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

Light-time computations for the BepiColombo radioscience experiment

The radioscience experiment is one of the on board experiment of the Mercury ESA mission BepiColombo that will be launched in 2014. The goals of the experiment are to determine the gravity field of Mercury and its rotation state, to determine the orbit of Mercury, to constrain the possible theories of gravitation (for example by determining the post-Newtonian (PN) parameters), to provide the spacecraft position for geodesy experiments and to contribute to planetary ephemerides improvement. This is possible thanks to a new technology which allows to reach great accuracies in the observables range and range rate; it is well known that a similar level of accuracy requires studying a suitable model taking into account numerous relativistic effects. In this paper we deal with the modelling of the space-time coordinate transformations needed for the light-time computations and the numerical methods adopted to avoid rounding-off errors in such computations.Comment: 14 pages, 7 figures, corrected reference

A Possibility to Build Isolated Masonry Housing in High Seismic Zones Using Rubber Seismic Isolators

New residential buildings in developing countries often have inadequate seismic protection, particularly for masonry. Such material is widely preferred because the cost and application are relatively cheap. To decrease the vulnerability, an interesting option is represented by seismic isolation, but the cost should remain relatively low, and this is the reason why rubber isolation with few pads remains the most suitable technical solution to adopt. In this study, we deal with a newly conceived low-cost seismic isolation system for masonry buildings relying on elastomeric bearings. The elastomeric isolator here proposed consists of few layers of rubber pads and fiber lamina, making it cheaper comparing to the conventional isolators. A detailed 3D finite element (FE) analysis to predict the behavior of the low-cost rubber isolator undergoing moderate deformations is carried out. A Yeoh hyperelasticity model with coefficients estimated through available experimental data is assumed for rubber pads. Having so derived the shear behavior, such isolation system is implemented at a structural level into a two stories masonry house prototype, identifying the 3D model with a damped nonlinear spring model, so making the FE analysis computationally inexpensive. For masonry, a concrete damage plasticity (CDP) model available in the commercial FE code Abaqus is adopted. A nonlinear static-pushover analysis is conducted to assess the performance of the isolated building. To simulate a realistic condition under seismic event, a ground motion data is applied to observe the dynamic behavior of the building by monitoring the damage level of masonry. Through a-posterior estimation, it is also possible to monitor the deformation of the isolators during the seismic excitation, to determine whether the isolator is capable of resisting shear deformations in different angles. According to the results obtained, quite good isolation is obtained with the system proposed, with immediate applicability at a structural level

A simple homogenized model for the non-linear analysis of FRP strengthened masonry structures

A two-step model able to predict the non-linear response of FRP strengthened threedimensional masonry structures is presented. In the first step, non-strengthened masonry is substituted by a macroscopically equivalent homogeneous material through a kinematic model based on finite elements and working on a heterogeneous assemblage of blocks. Non-linearity is concentrated exclusively on joints reduced to interfaces, exhibiting a frictional behaviour with limited tensile and compressive strength with softening. The homogenized stress-strain behaviour evaluated at the meso-scale is then implemented at a structural level in a finite element non-linear code, relying on an assemblage of rigid infinitely resistant six-noded wedge elements and non-linear interfaces, with deterioration of the mechanical properties. FRP reinforcing strips are modelled through rigid triangles and non-linear interfaces between adjoining triangles. Delamination from the support is accounted for, by modelling FRP-masonry bond by means of non-linear softening triangular interfaces. Italian code CNR DT 200 0 formulas are used to evaluate peak interface tangential strength and post peak behaviour. A structural examples relying into a masonry deep beam is presented for validation purposes.(undefined

Heterogeneous and homogenized FE models for the limit analysis of out-of-plane loaded masonry walls

In the present contribution, a full 3D heterogeneous and a 2.5D homogenized kinematic FE limit analysis approach are employed for the evaluation of collapse loads and failure mechanisms of both running and English bond masonry slabs simply supported at the edges and out-of-plane loaded. Information at failure given by the full 3D approach underlines that particular care should be used in the evaluation of collapse loads with 2.5D approaches in case of multi-wythes panels.(undefined

Monte Carlo homogenized limit analysis model for randomly assembled blocks in-plane loaded

A simple rigid-plastic homogenization model for the limit analysis of masonry walls in-plane loaded and constituted by the random assemblage of blocks with variable dimensions is proposed. 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. The model is characterized by a few material parameters, is numerically inexpensive and very stable. A sub-class of elementary deformation modes is a-priori chosen in the REV, mimicking typical failures due to joints cracking and crushing. Masonry strength domains are obtained equating the power dissipated in the heterogeneous model with the power dissipated by a fictitious homogeneous macroscopic plate. Due to the inexpensiveness of the approach proposed, Monte Carlo simulations can be repeated on the REV in order to have a stochastic estimation of in-plane masonry strength at different orientations of the bed joints with respect to external loads accounting for the geometrical statistical variability of blocks dimensions. Two cases are discussed, the former consisting on full stochastic REV assemblages (obtained considering a random variability of both blocks height an length) and the latter assuming the presence of a horizontal alignment along bed joints, i.e. allowing blocks height variability only row by row. The case of deterministic blocks height (quasi-periodic texture) can be obtained as a subclass of this latter case. Masonry homogenized failure surfaces are finally implemented in an upper bound FE limit analysis code for the analysis at collapse of entire walls in-plane loaded. Two cases of engineering practice, consisting on the prediction of the failure load of a deep beam and a shear wall arranged with random texture are critically discussed. In particular, homogenization results are compared with those provided by a heterogeneous approach. Good agreement is found both on the failure mechanism and on the distribution of the collapse load

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

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

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