## 4 Numerical Simulation of the RC Frame--Infill Concrete Panel Interaction
These values were found from back analysis by utilizing these diagonal compression-tension tests (Figure 19a) and were employed for numerically simulating the behaviour of the RC infilled frames presented in Section 3. Figure 18: Horizontal displacement versus horizontal load cyclic response of specimens Bare Jacket1,2. As can be seen in Figure 20a, good agreement is reached between the numerical and measured diagonal compression-tension response through this nonlinear numerical simulation for these square panels tested in diagonal compression-tension. Based on this good agreement, the same numerical simulation is followed for the infilled RC frame specimens investigated in this study. This numerical simulation reproduced as a limit state the plastified region depicted in Figure 20b (where the \"x\" signs denote the areas reaching the adopted tensile limit state), which is along the same main vertical diagonal region depicted in Figure 19a. Figure 19: (**a**) Observed damage. (**b**) Numerically simulated diagonal tension cracking of square concrete panel. (**b**) Assumed combined compression–tension failure envelope included in the numerical simulation of the square concrete panel subjected to diagonal compression as well as of RC the infilled panels. Figure 20: Diagonal compression–tension. (**a**) Comparison between observed and numerically simulated behaviour of the tested square concrete panels. (**b**) Predicted plastified regions from the numerical simulation. 3. A first group of nonlinear link elements, which could not sustain any tension, was used to numerically simulate the possibility of the concrete infill to be separated from the surrounding frame. These elements could transfer shear at the interface when no separation was detected [44]. An additional second group of nonlinear link elements simulated the steel ties at their exact location, and they could transfer axial tensile and shear forces at the interface between the shell finite elements representing the infill panel and the linear finite elements simulating the surrounding frame. The nonlinear behaviour in axial tension of these link elements was derived from their diameter (8 mm) and yield stress (570 Mpa). These link elements were also provided with nonlinear properties to simulate the dowel action (shear force transfer) at this interface. A number of provisions with relevant empirical formulas are included in [67] that describe the nonlinear shear transfer mechanism between two concrete parts connected with a steel tie. In addition, an extensive experimental sequence was carried out at Aristotle University in an effort to quantify this shear force transfer for steel ties of diameters varying from 8 mm, identical to the ones depicted in Figure 11, up to 14 mm [60; 61; 62]. The used experimental setup is depicted in Figure 21 wherein a portion of a jacketed column together with a portion of the RC infill connected with these steel ties are subjected to combined loads indicated by the red arrows. All steel ties used in this experimental sequence as well, as for the specimens reported in Section 3, were embedded within the concrete volume in a way which prohibited any undesirable pullout. Twenty-three specimens were built at the same time with the same geometry and the same quality concrete. Three of these specimens were built without any steel ties whereas the rest were provided with steel ties of the same steel grade having diameters varying of 8 mm, 10 mm, 12 mm, and 14 mm. Figure 21b depicts a cross-section of such specimens wherein the upper and lower parts of the \"old\" column are rigidly supported by the strong reaction frame. Initially, the load normal to the interface was applied as indicated in this figure, keeping its amplitude constant during one test. Then, a cyclic load with a direction parallel to the interface was applied with an eccentricity of 20 mm from this interface. The amplitude of this load was gradually increased until a limit state condition was reached.