Adaptive search space decomposition method for pre- and post-buckling analyses of space truss structures

The paper proposes a novel adaptive search space decomposition method and a novel gradient-free optimization-based formulation for the pre- and post-buckling analyses of space truss structures. Space trusses are often employed in structural engineering to build large steel constructions, such as bridges and domes, whose structural response is characterized by large displacements. Therefore, these structures are vulnerable to progressive collapses due to local or global buckling effects, leading to sudden failures. The method proposed in this paper allows the analysis of the load-equilibrium path of truss structures to permanent and variable loading, including stable and unstable equilibrium stages and explicitly considering geometric nonlinearities. The goal of this work is to determine these equilibrium stages via optimization of the Lagrangian kinematic parameters of the system, determining the global equilibrium. However, this optimization problem is non-trivial due to the undefined parameter domain and the sensitivity and interaction among the Lagrangian parameters. Therefore, we propose to formulate this problem as a nonlinear, multimodal, unconstrained, continuous optimization problem and develop a novel adaptive search space decomposition method, which progressively and adaptively re-defines the search domain (hypersphere) to evaluate the equilibrium of the system using a gradient-free optimization algorithm. We tackle three benchmark problems and evaluate a medium-sized test representing a real structural problem in this paper. The results are compared to those available in the literature regarding displacement–load curves and deformed configurations. The accuracy and robustness of the adopted methodology show a high potential for gradient-free algorithms to analyze space truss structures

New Frontiers on Seismic Modeling of Masonry Structures

An accurate evaluation of the non-linear behavior of masonry structural elements in existing buildings still represents a complex issue that rigorously requires non-linear finite element strategies difficult to apply to real large structures. Nevertheless, for the static and seismic assessment of existing structures, involving the contribution of masonry materials, engineers need reliable and efficient numerical tools, whose complexity and computational demand should be suitable for practical purposes. For these reasons, the formulation and the validation of simplified numerical strategies represent a very important issue in masonry computational research. In this paper, an innovative macroelement approach, developed by the authors in the last decade, is presented. The proposed macroelement formulation is based on different, plane and spatial, macroelements for the simulation of both the in-plane and out-of-plane behavior of masonry structures also in presence of masonry elements with curved geometry. The mechanical response of the adopted macroelement is governed by non-linear zero-thickness interfaces, whose calibration follows a straightforward fiber discretization, and the non-linear internal shear deformability is ruled by equivalence with a corresponding geometrically consistent homogenized medium. The approach can be considered as "parsimonious" since the kinematics of the adopted elements is controlled by very few degrees of freedom, if compared to a corresponding discretization performed by using non-linear finite element method strategies. This innovative discrete element strategy has been implemented in two user-oriented software codes 3DMacro (Caliò et al., 2012b) and HiStrA (Historical Structures Analysis) (Caliò et al., 2015), which simplify the modeling of buildings and historical structures by means of several wizard generation tools and input/output facilities. The proposed approach, that represents a powerful tool for the structural assessment of structures in which the masonry plays a key role, is here validated against experimental results involving typical masonry monumental substructural elements and numerical results involving real-scale structures

A macro‐model for describing the in‐plane seismic response of masonry‐infilled frames with sliding/flexible joints

Masonry infill walls are among the most vulnerable components of reinforced concrete (RC) frame structures. Recently, some techniques for enhancing the performance of the infills have been proposed, aiming at improving both the global and the local behaviour of the infilled frame structure. Among the most promising ones, there are those that aim to decouple or reduce the infill-frame interaction by means of flexible or sliding joints, relying respectively on rubber or low-friction materials at the interface between horizontal subpanels or between the panels and the frame. Numerous models have been developed in the last decades for describing the seismic response of masonry-infilled RC frames, but these have focused mainly on the case of traditional infills. This study aims to fill this gap by proposing a two-dimensional macro-element model for describing the in-plane behaviour of RC infilled frames with flexible or sliding joints. The proposed modelling approach, implemented in OpenSees, is an extension of a discrete macro-element previously developed for the case of traditional infill panels. It is calibrated and validated in this study against quasi-static tests from the literature, carried out on masonry-infilled RC frames with sliding and rubber joints. The study results show the capabilities of the proposed modelling approach to evaluate the benefits of using flexible joints in terms of minimising the negative effects of the interaction between infill and RC frame and limiting the increase of global stiffness of the system with respect to the bare frame condition

A NEW 3D-ADAPTIVE DISCRETE INTERFACE FOR MODELING THE TORSION BEHAVIOR OF MASONRY CONTACT JOINTS

The numerical modelling of the torsion behaviour of masonry block joints represents a key aspect for the assessment of the out-of-plane response of masonry walls. However, it repre-sents a challenging computational issue due to the high non-linear coupling between the tor-sion and other internal forces (shear, bending moment and axial load), meaning the necessity to use complex 3D non-linear constitutive laws. The limit analysis-based approaches represent efficient and reliable numerical tools to predict the ultimate torsion load including the interaction with shear and bending moment. Within this framework, few researchers have proposed and experimentally validated continuous and discrete contact models, able to predict the ultimate strength of masonry contact joints. These models are successfully employed to develop high-detailed simulations of 3D dry-jointed masonry block structures subjected to lateral in-plane and out-of-plane actions. Nonetheless, the limit analysis is not able to characterize the non-linear response of masonry walls prior to collapse and to predict the evolution of plastic damage and the ductility resources, if available. Aiming at overcoming such a limit, this paper introduces a new 3D adaptive discrete interface, able to simulate the non-linear torsion-shear behaviour of masonry joints. The interface consists of four springs whose orientation is updated during the analysis, following an incremental iterative Newton-Raphson algorithm taking into account the current position of the torsion centre of the interface. The ultimate torsion-shear capacity domain obtained by the proposed model is compared with limit-analysis predictions and experimental data available in the literature. The results highlight the ability of the new interface to effectively reproduce the non-linear behaviour and the ultimate strength of joints subjected to different loading combinations

Emerg Infect Dis

Escherichia coli strains of nonenteropathogenic serogroups carrying eae but lacking the enteropathogenic E. coli adherence factor plasmid and Shiga toxin DNA probe sequences were isolated from patients (children, adults, and AIDS patients) with and without diarrhea in Brazil. Although diverse in phenotype and genotype, some strains are potentially diarrheagenic.2004870

A discrete‐element approach accounting for P‐Delta effects

This paper presents a discrete macro-element accounting for P-Delta effects to describe the rocking response of masonry walls subjected to out-of-plane (OOP) loadings. Both constitutive and geometric nonlinearities are considered within the discrete macro-element method (DMEM), which is a modeling approach characterized by a very low computational cost compared to traditional distinct element method (DEM) and detailed finite element (FEM) strategies. OOP failure mechanisms are the main cause of severe damage for unreinforced masonry (URM) buildings, subjected to seismic actions. These mechanisms are generally activated by low seismic excitation and displacements. However, after their activation, they can evolve towards large displacements related to rigid-block-like kinematics that strongly affects the nonlinear mechanical response. Therefore, geometric nonlinearities, often ignored, should be included in the analyses. A new simplified, still accurate P-Delta formulation is presented, according to which the global equilibrium is imposed by referring to the undeformed system configuration, avoiding assembling the geometric stiffness matrix. Namely, the system load vector is updated at each step of the analysis, accounting for the additional moments generated by the in-plane compression forces acting on the macro-elements in the deformed configuration. The proposed model is validated against closed-form analytical solutions of rigid-block benchmarks in large displacements and the results of experimental tests already available in the literature. In addition, extensive parametric analyses are performed to investigate the role of different mechanical and geometric parameters characterizing the ultimate non-linear response of masonry walls subjected to horizontal forces. The results show how the proposed model, including P-Delta effects, accurately predicts the non-linear rocking response of masonry walls until the attainment of the unstable configuration