# Experimental Study on the Performance of Polyurethane-Steel Sandwich Structure under Debris Flow
## 4 Conclusions  
This paper proposes a creative polyurethane-steel sandwich composite structure to strengthen the impact resistance of buildings under debris flow. To make a comparative experimental study of the dynamic behaviors of such a polyurethane-steel sandwich structure and steel frame structure under the impact of simulated debris flow, a series of impact loading tests are designed and carried out. By analyzing the test results, the following conclusions can be drawn, which can provide a reference for future engineering applications:  
(1) The steel frame structure mainly depends on the impacted column to resist the impact loading.  
For steel frame structure, large local damage occurs under high or continuous impact loading; the local bending resistance of the impacted column decreases sharply, and instability may occur; furthermore, the mechanical behaviors and structural load-transferring path change, and the catenary action on the impacted column is quite obvious; the impact force is primarily borne by the impacted column, and the deformation is mainly concentrated on the impacted column.  
(2) Under debris flow impact, the polyurethane-steel sandwich structure exhibits superior performance to resist the impact loading.  
Under lower debris flow impact intensity, its deformation damage is minor, and does not affect its serviceability; under large and continuous impact loading, depending on the low compression performance of the polyurethane elastomer filled in column, it can coordinate the deformation of the column so as not to cause a great dent in local parts; it can ensure the bending resistance and vertical bearing capacity of column not decrease significantly, transferring the impact force more reliably to the whole structure; also, the dissipation of impact energy depends on overall force and deformation; meanwhile, the stress and strain of polyurethane-steel sandwich structure develop more uniformly.  
In conclusion, the experimental study indicates that the polyurethane-steel sandwich structure can relieve the impact effect in time and space, and remarkably strengthens the overall mechanical performance of the whole structure under debris flow, compared with traditional steel frame structure. For the newly-built structures, the polyurethane-steel sandwich structures are favorable, which has bright prospects and deserves further study for its application in civil engineering. However one should be aware of that this is still the preliminary stage of the engineering design. During the next stages a more detailed parametric analysis for design would be necessary.  
Financial support from the National Key Technology R&D Program through grant 2014 BAL05B01 is highly appreciated.  
[PERSON] conceived the new structural system, designed the work and revised the paper; [PERSON] performed the experimental study and theoretical analysis, drafting the paper; [PERSON] proposed the method, analyzed the data and revised the paper critically for important intellectual content. All authors approve the final version to be published and have agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.  
The authors declare no conflict of interest.