# Experimental Study on the Performance of Polyurethane-Steel Sandwich Structure under Debris Flow
## 3 Test Phenomena and Analysis
### Response of the Impacted Column  
After the end of each impact, the local deformation of steel frame structure is significantly larger than that of the polyurethane-steel sandwich structure. The column's impacted surface of steel frame structure appeared deep dent and its both sides warped badly. Its local bending deformation is quite large. As shown in Figure 4, the column surface near the impact position of the steel frame structure is slightly cracked after case G3.  
According to Figure 5a, it is noteworthy that the total value and incremental value of Model A's dent-depth are far greater than those of Model B's. This is because the columns of the polyurethane-steel sandwich structure are filled with a polyurethane elastomer that has low compression characteristics, which strengthens the structural integrity, hence, it can coordinate the whole force and deformation in the column. The polyurethane elastomer is bonded with the steel column to ensure a reliable connection, and it could transmit the shear force further and ensure the integrity of column against the impact force. Meanwhile, it also acts as a buffer against the impact force.  
Figure 5b illustrates the displacement time-history of the impact position's back surface under case G6. The results clearly demonstrate that the displacement of Model B at that position is much larger than that of Model A. The reason for this phenomenon is that polyurethane elastomers are fully extruded, and the impact force is transmitted to the column's back surface by polyurethane elastomer. As to steel frame structure, its column completely relied on the side plate to transmit the impact force to its back surface, and its load transferring path is not direct, forming quite a large dent on the impact surface and a relatively small displacement on its back surface.  
By comparing Figure 5a,b, at the end of case G6, it is evident that the displacement of the impact surface of Model A is relatively large while that of its back surface is small. However, the law of Model B is opposite to Model A. Accordingly, after case G6, the distance between the impact position and its back surface of Model A column is smaller than that of Model B's column, and the difference between the two models is about 30 mm. The decrease of distance between the impact position and its back surface means the decrease of section's bending arm and bending modulus. Consequently, the bending resistance of steel frame structure's column is evidently reduced. Furthermore, under the  
Figure 4: Photo of the impacted column after case G3: (**a**) Model A; and (**b**) Model B.  
impact loading of debris flow, the midspan of the column is the key position to resisting the bending moment. The increase of the horizontal displacement at the midspan of column would also cause the second-order effect of gravity load to increase remarkably, even triggering the overall collapse of the structure, causing serious economic losses and casualties.  
Figure 6 shows the maximum normal strain at back surface of the impact position and the maximum shear strain at the corresponding side plate of the impacted column. It can be seen that the maximum normal strain of Model B at the back surface of the impact position is larger than that of Model A; for the maximum shear strain of the side plate, that of Model A is relatively larger. According to the analysis of the previous test phenomenon and the measured displacement, owing to the low compressibility of polyurethane elastomer and the reliable bonding between the column and polyurethane elastomer, the impact force on the impacted column could be transmitted directly to the opposite plate, thus the overall mechanical performance is effectively improved. However, the impact force of steel frame structure is directly transferred to the side plate, hence, its shear strain is relatively large.