There are few study which focused on the fall of cargoes, while many automated warehouses shut down because of the fall of cargoes in past earthquakes. Here, the authors conducted experiments on them. These experimental studies were to investigate the slip of cargoes and the characteristics of vibration which is the main cause of the fall of cargoes.
First of all, we examined the failures of cargoes on the upper part of the warehouse. As the results of the experiment, movements of cargoes were classified into four failure modes; slip mode, collapse mode, overturn mode, slip-collapse mixed mode. Above all, we focused on the slip mode. The reason is that other modes appeared for five or six layers, while it is impossible to stack these cargoes because of the corrugated carton strength which is provided by JIS and that the cargoes are encouraged to wrap with shrink.
Second, we examined the movement of cargo slipping. It was assumed that the movement of cargoes and palettes were not the same, because of differences between the acceleration phases at top of cargoes and those of palettes. Moreover, coefficients of the friction were classified into static friction and dynamic friction, from the time difference between the rise of horizontal force and start of cargo slipping.
Next, we showed the judgement method of the fall of cargoes (in the slip mode) and verified that the results of experiments can be explained with the displacement of cargoes and overturning moment of cargoes.
Then we investigated the characteristics of the slip and the vibraiton of cargoes, which is the main causes of cargoes falling, especially for the slip mode. First, we showed that the vibration mode of cargoes assessed primary vibration mode by taking the displacement distribution of cargoes into consideration. For the slipping characteristics, the coefficients of friction were evaluated. The coefficient of static friction varied from 0.4～0.6 although we used the same palette. The coefficient of dynamic friction increases with the height of cargoes. For finding out the main factor that influence the volume of slip of cargoes in characteristics of input wave, we confirmed the relation between cargoes displacement and support beem acceleration as well as that between the cargoes displacement and support beem the velocity. The displacement of cargoes had the better relation with the support beem the velocity than the acceleration. For the vibration characteristics, we confirmed the relation between the maximum acceleration of palettes, the natural frequency and the damping coefficient calculated with system identification.
The slip of cargoes and vibration characteristics which were the main causes of cargoes falling and associated coefficient of friction, natural frequency and damping coefficient, were evaluated experimentally. In the future, we will use the model which instructs the results presented in this paper, and investigate the effectiveness of the countermeasure to avoid the fall of cargoes. Moreover, we will verify the prediction method of cargoes falling using these results.
