Efficiency of a wide-area survey in achieving short- and long-term warning for small impactors

We consider a network of telescopes capable of scanning all the observable sky each night and targeting Near-Earth objects (NEOs) in the size range of the Tunguska-like asteroids, from 160 m down to 10 m. We measure the performance of this telescope network in terms of the time needed to discover at least 50% of the impactors in the considered population with a warning time large enough to undertake proper mitigation actions. The warning times are described by a trimodal distribution and the telescope network has a 50% probability of discovering an impactor of the Tunguska class with at least one week of advance already in the first 10 yr of operations of the survey. These results suggest that the studied survey would be a significant addition to the current NEO discovery efforts

Simple homogenized model for the non-linear analysis of FRP strengthened masonry structures. Part II : structural applications

The homogenized masonry nonlinear stress-strain curves obtained through the simple micromechanical model developed in the first part of the paper are here used for the analysis of strengthened masonry walls under various loading conditions. In particular, a deep beam and a shear wall strengthened with fiber-reinforced polymer (FRP) strips are analyzed for masonry loaded in-plane. Additionally, single and double curvature masonry structures strengthened in various ways, namely a circular arch with buttresses and a ribbed cross vault, are considered. For all the examples presented, both the nonstrengthened and FRP–strengthened cases are discussed. Additional nonlinear finite-element analyses are performed, modeling masonry through an equivalent macroscopic material with softening to assess the present model predictions. Detailed comparisons between the experimental data, where available, and numerical results are also presented. The examples show the efficiency of the homogenized technique with respect to (1) accuracy of the results; (2) low number of finite elements required; and (3) independence of the mesh at a structural level from the actual texture of masonry

Simple homogenized model for the non-linear analysis of FRP strengthened masonry structures. Part I : theory

A suitable and simple two-step model able to predict the non-linear response of FRP strengthened 13 three-dimensional masonry structures is presented. In the first step, non-strengthened masonry is 14 substituted by a macroscopically equivalent homogeneous material through a kinematic model 15 based on finite elements and working on a heterogeneous assemblage of blocks. Non-linearity is 16 concentrated exclusively on joints reduced to interfaces, exhibiting a frictional behavior with 17 limited tensile and compressive strength with softening. The homogenized stress-strain behavior 18 evaluated at the meso-scale is then implemented at a structural level in a finite element non-linear 19 code, relying on an assemblage of rigid infinitely resistant six-noded wedge elements and non-linear 20 interfaces, exhibiting deterioration of the mechanical properties. FRP reinforcing strips are modeled 21 through rigid triangles and non-linear interfaces between adjoining triangles. Delamination from the 22 support is accounted for, by modeling FRP-masonry bond by means of non-linear softening 23 triangular interfaces. Italian code CNR DT 200 (2004) formulas are used to evaluate peak interface 24 tangential strength and post peak behavior. In this first part, the theoretical base of the model and 25 the non-linear stress strain behavior at a cell level are discussed. Structural examples will be 26 analyzed in the accompanying paper devoted to the structural scale

FE homogenized limit analysis code for masonry buildings

In this paper, a FE homogenized limit analysis code for the collapse analysis of 3D masonry buildings subjected to horizontal actions is presented. In the code, masonry is modelled through a fictitious macroscopic homogeneous material. Masonry macroscopic mechanical properties are obtained by means of a recently presented equilibrated limit analysis approach performed on a suitable unit cell, which generates the entire structure by repetition. Masonry homogenised failure surfaces are then implemented in the 3D code here outlined. With respect to previously presented models, the algorithm allows to analyze real scale buildings for coupled in-plane and out-of-plane actions. The possible presence of steel, RC and ring beams is also considered introducing in the numerical model two-node beam elements. A relevant 3D structural example consisting of a masonry school subjected to horizontal actions is treated. Full sensitivity analyses and a comparison with results obtained with a commercial elasto-plastic software are also presented to validate the model proposed

Blast analysis of enclosure masonry walls using homogenization approaches

A simple rigid-plastic homogenization model for the analysis of enclosure masonry walls sub- jected to blast loads is presented. The model is characterized by a few material parameters, is numerically inexpensive and very stable, and allows full parametric studies of entire walls subject to blast pressures. With the aim of considering the actual brickwork strength along vertical and horizontal axes, masonry out-of-plane anisotropic failure surfaces are obtained by means of a compatible homogenized limit analysis approach. In the model, a 3D system of rigid infinitely strong bricks connected by joints reduced to interfaces is identified with a 2D Kirchhoff-Love plate. For the joints, which obey an associated flow rule, aMohr-Coulomb fail- ure criterion with a tension cutoff and a linearized elliptic compressive cap is considered. In this way, the macroscopic masonry failure surface is obtained as a function of the macroscopic bending, torque, and in-plane forces by means of a linear programming problem in which the internal power dissipated is minimized. Triangular Kirchhoff-Love elements with linear in- terpolation of the displacements field and constant moment within each element are used at a structural level. In this framework, a simple quadratic programming problem is obtained to analyze entire walls subjected to blast loads. The multiscale strategy presented is adopted to predict the behavior of a rectangular wall supported on three sides (left, bottom, and right) representing an envelope wall in a building and subjected to a standardized blast load. The top edge of the wall is assumed unconstrained due to an imperfect connection (often an inter- layer material is used to prevent damage in the in-fill wall). A comparison with a standard elastic-plastic heterogeneous 3D analysis conducted with a commercial FE code is also pro- vided for a preliminary verification of the procedure at a structural level. The good agreement found and the very limited computational effort required for the simulations conducted with the presented model indicate that the proposed simple tool can be used by practitioners for the safety assessment of out-of-plane loaded masonry panels subjected to blast loading. An ex- haustive parametric analysis is finally conducted with different wall thicknesses, joint tensile strengths, and dynamic pressures, corresponding to blast loads (in kilograms of TNT) ranging from small to large

Homogenized non-linear dynamic model for masonry walls in two-way bending

A simple homogenization approach accounting for mortar joint damaging is presented, suitable to analyse entire panels in two-way bending in the non-linear dynamic field. A rectangular running bond elementary cell (RVE) is subdivided into several layers along the thickness and, for each layer, a discretization where bricks are meshed with plane-stress three-noded triangular elements and joints are reduced to interfaces with damaging behaviour is assumed. Non linearity is due exclusively to joints cracking, which exhibit also a frictional behaviour with limited tensile and compressive strength with softening. A damaging material is utilized for joints in order to properly take into account the actual opening and closure of cracked mortar under cyclic loads. Finally, macroscopic curvature bending moment diagrams are obtained integrating along the thickness inplane micro-stresses of each layer. Homogenized masonry flexural response under load-unload conditions is then implemented at a structural level in a FE non-linear code based on a discretization with rigid three-noded elements and elasto-damaging interfaces where elastic and inelastic deformation is allowed only for flexural actions. The two step model proposed is validated both at a cell and structural level, comparing results obtained with both experimental data and existing macroscopic numerical approaches available in the literature

A simplified homogenized limit analysis model for randomly assembled blocks out-of-plane loaded

A kinematic rigid-plastic homogenization model for the limit analysis of masonry walls arranged in random texture and out-of-plane loaded is proposed. The model is the continuation of a previous work by the authors in which masonry in-plane behavior was investigated. In the model, blocks constituting a masonry wall are supposed infinitely resistant with a Gaussian distribution of height and length, whereas joints are reduced to interfaces with frictional behavior and limited tensile and compressive strength. Block by block, a representative element of volume (REV) is considered, constituted by a central block interconnected with its neighbors by means of rigid plastic interfaces. Two different classes of problems are investigated, the first consisting of full stochastic REV assemblages without horizontal and vertical alignment of joints, the second assuming the presence of a horizontal alignment along bed joints, i.e. allowing blocks height variability only row by row. A sub-class of elementary deformation modes is a-priori chosen in the REV, mimicking typical failures due to joint cracking and crushing. The model is characterized by a few material parameters and it is therefore particularly suited to perform large scale Monte Carlo simulations. Masonry strength domains are obtained equating the power dissipated in the heterogeneous model with the power dissipated by a fictitious homogeneous macroscopic plate. A stochastic estimation of out-of-plane masonry strength domains (both bending moments and torsion are considered) accounting for the geometrical statistical variability of blocks dimensions is obtained with the proposed model. The case of deterministic block height (quasi-periodic texture) can be obtained as a subclass of this latter case. As an important benchmark, the case in which joints obey a Mohr-Coulomb failure criterion is also tested and compared with results obtained assuming a more complex interfacial behavior for mortar. Masonry homogenized failure surfaces are finally implemented in an upper bound Finite Element (FE) limit analysis code. Firstly, to validate the model proposed, two small scale structural examples of practical interest are considered, relying in masonry panels in two-way out-of-plane bending. In both cases, failure load distributions and failure mechanisms provided by the homogenization model are compared with those obtained through a heterogeneous approach. Finally, in order to show the capabilities of the approach proposed when dealing with large scale structures, the ultimate behavior prediction of a Romanesque masonry church façade located in Portugal and arranged in irregular texture is presented. Comparisons with Finite Element heterogeneous approaches and “at hand” calculations show that reliable predictions of the load bearing capacity of real large scale structures may be obtained with a very limited computational effort

Blood Oxygenation Level-Dependent MRI to Assess Renal Oxygenation in Renal Diseases: Progresses and Challenges.

BOLD-MRI (blood oxygenation-level dependent magnetic resonance imaging) allows non-invasive measurement of renal tissue oxygenation in humans, without the need for contrast products. BOLD-MRI uses the fact that magnetic properties of hemoglobin depend of its oxygenated state:: the higher local deoxyhemoglobin, the higher the so called apparent relaxation rate R2(*) (sec(-1)), and the lower local tissue oxygen content. Several factors other than deoxyhemoglobin (such as hydration status, dietary sodium intake, and susceptibility effects) influence the BOLD signal, and need to be taken into account when interpreting results. The last 5 years have witnessed important improvements in the standardization of these factors, and the appearance of new, highly reproducible analysis techniques of BOLD-images, that are reviewed in this article. Using these new BOLD-MRI analysis techniques, it has recently been shown that persons suffering from chronic kidney diseases (CKD) have lower cortical oxygenation than normotensive controls, thus confirming the chronic hypoxia hypothesis. The acute alterations in R2(*) after the administration of furosemide are smaller in CKD, and represent an estimate of the oxygen-dependent tubular transport of sodium. BOLD-MRI-alone or in combination with other functional MRI methods- can be used to monitor the renal effects of drugs, and is increasingly used in the preclinical setting. The near future will tell whether or not BOLD-MRI represents a new tool to predict renal function decline an adverse renal outcome

Long-term impact risk for (101955) 1999 RQ36

The potentially hazardous asteroid (101955) 1999 RQ36 has the possibility of collision with the Earth in the latter half of the 22nd century, well beyond the traditional 100-year time horizon for routine impact monitoring. The probabilities accumulate to a total impact probability of approximately 10E-3, with a pair of closely related routes to impact in 2182 comprising more than half of the total. The analysis of impact possibilities so far in the future is strongly dependent on the action of the Yarkovsky effect, which raises new challenges in the careful assessment of longer term impact hazards. Even for asteroids with very precisely determined orbits, a future close approach to Earth can scatter the possible trajectories to the point that the problem becomes like that of a newly discovered asteroid with a weakly determined orbit. If the scattering takes place late enough so that the target plane uncertainty is dominated by Yarkovsky accelerations then the thermal properties of the asteroid,which are typically unknown, play a major role in the impact assessment. In contrast, if the strong planetary interaction takes place sooner, while the Yarkovsky dispersion is still relatively small compared to that derived from the measurements, then precise modeling of the nongravitational acceleration may be unnecessary.Comment: Reviewed figures and some text change