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The load-bearing system model of existing buildings is created by using rod finite element in column and beam elements, and shell finite element in slab and curtain elements. Beams and columns are modeled in three dimensions, taking into account all six degrees of freedom, three translational and three rotational freedoms. In rod finite elements, there are six internal forces corresponding to all six degrees of freedom. These internal forces are the axial force defined according to the local axis of the bar, and the shear forces V2, V3 and the torsion moment and the bending moments M2, M3. The 6 internal forces, 6 node displacement and deformation values obtained as a result of three dimensional analysis are calculated. ( Çubuk Eleman Lokal Aksları Element Local Axes)
Floors and basement walls, which are considered to behave linearly in the performance analysis of existing structures , are modeled with shell finite elements. The shell is modeled in three dimensions, taking into account all six degrees of freedom at the nodes of finite elements. ( Kabuk Elemanların Lokal Aksları Local Axes of Shell Elements)
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If the earthquake calculation is made with the linear method specified in TBDY Section 15.5 , the unit deformation and plastic rotation demands are obtained by using the internal force, deformation and joint displacement values of the structural system elements modeled in three dimensions without defining plastic hinges.
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If the earthquake calculation is made with the nonlinear methods specified in TBDY Section 15.6 , the unit deformation and plastic rotation demands are obtained by determining the plastic rotation values of the plastic joints at the element ends . Axial force and biaxial bending interaction are taken into account in the definition of plastic hinges. The following picture shows the yield surfaces for a plastic joint in an existing structure, consisting of axial force and biaxial bending interaction.
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