# Dynamic Behavior Analysis of I-Shaped RC Beams under Combined Blast and Impact Loads
## 5 Conclusions  
In this paper, the I-shaped RC beam widely used in highway bridges was taken as the analysis object, and the explicit dynamic analysis software LS-DYNA was used to numerically analyze the dynamic response and failure behavior of the structure under combined blast and impact loads. There is no relevant research on this type of structure in the existing literature. To this end, we carried out explosion experiments on the I-shaped RC beams, obtained an effective numerical model, and carried out numerical calculations under combined loads. It was found that different loading sequences have an important influence on the dynamic response of the structure. When the impact load was first applied to the structure, more severe concrete damage and larger mid-span displacements occurred in the depth direction of the beam. However, when the blast load was first applied to the structure, the concrete in the span direction of the beam was damaged in a longer range, while the large-scale concrete spalling in the depth direction of the beam did not occur. This was mainly due to the different mechanisms of blast and impact loads. When the impact load was applied to the structure in the form of concentrated force, the impact area will easily form local flexural-shear stress, and the subsequent explosion load will cause the combined stress to spread. The application mechanism of blast load was uniform load, and the structure will not form obvious local stress before being subjected to impact load. The concrete tension zone has the greater resistance to cracking. In addition, a parametric analysis was carried out, including some important structural parameters such as span, depth, and reinforcement configuration. It was found that the maximum mid-span displacement of the beam decreases when the beam span decreases or the beam depth increases. However, when the reinforcement ratio of longitudinal or transverse reinforcement increases, the maximum mid-span displacement of the beam  
Figure 16: Time-history curves of mid-span displacement of beams under different transverse reinforcement spacing. Figure 15: Damage states of beams under combined loads of different transverse reinforcement spacing. decreases. It is worth noting that the loading sequences of the different combined loads have different sensitivities to the structural parameters. When the span, depth, transverse reinforcement ratio, and longitudinal reinforcement ratio change within a certain range, the maximum mid-span displacement changes are 52.08%, 42.31%, 39.66%, and 47.62%, respectively, under the impact-blast load. Under the blast-impact load, the displacement changes are 37.93%, 36.67%, 54.35%, and 16.00%, respectively. Therefore, the structure is more sensitive to changes in the structural parameters when it is first subjected to impact loads. Based on the above research, the work that can be continued in the future is as follows:  
(1) Empirical formula of residual capacity: through the restart function of the finite element software LS-DYNA, apply a flexural moment to the structure after the combined loads to obtain the residual capacity of the structure. According to the parameter analysis, the empirical formula is obtained. (2) Dynamic analysis of I-shaped composite girder bridges: establish a numerical analysis model for I-shaped composite girder bridges. Based on the dynamic response of the components under the combined loads, the dynamic response or collapse analysis of I-shaped composite girder bridges is further studied. (3) Different types of combined loads: In addition to the extreme loads of blast and impact, bridge structures may still face other forms of combined loads. For example, when a tanker truck explodes, the structure may be subjected to both blast and fire loads. All authors significantly contributed to the scientific study and writing.