# Experimental Study on the Performance of Polyurethane-Steel Sandwich Structure under Debris Flow
## 3 Test Phenomena and Analysis
### Overall Structural Deformation  
In Figure 11, the overall deformation of the impacted column and its connected members at the end of case G6 are shown. It clearly demonstrates that the rotation angle between the impacted column and its connected beam of Model A is larger than that of Model B. Combining the analysis in Section 3.1, since the excessive local dent of Model A's impacted column leads to the decrease of bending resistance capacity, the catenary action of column plays a more important role in resisting the impact. At the same time, the constraint capacity of Model A's column on its beam is weak. Therefore, the angle between the beams and columns is relatively large.  
As depicted in Figure 12, the vertical deformations of Model A and Model B at the end of all impact cases are measured as approximately 14 mm and 6 mm, respectively. This is because, as for the steel frame structure, the excessive local dent of the impacted column makes the catenary action more obvious, which results in increased column tension, and the displacement of the upper structure increases under the action of tension. However, the catenary action of polyurethane-steel sandwich structure is alleviated, and the displacement is mainly caused by the bending deformation.  
Figure 11: Photo of the impacted column after case G6. (**a**) Model A; and (**b**) Model B.  
Figure 12: _Cont._Figure 13a,b displays the maximum normal strain of the B/2 and A/3 column base, which is adjacent to the impacted column. It is noted that because of the polyurethane elastomer's role in transmitting force and enhancing integrity, the maximum normal strain at these positions of the steel frame structure is smaller than that of polyurethane-steel sandwich structure. Correspondingly, the normal stress at these positions of the steel frame structure is relatively small. Figure 13c shows the maximum normal strain at the beam bottom flange near the beam-column joint, which is directly connected with the impacted column. This demonstrates that, under each impact, for the polyurethane-steel sandwich structure, the absolute increase of normal strain at that position is larger than that of steel frame structure (the negative strain represents the compression of beam). In other words, the larger the impact force, the better the performance of polyurethane-steel sandwich structure, which is obviously beneficial to practical engineering applications. These results mainly occur because the column of polyurethane-steel sandwich structure has a greater ability to restrain the beam, resulting in a greater bending moment of the beam, and the bending moment here would cause the compression of the bottom flange.  
Figure 14 shows the displacement, velocity and acceleration time-history at the top of each floor of the A/3 column under case G2. From these figures, it can be seen that for polyurethane-steel sandwich structure (Model B), both the displacement, velocity, and acceleration time-history at the top of each floor of the A/3 column are greater than the corresponding time-history at the corresponding position of steel frame structure (Model A), indicating that polyurethane-steel sandwich structure has better integrity and excellent impact transmission capability. This is consistent with other phenomena in the paper, such as Figure 9a,b.  
Figure 12: The vertical deformations at the end of case G6. (**a**) Model A; and (**b**) Model B.  
Figure 13: _Cont._  
Figure 14: Response of the A/3 column in case G2: (**a**) displacement time-history at the top of the first floor; (**b**) displacement time-history at the top of the second floor; (**c**) velocity time-history at the top of the first floor; (**d**) velocity time-history at the top of the second floor; (**e**) acceleration time-history at the top of first floor; and (**f**) acceleration time-history at the top of the second floor.  
Figure 13: The maximum normal strain under each impact. (**a**) B/2 column base; (**b**) A/3 column base; and (**c**) the A end of the 2/A-B beam at the first floor.