## 4 Numerical Simulation of the RC Frame--Infill Concrete Panel Interaction  
Summary results of a numerical simulation are presented, predicting the behaviour experimentally studied in Section 3 infilled RC frames. In this numerical simulation, the same methodology presented in detail by [PERSON] and [PERSON] [44] is followed employing commercial software [68; 69]. The following important mechanisms were included in this simulation [44; 60; 61; 62]. 1. The possibility of either the two columns or the beam of the RC frame to form plastic hinges at their ends. To this end, the surrounding frame is simulated with linear frame elements based on the relevant cross-sections detailing specific locations for plastic hinges with nonlinear properties (bending moment against rotation) obtained from the cross-sectional reinforcing details and the mechanical properties of the concrete and the longitudinal reinforcement (Figure 6, Figure 10 and Table 1). This was done by employing an in-house-developed software based on the work by [PERSON] and [PERSON] [70] and the RC detailing [71]. Next, use was made of a number of commercial software packages that could simulate numerically the non-linear mechanism of the frame depicted in Figure 17 for specimen Bare jacket1 [60; 61; 62]. These commercial packages were employed in a combined way during the various stages of the numerical investigation [68; 69]. In these numerical analyses, in order to trace the nonlinear solution path, a combined incremental/Newton-Raphson equilibrium iterative procedure included in these commercial packages has been utilized. The solution process follows a step-by-step incremental  
Figure 16: Horizontal load cyclic response versus sliding or separation at the contact surface of the concrete infill panel and surrounding frame for specimens UC-IP 1, UC-IP 2, and RC-IP 3. Figure 17: Numerical simulation of the surrounding RC frame with the location of the plastic hinges. static material nonlinear analysis adopting initially a constant displacement increment. A line search technique was adopted together with a number of convergence parameters in an effort to improve the convergence rate and to achieve equilibrium from one step to the next. The resulting envelope curve in terms of a numerically predicted horizontal load against the top beam displacement is compared with the corresponding experimental cyclic response in Figure 18 demonstrating reasonable agreement. 2. The same numerical simulation for the RC frame described before is also used here when forming the numerical model of the RC frame together with the UC-RC infill panels. The concrete infills were simulated with shell elements in all cases, together with a failure envelope for numerically predicting the formation of nonlinear limit states within the concrete infill (modified Von-Mises, Figure 19b) which was available in the used commercial software [68]. To quantify the parameters of this failure envelope, a number of square concrete panels with dimensions 730 mm \(\times\) 730 mm and a thickness of 50 mm were constructed with the same concrete mix and at the same time with the concrete panels of the specimens presented in Section 3. These panels were next subjected to diagonal compression-tension (Figure 19a). Two of these diagonal compression panels were unreinforced (similar to UC-IP 1 and UC-IP 2) and the other two were reinforced with the same reinforcement as RC-IP 3 (see Figure 11). Figure 19a depicts the typical diagonal cracking, which developed along the main vertical diagonal during these tests [72; 73; 74; 75]. This testing arrangement was numerically simulated by adopting the same failure envelope (Figure 19b) but with different sets of values--one set for the unreinforced panels and another set for the reinforced square panels. The values shown in Figure 19b for uniaxial compression or tension limits, equal to fc = 22 MPa and ft = 0.22 MPa, correspond to the unreinforced panel. The corresponding values adopted for the reinforced panel are equal to fc = 26 MPa and ft = 0.26 MPa.