Article
Modeling of the seismic behavior of shearcritical reinforced concrete columns
Department of Civil & Env. Eng., University of Houston, Houston, TX 77204, USA
Engineering Structures (Impact Factor: 1.84). 11/2010; 32(11):36013615. DOI: 10.1016/j.engstruct.2010.08.004 ABSTRACT
Inelastic failure of reinforced concrete (RC) structures under seismic loadings can be due either to loss of flexural, shear, or bond capacity. This paper describes the formulation of an inelastic nonlinear beam element with axial, bending, and shear force interaction. The element considers shear deformation and is based on the section discretization into fibers with hysteretic models for the constituent materials. The steel material constitutive law follows the Menegotto–Pinto model. The concrete model is based on a smeared approach of cracked continuous orthotropic concrete with the inclusion of Poisson effect. The concrete model accounts for the biaxial state of stress in the directions of orthotropy in accordance with the Softened Membrane Model, in addition to degradation under reversed cyclic loading. The shear mechanism along the beam is modeled using a Timoshenko beam approach. Transverse strains are internal variables determined by imposing equilibrium between concrete and transverse reinforcements. Element forces are obtained by performing equilibrium based numerical integration on section axial, flexural and shear behaviors along the length of the element. Dynamic behavior was accounted for by adopting the wellknown Newmark approach. Rayleigh damping was assumed to simulate the damped behavior under seismic excitations. In order to establish the validity of the proposed model correlation studies were conducted between analytical results and experimental data of RC columns tested under the shake table.

 "Relevant proposals are the models from Ranzo (2000), Petrangeli et al. (1999), Ceresa et al. (2009), Navarro (2009), Güner (2008), Mohr et al. (2010) and Mullapudi and Ayoub (2010). These proposals differ from each other in the shear kinematic assumptions taken at the sectional level, in the type of multiaxial constitutive equations and also in the approach of stiffness or flexibility basis of the FE formulation. "
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ABSTRACT: Purpose – A nonlinear finite element (FE) beamcolumn model for the analysis of reinforced concrete (RC) frames with due account of shear is presented in this paper. The model is an expansion of the traditional flexural fibre beam formulations to cases where multiaxial behaviour exists, being an alternative to plane and solid FE models for the nonlinear analysis of entire frame structures. The paper aims to discuss these issues. Design/methodology/approach – Shear is taken into account at different levels of the numerical model: at the material level RC is simulated through a smeared cracked approach with rotating cracks; at the fibre level, an iterative procedure guarantees equilibrium between concrete and transversal reinforcement, allowing to compute the biaxial stressstrain state of each fibre; at the section level, a uniform shear stress pattern is assumed in order to estimate the internal shear stressstrain distribution; and at the element level, the Timoshenko beam theory takes into account an average rotation due to shear. Findings – The proposed model is validated through experimental tests available in the literature, as well as through an experimental campaign carried out by the authors. The results on the response of RC elements critical to shear include displacements, strains and crack patterns and show the capabilities of the model to efficiently deal with shear effects in beam elements. Originality/value – A formulation for the nonlinear shearbending interaction based on the fixed stress approach is implemented in a fibre beam model. Shear effects are accurately accounted during all the nonlinear path of the structure in a computationally efficient manner. 
 "Beamcolumn elements, on the other hand, proved to be efficient models for simulating the flexural behavior of concrete structures and are computationally inexpensive. By including shear deformations, these elements can simulate accurately reinforced concrete walls dominated by both, shear and flexural behavior, as well as axial loads [1]. Earlier beamcolumn elements were based on plastic hinge formulations. "
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ABSTRACT: a b s t r a c t Reinforced concrete (RC) sheardominant walls can fail suddenly at lower ductility levels, which can lead to catastrophic damage. Accurate modeling of sheardominant RC walls is therefore essential. In this paper, fiber beam elements, which are proven to be computationally very efficient, were developed to model the behavior of thinwalled RC shear walls. Concrete and steel were considered as separate materials, and are combined at the section level to describe the behavior of the reinforced concrete member. Concrete was modeled as an orthotropic material in which the principal directions of total stresses were assumed to coincide with the principal directions of total strains, thus changing the directions continuously during the loading. The constitutive model follows the Softened Membrane Model (SMM) in which the compressive strength of concrete is reduced as a function of the lateral strain. The model was subsequently used to conduct a series of numerical studies to evaluate the effect of several parameters affecting the nonlinear behavior of the shear dominated wall. These parameters include the aspect ratio, the transverse reinforcement ratio, the axial force, and the concrete compressive strength. These studies resulted in several important conclusions regarding the global and local behavior of wall systems.  [Show abstract] [Hide abstract]
ABSTRACT: Severe infections at the forearm level are difficult to treat not only in terms of sterilization but also in terms of functional restitution. Traditional radical debridement is very important, however, reconstruction of the excised tissues sometimes meets with difficulty when there is used of such conventional techniques. At the forearm level, local flaps generally are not sufficient in covering big defects. Conventional bone grafts may be resorbed or they cannot help healing when placed in infected and hypovascular tissue bed. Therefore, bone reconstruction is a real challenge. Development of microsurgical techniques has increased the possibilities of treatment when those severe infections occur. Reconstruction of large soft tissue defects can be achieved by choosing the appropriate free flap. Vascularized fibular grafts allow the use of a segment of diaphyseal bone which is structurally similar to the radius and the ulna, and of length that would suffice to reconstruct most skeletal defects. In the upper limbs the vascularized fibular graft is indicated for patients in whom conventional bone grafting has failed or large bone defects are present (extending beyond 5 cm). When contemporary soft tissue reconstruction is needed, the fibula may give osteocutaneous and osteomyocutaneous grafts to be transferred. We report the results of a series of 22 cases of severe chronic osteomyelitis of the radius and/or the ulna treated with free vascularized fibula bone grafts. All patients were reviewed at a mean followup of 3 years (10–93 months); in all cases the infection never recurred. We report only one bone resorption, in the case of a doublebarrel fibular transfer, which probably occurred due to vascularization failure. Even in this case, the patient was able to resume previous occupation.
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