# Numerical Simulation on Dynamic Behavior of Slab-Column Connections Subjected to Blast Loads
## 3 Numerical Analysis
### Modeling of Specimens  
In order to obtain accurate numerical analysis results, it is necessary to establish an accurate material model. In this study, Mat_072R3 was selected from the concrete material models provided by the analysis program LS-DYNA. This material model reflects the strain-rate effect and has already been found in several studies in the literature to be suitable for analyzing concrete structures under high strain-rate [47; 48; 49]. However, Mat_072R3 was unable to exhibit local damage caused by explosions, such as crater spalls, which are associated with structural failure and erosion [50]. Therefore, to simulate these characteristics, LS-DYNA's \"Add_Erosion keyword\" option was applied to the concrete material model. To model the reinforcing bars, LS-DYNA's Mat_024 was applied, which is defined as an elastic-plastic material with arbitrary stress-strain curve and an arbitrary strain-rate dependency. The fracture of Mat_024 is based on plastic deformation [49].  
Numerical analysis results may vary depending on the mesh size of the element [51; 52]. According to the previous studies, when simulating a structure subjected to an explosive load, a mesh size of 25 to 30 mm led to the analysis results most similar to the experimental results [33; 53]. In this study, before the main analysis was conducted, various mesh sizes were evaluated in terms of accuracy and efficiency of analysis. In the preliminary analysis, the displacement, stress of reinforcing bars, and fracture shape were investigated in the same way as in the main analysis. Considering the analysis results and the time required for analysis, a mesh size of 20 to 25 mm is considered to be the most reasonable. Therefore, in this study, the concrete mesh is composed of 25 mm cubic, 8-node solid elements.  
The interaction between concrete and reinforcing bars has a great influence on the behavior of RC structures. In particular, interactions such as bond-slip are very difficult to simulate. A method of tying nodes was recommended to simulate the structure's actual behavior and to provide the simplicity of analysis [48; 54]. In this study, the nodes of the reinforcing bar and concrete are connected to each other to provide accurate structural performance.  
The one-point integration method used in this analysis is effectively applied to dynamic analysis due to its relatively short analysis time. However, there is a risk of creating a zero-energy state that causes a negative volume or creating an element that behaves differently from the actual behavior. In the numerical analysis, the volume of solid elements is generally reduced when subjected to the compressive pressure, but when zero energy is generated inside the solid elements, the negative volume occurs due to the abnormal operation of the element, resulting in an increase in volume, as shown in Figure 2. In this case, the amount of internal energy loss is called hourglass energy. When the hourglass energy is largely generated, it is difficult to ensure the accuracy of the analysis [55; 56; 57]. Therefore, LS-DYNA's \"Hourglass\" option, which can control the accuracy of analysis due to this phenomenon, was applied to the material model [48; 58].