Afsin Saritas

Frame Element for Metallic Shear-Yielding Members under Cyclic Loading

Modeling the energy dissipation capacity of shear-yielding members is important in the evaluation of the seismic response of earthquake resistant structural systems. This paper presents the model of a frame element for the hysteretic behavior of these members. The model is based on a three-field variational formulation with independent displacement, stress, and strain fields. The displacement field is based on the Timoshenko beam theory. The nonlinear response of the element is derived from the section integration of the multiaxial material stress-strain relation at several control points along the element, thus accounting for the interaction between normal and shear stress and the spread of inelastic deformations in the member. With the derivation of the axial force-shear-flexure interaction of short members from the material response the proposed model is general, in contrast to existing concentrated plasticity models that require parameter calibration for different loading and support conditions. Furthermore, the model does not suffer from shear locking and does not require mesh refinement for the accurate representation of inelastic member deformations. Correlation studies of analytical results with available experimental data of the hysteretic behavior of shear-yielding members confirm the capabilities of the proposed model. Its computational efficiency makes it suitable for large scale simulations of the earthquake response of structures with shear-yielding members.

A Beam Finite Element for Shear-Critical RC Beams

The proposed beam finite element is capable of describing the response of reinforced concrete members under the interaction of axial force, shear, and bending moment. The element is based on a mixed formulation and does not exhibit the shear locking problems of displacement-based elements. The material model of the Modified Compression Field Theory by Vecchio and Collins (1986) was used to describe the biaxial response of the concrete at monitoring points across several monitoring sections along the element axis. With this model the proposed beam element is able to capture well the overall monotonic response of shear critical beams, while equally satisfactory agreement is obtained with local response measures, such as crack orientations. In spite of this promising agreement with experimental results the numerical performance of the material model was rather slow for the lack of a consistent tangent stiffness matrix. This fact coupled with the material models inability to represent cyclic loading point out the need for a numerically consistent, robust and efficient constitutive model.

Free vibration characteristics of a 3d mixed formulation beam element with force-based consistent mass matrix

In this analytical study, free vibration analyses of a 3d mixed formulation beam element are performed by adopting force-based consistent mass matrix that incorporates shear and rotary inertia effects. The force-based approach takes into account the actual distribution of mass of an element in the derivation of the mass matrix. Moreover, the force-based approach enables accurate determination of free vibration frequencies of members with varying geometry and material distribution without any need for specification of different displacement shape functions for each individual case. This phenomenon is justified by comparing free vibration frequencies of cantilever beams that have circular and rectangular cross-sections and various mass distribution configurations. Vibration frequencies of the mixed formulation element are compared with the frequencies obtained from closed-form solutions and finite element analyses. Fundamental frequency is computed with only one element per member span and higher order frequencies are determined with two or four elements with considerable accuracy by employing 3d mixed element and force-based consistent mass matrix.

Hybrid finite element for analysis of functionally graded beams.

ABSTRACT A hybrid finite element model is presented, where stiffness and mass distributions over a beam with functionally graded material (FGM) are accurately modeled for both elastic and inelastic material responses. Von Mises and Drucker-Prager plasticity models are implemented for metallic and ceramic parts of FGM, respectively. Three-dimensional stress-strain relations are solved by a general closest point projection algorithm, and then condensed to the dimensions of the beam element. Numerical examples and verification studies on a proposed element demonstrate accuracy and robustness under inelastic material response as well as capturing fundamental, higher, and mix modes of vibration frequencies and shapes.

Assessing the Effects of Mechanical Preventive Measures on Alkali-Silica Reaction Expansion with Accelerated Mortar Bar Test

This paper presents a modification to the apparatus of the mortar bar test of ASTM C490 in order to assess and compare the influence of mechanical preventive measures in reducing alkali-silica reaction (ASR) expansion in concrete with respect to the traditional measures. For this purpose, reinforced mortar specimens with or without pre-stressing force applied were examined and the effects of the reinforcement ratio and pre-stressing force on ASR-induced expansion and cracking were studied through the accelerated mortar bar test method of ASTM C1260. The mechanical preventive measure was also compared with a traditional preventive measure by using fly ash. The results show that reinforcement and pre-stressing force play a role in limiting expansion and cracking due to ASR and that the developed test apparatus in this study can be used for measurements.