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An enhanced multi-fiber beam–column element suitable for the analysis of reinforced concrete members including the torsional effect is presented in this paper. The model is developed based on displacement formulation with the assumptions of small displacement. The section kinematics is based on the assumptions of a two-node Timoshenko beam and enhanced by introducing additional degrees of freedom at each section in order to take into account the warping phenomenon. For this, a system of fixed points is created and interpolated by Lagrange functions and polynomials. To take into account the effect of stirrups, a discretization of control sections into regions following its material response is applied. The basic assumption of the modified compression field theory with a secant-stiffness formulation is used to represent the constitutive material model for reinforced concrete. The model is validated by comparison with analytical solutions and several experimental tests. The simulations include a variety of monotonic load conditions under bending, shear and torsion for specimens with rectangular sections.

Purpose - The purpose of this paper is to present eight local elasto-plastic beam element formulations incorporated into the corotational framework for two-noded three-dimensional beams. These formulations capture the warping torsional effects of open cross-sections and are suitable for the analysis of the nonlinear buckling and post-buckling of thin-walled frames with generic cross-sections. The paper highlights the similarities and discrepancies between the different local element formulations. The primary goal of this study is to compare all the local element formulations in terms of accuracy, efficiency and CPU-running time.Design methodology approach - The definition of the corotational framework for a two-noded three-dimensional beam element is presented, based upon the works of Battini .The definitions of the local element kinematics and displacements shape functions are developed based on both Timoshenko and Bernoulli assumptions, and considering low-order as well as higher-order terms in the second-order approximation of the Green-Lagrange strains. Element forces interpolations and generalized stress resultant vectors are then presented for both mixed-based Timoshenko and Bernoulli formulations. Subsequently, the local internal force vector and tangent stiffness matrix are derived using the principle of virtual work for displacement-based elements and the two-field Hellinger-Reissner assumed stress variational principle for mixed-based formulations, respectively. A full comparison and assessment of the different local element models are performed by means of several numerical examples.Findings - In this study, it is shown that the higher order elements are more accurate than the low-order ones, and that the use of the higher order mixed-based Bernoulli element seems to require the least number of FEs to accurately model the structural behavior, and therefore allows some reduction of the CPU time compared to the other converged solutions; where a larger number of elements are needed to efficiently discretize the structure.Originality value - The paper reports computation times for each model in order to assess their relative efficiency. The effect of the numbers of Gauss points along the element length and within the cross-section are also investigated.

Modern standards for constructions in seismic zones allow the construction of buildings able to dissipate the energy of the seismic input through an appropriate location of cyclic plastic deformations involving the largest possible number of structural elements, forming thus a global collapse mechanisms without failure and instability phenomena both at local and global level. The key instrument for this purpose is the capacity design approach, which requires an appropriate selection of the design forces and an accurate definition of structural details within the plastic hinges zones, prescribing at the same time the oversizing of non-dissipative elements that shall remain in the elastic field during the earthquake. However, the localization of plastic hinges and the development of the global collapse mechanism is strongly influenced by the mechanical properties of materials, which are characterized by an inherent randomness. This variability can alter the final structural behaviour not matching the expected performance. In the present paper, the influence of the variability of material mechanical properties on the structural behaviour of steel and steel/concrete composite buildings is analyzed, evaluating the efficiency of the capacity design approach as proposed by Eurocode 8 and the possibility of introducing an upper limitation to the nominal yielding strength adopted in the design.

In Europe, the simultaneous introduction of new energy efficiency standards and majored seismic requirements has lead concrete constructors to find innovative solutions. In case of internal insulation, slab to wall connections have to be designed to maintain continuity of the insulation in structural connections while ensuring the transfer of shear forces from the slab to the wall. For this purpose, INGENOVA, a French civil engineering office, has developed a robust and ductile system, called SLABE®: stainless steel Z-shaped profiles are used as shear keys, creating an innovative hybrid concrete connection. An important research program has been carried out in order to characterize the SLABE® system behaviour under cyclic loads. Nearly true scale specimens including concrete slabs and walls have been tested in order to determine the stiffness and resistance under cyclic horizontal and vertical shear actions. Tests demonstrate a very stable behaviour of the system up to its theoretical yielding capacity and a large reserve of ductility due to the material of steel members. At last the effect of the SLABE on the overall seismic answer of buildings has been investigated numerically, in order to predict both the force redistribution between external and internal concrete walls, and the magnitude of horizontal forces transmitted by the shear link.

Modern seismic codes recommend the design of ductile structures able to absorb seismic energy through high plastic deformation. Since seismic ductile design relies on an accurate control of plastic hinges formation, which mainly depends on the distribution of plastic resistances of structural elements, efficiency of the design method strongly depends on the actual mechanical properties of materials. The objective of the present contribution is therefore to assess the impact of material variability on the performance of capacity-designed steel-concrete composite moment resisting frames.

Purpose - The purpose of this paper is to present eight local elasto-plastic beam element formulations incorporated into the corotational framework for two-noded three-dimensional beams. These formulations capture the warping torsional effects of open cross-sections and are suitable for the analysis of the nonlinear buckling and post-buckling of thin-walled frames with generic cross-sections. The paper highlights the similarities and discrepancies between the different local element formulations. The primary goal of this study is to compare all the local element formulations in terms of accuracy, efficiency and CPU-running time. Design/methodology/approach - The definition of the corotational framework for a two-noded three-dimensional beam element is presented, based upon the works of Battini .The definitions of the local element kinematics and displacements shape functions are developed based on both Timoshenko and Bernoulli assumptions, and considering low-order as well as higher-order terms in the second-order approximation of the Green-Lagrange strains. Element forces interpolations and generalized stress resultant vectors are then presented for both mixed-based Timoshenko and Bernoulli formulations. Subsequently, the local internal force vector and tangent stiffness matrix are derived using the principle of virtual work for displacement-based elements and the two-field Hellinger-Reissner assumed stress variational principle for mixed-based formulations, respectively. A full comparison and assessment of the different local element models are performed by means of several numerical examples. Findings - In this study, it is shown that the higher order elements are more accurate than the low-order ones, and that the use of the higher order mixed-based Bernoulli element seems to require the least number of FEs to accurately model the structural behavior, and therefore allows some reduction of the CPU time compared to the other converged solutions; where a larger number of elements are needed to efficiently discretize the structure. Originality/value - The paper reports computation times for each model in order to assess their relative efficiency. The effect of the numbers of Gauss points along the element length and within the cross-section are also investigated.

This paper deals with the causes and the development of weak storey mechanisms in concentrically braced frames subjected to seismic action. In order to investigate this phenomenon, different braced frames were designed in accordance with Eurocode provisions. The design parameters and the obtained structures are presented in the paper. Later the results of a comprehensive nonlinear time history analysis carried out with various accelerograms are shown emphasizing the presence of weak storeys. The connection between the behaviour of the brace and the occurrence of the weak storey is investigated. It is described how the plastic deformation of the diagonals and the development of the weak storey are related. A theoretical influence of the brace deformation on the modal response of the braced frames is introduced and also proven by means of signal processing of the displacement time series of the numerical experiments. In the last chapter a possible redesign method is introduced to rule out the occurrence of weak stories.