# Dynamic Behavior Analysis of I-Shaped RC Beams under Combined Blast and Impact Loads
###### Abstract
High energy-absorbing pads were used to protect RC beams subjected to impact loads and a simplified equation for the maximum deflection of beams with pads was established [20]. From the above studies, it can be concluded that the main reason for the difference in the dynamic response of structures caused by blast loads and impact loads is that the load application mechanisms are different. The shock wave of the explosion load is applied to the structure in the form of a uniform load. When the explosion energy is large enough, the concrete on the blast surface of the beam will be damaged under compression, and the concrete on the back blast surface will be damaged due to tension. However, RC beams may undergo flexural failure, shear failure, or flexural-shear failure under different conditions. This mainly depends on the energy of the blast load, which is generally related to the proportional distance \(Z\) of the explosive. \[Z=\frac{R}{W^{1/3}} \tag{1}\]  
where \(R\) is the distance from the center of the explosive to the structure and \(W\) is the weight of the explosive. However, the impact load is applied to the structure in the form of the concentrated load, and the dynamic response of the structure is closely related to the impact rate and impact duration. Flexural failure can occur when a structure is subjected to low rate but long duration impact loading, similar to static loading. Under high-rate and short-duration impact loads, structures may undergo localized shear failure. At moderate rates, combined shear and flexural failure may occur. Under high-rate and short-duration impact loads, structures may undergo localized shear failure. At moderate rates, combined shear and bending failure may occur. Many scholars have conducted much research on the damage mechanism and failure behavior of RC beams under impact or blast loads. However, in reality, except for the single load, the bridge structure may also be subjected to combined blast and impact loads. For example, when a car or ship carrying explosives hits a beam or column, the structure is subjected to impact loads that may subsequently cause blast loads [21; 22]. When the high-speed projectile explodes, the structural fragments of some explosive objects may be applied to the structure in the form of impact load after the explosion [23]. Therefore, it is very important to analyze the dynamic response and failure behavior of bridge structures under combined loads. At the same time, the I-shaped RC beam, as a hollow member, has a slab-rib-slab structure. Because of its structural advantages such as saving materials and reducing self-weight, it is widely used in highway bridges. However, there are no reports on its research under extreme loads. Therefore, the main purpose of this paper was to use explicit dynamic analysis software LS-DYNA to numerically analyze the dynamic mechanical response and failure behavior of I-shaped RC beams under combined loads. The key factors affecting the dynamic response of I-shaped RC beams under combined loads were evaluated, in order to provide a theoretical basis for the protection design, prevention strategies, and improvement methods of this type of structure. To this end, we have carried out the following work:  
(1) Numerical model verification under single load: Carry out explosion experiments on I-shaped RC beams to obtain an effective numerical analysis model. The numerical model with the same material model was verified experimentally under impact loading. (2) Numerical calculation under combined loads: Based on the verification of the numerical model, the dynamic response of the I-shaped RC beam under combined loads of near-field blast loads and medium-rate impact loads was analyzed. (3) Parameter analysis: The impact of different load application sequences and different structural parameters on the vulnerability of the structure was evaluated.