5TH International Congress on Technology - Engineering & Science - Kuala Lumpur - Malaysia (2018-02-01)

Numerical Investigation Of One-way Reinforced Concrete Slab Subjected To Underwater Explosion

One of the main issues in the design of marine and hydraulic structures is the stability of these structures subjected to underwater explosion loading (UNDEX). To evaluate the above structures under UNDEX loading, shock waves and bubbles which are produced by gas emission during the explosion should be considered. The energy released during the explosion has three forms: shock wave, bubbles and heat which 57% of the total energy is wasted by the shock waves, 37% by the bubbles and 6% by heat [1]. For the first time in the field of UNDEX in 1948, Cole showed that explosion is due to the release of energy in a very short time, and causes energy to generate unusual pressure. This pressure will cause irreparable damage [2]. In 1998, the dynamic response of the ship (Catamaran-Hull) under UNDEX loading [3] was evaluated, and a metal plate with stiffeners under non-contact underwater loading was assessed in 2014 [4]. Chen et al. studied the failure of a concrete gravity dam under UNDEX loading in a reservoir [5]. Wang et al. investigated Experimental and numerical studies of a ship structure under shock waves and gas bubble vibration. They identified bubbles as the main cause of ship damage and admitted that the result accuracies in a simulation with the Abaqus/CAE finite element software are acceptable. [1]. In the present study, by using the Abaqus/CAE finite element software, the experimental model of Wang et al. is simulated (Figure 1). This simulation is performed under 5 gr of spherical explosive mass at a distance of 30 cm away from the center of a box-shaped structure, which is floated on the surface of the water (Figure 2). The above model is studied with the specific mechanical properties (Table 1) under three different element sizes. Finally, a 2.5 cm element size is selected for the water part with a minimum error for peak pressure (Figure 3 and 4). The peak pressure at the water level (Point-1) from traditional experiments and numerical simulations are compared with the present study. The relative error of this numerical simulation is 0.97% (Table 2). Then a one-way reinforced concrete slab that is in contact with water is simulated, and the displacement under UNDEX at the concrete slab center which is in contact with water is calculated. Reinforced slab size is 100 cm × 100 cm × 4 cm, and it has a rebar mesh at the center of slab thickness. The diameter of the rebars is 6 mm and their spaces are 7.5 cm (Figure 5). Table 3 shows the mechanical properties of the RC slab. This slab is analyzed under the TNT explosive mass of 10, 20, 30, 40 and 50 gr which are placed 40 cm away from the RC slab center. To study the dynamic response of the concrete slab under uniaxial compression and tension, the modified Popovics and the Belarbi-Hsu models are used respectively (Table 4). To characterize the dynamic behavior of steel rebars, Johnson-Cook model is exploited for high strain and high-temperature steels (Table 5). Under 10 gr of the explosive mass, shock waves impact the RC slab center (Figure 6), so promptly, the acceleration reaches its maximum value in the slab center, and its central displacement increases. Then, the acceleration rapidly decreases, and in the center of RC slab negative pressure, which is due to bubbles collision to the slab surface, is produced, so maximum displacement is reduced (Figure 7). With increasing the explosive weight from 10 to 20 gr, the central displacement of the slab increases considerably, but by increasing the explosive weight from 40 to 50 gr, the changes of this displacement is reduced significantly (Figure 8). According to these results, it can be said that RC structures displacement under a small amount of explosive weight will show a fluctuation around zero. However, by increasing the amount of explosive weight it always has positive displacement.
Maziar Fahimi-Farzam, Saeid Pourhosein, seyyed Ali Mousavi Gavgani