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Direct determination of influence lines and surfaces by F.E.M.
Orakdogen, Engin,Girgin, Konuralp Techno-Press 2005 Structural Engineering and Mechanics, An Int'l Jou Vol.20 No.3
In this study, element loading matrices are defined for static application of classical M$\ddot{u}$ller-Breslau principle to finite element method. The loading matrices are derived from existing element matrices using Betti's law and known governing equations of F.E.M. Thus, the ordinates of influence lines and influence surfaces may be easily obtained from structural analysis for the loading matrices derived from governing equations, instead of through introduced unit force or displacement techniques. An algorithm for a computer program and comparative numerical examples are also presented to illustrate the procedure for determination of influence line and surface ordinates.
A matrix displacement formulation for minimum weight design of frames
Orakdogen, Engin Techno-Press 2002 Structural Engineering and Mechanics, An Int'l Jou Vol.14 No.4
A static linear programming formulation for minimum weight design of frames that is based on a matrix displacement method is presented in this paper. According to elementary theory of plasticity, minimum weight design of frames can be carried out by using only the equilibrium equations, because the system is statically determinate when at an incipient collapse state. In the present formulation, a statically determinate released frame is defined by introducing hinges into the real frame and the bending moments in yield constraints are expressed in terms of unit hinge rotations and the external loads respectively, by utilizing the matrix displacement method. Conventional Simplex algorithm with some modifications is utilized for the solution of linear programming problem. As the formulation is based on matrix displacement method, it may be easily adopted to the weight optimization of frames with displacement and deformation limitations. Four illustrative examples are also given for comparing the results to those obtained in previous studies.
Nermin Orakdogen 한국고분자학회 2014 Macromolecular Research Vol.22 No.1
Copolymeric hydrogels of N,N-dimethylaminoethyl methacrylate (DMAEMA) and N-isopropylacrylamide(NIPA) of various monomer ratios were evaluated as thermo-responsive and pH-responsive systems for thedevelopment of controlled-release and targeted-delivery devices. The swelling properties were investigated with differenttemperature, pH, and monomer feed ratios. The results show that the temperature-dependent and pH-dependentphase transition of poly(N,N-dimethylaminoethyl methacrylate-N-isopropylacrylamide) (P(DMAEMA-NIPA)) copolymerichydrogels can be controlled by changing the amount of NIPA units in the network chains. In experiments todetermine the temperature-dependent swelling of copolymeric hydrogels in water, it was found that the swellingratio rapidly decreases as the temperature increases between 35 and 70 ℃. To characterize the network structure ofthe copolymeric hydrogels corresponding to effective cross-linking density and average network chain length,uniaxial compressive mechanical testing was carried both after the preparation of hydrogels and after their equilibriumswelling in water. The data obtained demonstrates that the resulting copolymeric hydrogels are promising asmaterials with tunable hydrophilicity-hydrophobicity and swelling behavior responsive to temperature and pH.
Calik-Karakose, Ulku H.,Orakdogen, Engin,Saygun, Ahmet I.,Askes, Harm Techno-Press 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.52 No.2
A four-noded curved shell finite element for the geometrically non-linear analysis of beams curved in plan is introduced. The structure is conceived as a sequence of macro-elements (ME) having the form of transversal segments of identical topology where each slice is formed using a number of the curved shell elements which have 7 degrees of freedom (DOF) per node. A curved box-girder beam example is modelled using various meshes and linear analysis results are compared to the solutions of a well-known computer program SAP2000. Linear and non-linear analyses of the beam under increasing uniformly distributed loads are also carried out. In addition to box-girder beams, the proposed element can also be used in modelling open-section beams with curved or straight axes and circular plates under radial compression. Buckling loads of a circular plate example are obtained for coarse and successively refined meshes and results are compared with each other. The advantage of this element is that curved systems can be realistically modelled and satisfactory results can be obtained even by using coarse meshes.
Torsional rigidity of arbitrarily shaped composite sections by hybrid finite element approach
Kutlu Darllmaz,Engin Orakdogen,Konuralp Girgin,Semih Kucukarslan 국제구조공학회 2007 Steel and Composite Structures, An International J Vol.7 No.3
The purpose of this study is to calculate the torsional rigidity of arbitrarily shaped composite sections on the basis of hybrid finite element approach. An analogy is used between the torsion problem and deformation of a plate which exhibits only shear behavior. In the analysis a simple hybrid finite element based on Hellinger-Reissner functional is presented and a set of numerical examples are performed to demonstrate and asses the performance of the developed element in practical applications.