buckling of plane slopes and three hinge buckling of curved slopes. For flexural buckling, Euler's formula is applied after making several simplifying assumptions regarding the geometry of a buckling slab. Three hinge buckling may occur if a slab is cut by cross-joints perpendicular to the slope. Stability of each of the two blocks is determined using equations of force and moment equilibrium, resulting in a series of equations describing the interblock forces required for stability. The applicability of the three hinge model was tested by applying the equations to a documented failure on an excavated slope. The method gave reasonable results.

Laboratory convection experiments: Effect of lateral cooling and generation of instabilities in the horizontal boundary layers

Abstract: Convection experiments are carried out in a tank with two isothermal heat sinks: The top plate and one of the sidewalls. Heating is supplied either by an isothermal bottom plate or by generation within the fluid. This situation is similar to that of the earth's subcontinental mantle in the presence of a neighboring subducting oceanic lithosphere. There, the mantle material loses heat not only to the base of the continental lithosphere but also to the cold dipping oceanic slab. The thermal structure of the convective fluid is observed by a variety of techniques, including differential interferometry and strioscopy. Two parameters characterize the observations: the usual ‘vertical’ Rayleigh number and a newly defined ‘lateral’ Rayleigh number. The lateral cooling induces a large roll with axis parallel to the cold wall. The variation of its width relative to the values of the two Rayleigh numbers has been determined. An application to the earth's upper mantle would predict rolls 5 times wider than high. The circulation in the large roll remains two-dimensional for Rayleigh numbers up to about 3 × 104. Beyond this value, boundary layer instabilities are observed within the persisting large roll. Their period has been determined. The interferometric method is shown to be very useful for visualizing other time-dependent processes such as the growth of the induced large rolls. This growth is rapid enough to allow us to propose the existence of such large rolls in the earth and argue that their action could have led to continental break-up.

Bent asymmetric dielectric slab waveguides: a detailed analysis

Abstract: Analytic expressions for the field and propagation constants of the mode along a uniformly curved three-layered slab waveguide are presented in terms of the power series of the curvature. To compare the accuracy of approximate solutions, numerical analysis is carried out. New bending loss formulas are also derived which consider the field deformation and the change of the propagation constant. It is shown that they are very accurate over a wide range of normalized frequency.

Analysis of Periodic Ferrite Slab Waveguides by Means of Improved Perturbation Method

Abstract: Periodic ferrite slab waveguides are analyzed by means of an improved perturbation method, and nonreciprocal leakage phenomena are shown theoretically. As an application of these phenomena, new planar isolators and circulators are proposed. Numerical examples are also provided.

The P velocity within the Tonga Benioff Zone determined from traced rays and observations

Abstract: P waves with travel time residuals between 0 s and −12 s are observed at regional stations in Samoa (AFI) and Raoul Island (RAO) for 39 earthquakes in Tonga with focal depths between 70 km and 300 km. These anomalously large residuals apparently are produced because seismic phases travel along the strike of the Tonga Benioff zone within the high-velocity subducted lithosphere for up to 1200 km before arriving at AFI and RAO. To eliminate erroneous residuals caused by poor event locations, we selected 11 stations and reread the available P times at these stations for the 39 events. These arrivals and (pP-P) intervals were used to relocate the events using a variant of the joint hypocenter determination method. Then the pattern of residuals at AFI and RAO (not used in the relocation) was compared to the pattern of residuals expected for various models of the subducted lithosphere, as determined by ray tracing. The observed pattern of residuals at AFI is consistent with the ray-traced models if some of the first arrivals are produced by rays traveling directly along the strike of the subducting lithosphere, and if others are produced by rays which reflect once off the upper surface of the subducting lithosphere before arriving at AFI. The observed residuals can be explained by a model where the P velocity in the subducted lithosphere is 8% higher than the velocity in the Herrin model. The residuals are fit even better by a layered slab model in which the seismic velocity is about 6% higher than the Herrin velocity at the upper surface of the subducted lithosphere and about 9% higher at the bottom of the slab. These velocity contrasts could be produced if the temperature in the slab was 700°C cooler than the surrounding mantle, and if there were no partially melted material within the slab. The existence of these anomalously large residuals suggests that the high velocity region in the upper 300 km of the mantle beneath Tonga must be fairly continuous over distances of 1000 km and more.

Automatic ice cube making apparatus

Abstract: Ice cube making apparatus of the type having a vertically disposed evaporator plate with a lattice structure on side thereof in which cubes are formed as water runs down the plate. The ice making cycle is controlled by a timer which is actuated by a pressure responsive switch in the refrigerant line and, upon timing out of the ice forming portion of the cycle, hot gas is delivered to the evaporator to detach the ice slab from the evaporator plate and a mechanical harvest plunger applies a uniform force to the ice slab to overcome capillary forces retaining said ice slab on the evaporator plate.

Simplified construction system

Abstract: A simplified box-like building is constructed one room at a time. Concrete slabs are used for walls, floor and ceiling, and are poured in a horizontal position, stacked vertically with bond-breaking layers between them to allow separation. After the foundation and stub wall are partially cured, the ceiling slab is poured with wedge-shaped edges clearing stub walls by approximately one inch. Tiltable wall slabs are then poured on top of the ceiling slab, each connected by hinges to its own stub wall. An accelerated light weight low-slump concrete is normally used, and makes possible a rapid nearly continuous pouring sequence. Erection of the building is also rapid. The light weight walls are tilted up by hand, pivoting about their base hinges. While they are temporarily held in position, the wall edge dowel reinforcement is bent out and single-side corner forms are positioned or expanded metal is attached to the dowels. The corner joint is sprayed with accelerated low-slump concrete to form a unit, and hollowed areas at the bases of walls are sprayed to form shear connections to the stub walls at the dowels. Removable light weight beams supporting winches are placed across the upper corners, and the ceiling slab is manually raised by winching into place. Dowel reinforcement bars are bent out and accelerated concrete joins the wall tops to the ceiling slab edges to complete the unit. The process is repeated for multi-story buildings and for multiple rooms on each floor. Due to the nature of the accelerated concrete, the process can be done nearly continuously by unskilled labor using only one piece of powered equipment which is the concrete mixer-pump-sprayer machine.

Building construction using hollow core wall slabs

Abstract: A concrete bond beam is employed in building constructions using concrete slabs having hollow core channels therein serving as vertical walls. The bond beam includes at least one opening therein and a pair of downwardly extending flanges. The bond beam is mounted on top of each slab with the top edges of the slabs being received between the flanges of the bond beam so that the opening therein is aligned with a selected core channel in the slab. Reinforcement rods extend both horizontally within the confines of the bond beam and vertically through the cores in the wall slabs and the openings in the bond beam. Structural concrete columns are formed in the wall slab cores surrounding the vertical reinforcement rods. The building thus contains a matrix of structural reinforced concrete beams running both horizontally and vertically throughout the construction.

Radiative transfer in an isotropically scattering two-region slab with reflecting boundaries

Abstract: The problem of radiative heat transfer in an absorbing, emitting, isotropically scattering two-layer slab with diffusely and specularly reflecting boundaries is solved by the FN method and results are presented for the transmissivity and reflectivity of the slab.

Electrophoresis system for multiple agarose slab gels

Abstract: The disclosure describes a horizontal slab gel electrophoresis apparatus. The latter consists of two spaced apart vessels which are each adapted to contain a running buffer. Each vessel is provided with an electrode for connection to a suitable power supply. At least one horizontal plate to contain a slab gel is disposed in bridging fashion over the first and second spaced apart vessels. Capillary contact should be established between one end of the slab gel and the running buffer which is present in the first vessel and also between the other end of the slab gel and the running buffer which is present in the second vessel. The method involves using this apparatus by adding a solution of a mixture of molecules of mixed molecular weights, such as DNA to wells formed in a horizontal slab gel after which the gel is treated by electrophoresis to produce localized bands each being characteristic of a specific molecular weight. This invention has the great advantage of enabling the electrophoresis to be carried out with a plurality of horizontal agarose slab gels while using only one power supply.

Slab for flooring

Abstract: A slab or block for making a floor comprises a plurality of tiles placed side by side and bonded face to face to a plate of plastic material. Heat insulating shavings such as vulcanized elastomer or wood shavings are dispersed in the plastic matrix of the plate. A textile backing such as a woven textile scrim or net is bonded to the second face of the plate.