The paper is concerned with the analytical description of a resistance mechanism, not considered in previous models, by which the hoops contribute to the shear capacity of RC columns with circular cross sections. The difference from previous approaches consists in observing that, because of deformation, the hoops change their original shape and, as a consequence, their slope does not match anymore the original one in the neighborhood of a crack. The model involves two parameters only, namely the crack inclination and the hoop strain in the neighborhood of a crack. A closed-form analytical formulation to correlate the average value of the crack width and the hoop strain is also provided. Results obtained using the proposed model have been compared with experimental data, and a satisfactory agreement is found

Passive strategies based on the introduction of energy dissipating devices into the structures have received considerable attention in recent years. Within this framework, as re-liable and cheap energy-dissipation devices, viscous fluid dampers have been largely used in seismic protection of industrial machines, technical equipments, buildings and bridges. Since the versatility of this passive protection system satisfactorily meets a wide range of require-ments, a reliable identification of their nonlinear mechanical behavior is of outstanding im-portance. This paper focuses on the parametric identification of fractional derivative based models for nonlinear viscous dampers by means of non-classical methods, which are uncon-ventional algorithms whose inner work is based on socially, physically and/or biologically inspired paradigms. Non-classical strategies are potentially powerful tools for solving com-plex identification problems because of their start-point independence, noise robustness and the capability in looking for the best solution in a global way. In contrast, it is important to highlight that they typically possess weak forms of convergence. For better assessing the cor-rectness of some non-classical methods in parametric identification of viscous dampers, we perform a large comparative analysis which involve the following soft computing based tech-niques: a multi-species genetic algorithm, six standard differential evolution algorithms and four swarm intelligence based algorithms (including a chaotic particle swarm optimization algorithm). A numerical study is initially conducted in order to investigate the general relia-bility of these methods. Moreover, the paper also provides some results about the parametric identification of nonlinear viscous dampers by using experimental data. A critical review of the obtained evidences is given in order to provide useful guidelines for similar engineering applications.

The paper addresses the lateral-torsional buckling for a special steel truss structure which is adopted to reinforce a particular class of composite steel-concrete beams. The investigated instability phenomenon deals with a transitory ultimate limit state that may occur when the truss structure is being assembled and is bearing the precast floor system, before the concrete has hardened. During this transitory stage the truss beam works as a steel structure and may exhibit local or global instability phenomena. It provides the reinforcement of the final composite steel-concrete beam once the concrete has hardened. The main result of this paper is an analytical approach for the estimation of the elastic critical moment which is required to calculate the final design lateral-torsional buckling resistance moment in accordance with the technical code in force (the European standard for steel structures is considered in this paper). The simplified analysis this paper presents leads to a closed-form solution for the elastic critical moment, thus providing a simple and rapid tool for the calculation of the lateral-torsional buckling resistance moment. A comparison to a finite element analysis has been performed to demonstrate the correctness of the proposed analytical formulation

The mechanical behavior of a random package of rigid particles, in which interparticle contact forces follow the Coulomb friction law, is analyzed with the aim of establishing a link between the microscopic frictional behavior of contacts, the equilibrium of particles and the macroscopic plasticity of this material. The concepts of kinematically admissible displacements and statically admissible forces are extended to this mechanical system and under a rather general assumption on particles shapes, it is shown that in the macro stress space the yield surface of this material is a cone. Further, the initial plastic flow of this material is examined in two particular cases and it is shown that in case of frictionless convex particles the plastic macro strain is associated, while in case of frictional identical spherical particles only its deviatoric part is associated. Finally a micromechanical derivation of the constitutive inequality relating the friction and dilatancy coefficients is given.

A new method for the limit analysis of discrete systems formed from dry rigid blocks with Coulomb-type (non-associative) contact interface laws is here exposed. The method resorts to a fictitious system whose cohesive-type contact interface laws depend on the axial forces of the real block system. Two theorems establish the connection between the collapse state of the frictional block assembly and that of the fictitious one. Based on this result, an original problem of mathematical programming is formulated to determine the minimum collapse load multiplier for block assemblies interacting through frictional interfaces. In the proposed formulation the complementarity conditions are not introduced as constraints but are obtained as Karush-Khun-Tucker conditions. Numerical applications demonstrate the potential of this approach for assessing the collapse load of masonry-like structures.