Issue 35

W. Xu et alii, Frattura ed Integrità Strutturale, 35 (2016) 481-491; DOI: 10.3221/IGF-ESIS.35.54 484 100 1700 100 Load point Load point 250 250 120 ф6@150 2ф10 2ф14 ф6@150 Figure 3: Size of test beam and reinforcement (1) Boundary condition Considering symmetry of the structure, half of the mid-span section of the beam is taken for modeling. Mid-span section is set to be symmetry constraint. Cushion block is placed at the position of support and support constraint is exerted on cushion block. Moreover, constraints in X and Y direction is exerted in the direction vertical to the beam. Free deformation of beam along beam direction is allowed. (2) Load application Self weight: the model considers self weight of the structure; specific gravity of concrete and steel are 25 kN/m 3 and 7.85 kN/m 3 respectively; acceleration of gravity is taken as 9.81 m/s 2 . Vertical load: as finite element analysis needs to consider descending branch of the stress-strain relationship curve, displacement loading is applied, in order to ensure calculation convegence. Vertical displacement is applied on loading site of the test beam until the beam breaks. Then the external force exerted is calculated according to load-displacement relationship of the loading point. Figure 4 : Nonlinear spring unit. The core area of frame joints is not equipped with stirrup; steel reinforcement and concrete are modeled using C3D8R unit (C3D8R refers to type of the unit; C refers to entity unit; 3D refers to three dimension; 8 refers to number of nodes in that unit; R means the unit is a reduced integration unit); to simulate the interaction between corroded steel reinforcement and concrete better when making three-dimensional finite element analysis on corroded steel reinforced concrete structural element, three nonlinear springs with different stiffness and vertical to each other (K x , K y , K z ) are placed between unit joint of steel reinforcement and unit joint of concrete. Stiffness matrix of the nonlinear spring e K is: