Study On Characteristics Of Flood Impact Loading And Damage Mechanism Of Rural Buildings

Flood disasters in mountainous areas of China are widespread.The flood damage to rural buildings often has a serious influence on the production and living of people.Under the background of national mountain flood disaster prevention and rural revitalization,focusing on the problem of flood control and disaster reduction,this paper investigated the loading characteristics,structural response,and damage mechanism of rural buildings in mountainous areas impacted by floods.It provides theoretical and practical support for comprehensively improving the flood risk management level and disaster resistance capacity of rural buildings.The main research contents and conclusions are as follows:(1)Combined with the field investigation results and relevant literature,the causes of flood disasters and the structural types of rural buildings in mountainous areas were analyzed.The main failure modes and the flood damage law of rural buildings impacted by floods were summarized and studied.(2)A total of 87 groups of tests of rural buildings impacted by floods were carried out to investigate the flood impact loading characteristics and the influence mechanism of the porosity P and the incident angleγ.The modified formula of the maximum impact force and the prediction formula of the the maximum impact moment were given.The results showed that whenγ=0°,the impact load decreased with increasing porosity,and the impact force reduction coefficientη_F could be considered as a linear positive correlation with the porosity.When P=0%,the impact force was negatively correlated with the incident angle(0°≤γ≤45°).Under the influence of the porosity and the incident angle,the reduction coefficientsη_F andη_Mshowed a W-shaped variation trend with the incident angle.The impact load on the building with openings and incident angles was large,and the openings should be made orthogonal to the flow and aligned front and rear to improve the flood resistance.The maximum impact force occurred when the water depth of the undisturbed flow reached 0.80 times the maximum water depth.The proportion of impulse before the maximum impact force in the total impulse was about 30-50%.The maximum impact moment was equivalent to the maximum impact force acting on 1.06 times the maximum water depth.(3)A total of 156 groups of tests were carried out to explore the physical mechanism of the shadowing effect.The modified formula of the maximum impact force considering the shadowing effect and quantitative relationship between the influence coefficient of the shadowing effect k_s and the relative longitudinal spacingλwere obtained.The results showed that the impact load and pressure considering the shadowing effect were negatively correlated to the aspect ratio of the front buildingα,positively correlated to the relative longitudinal spacingλ,and there was no appreciable trend for the orientation of the front buildingθ.The shadowing effect depends on the confluence state of the branches and the development of the flood flow reconstruction.When the building spacing was small,relative optimal values ofθwere 50°,45°,and 0°for the front building withα=0.5,1.0,and 1.5,respectively,but when the building spacing was large,there was no obvious optimal value ofθ.The shadowing effect was negatively correlated to the flood intensity.Large building spacing could delay the occurrence of the maximum impact load and slightly increase the proportion of impulse before the maximum impact force in the total impulse.The shadowing effect could slightly reduce the impact arm.(4)The structural response and damage process of a single rural building impacted by floods in 42 disaster scenarios were simulated,and the damage characteristics were explored.The physical vulnerability was analyzed,and the damage mechanism of rural buildings impacted by floods considering the porosity and the incident angle was expounded.The results showed that the equivalent stress of the building impacted by floods followed such patterns:in the vertical direction,it was higher at the bottom,and in the horizontal direction,it was higher at the rear.The opening could reduce the impact load,but it might also reduce the local structural strength and the overall bearing capacity.The flood impact loading mechanism on oblique buildings was relatively gentle.When the incident angle of the building was about 45°or the incident angle of the building with openings was large,the physical vulnerability was large.The physical vulnerability depends on the magnitude of the impact load and the strength of the structural lateral resistance hierarchy.(5)The structural response and damage process of rural building clusters impacted by floods in 117 disaster scenarios were simulated,and damage characteristics of rural buildings considering the shadowing effect were analyzed.The shadowing effect was quantified and the influence mechanism was revealed.The results showed that the impulsive force,the maximum impact force,and the equivalent stress of the building considering the shadowing effect were small.The smaller the relative longitudinal spacing and porosity,the stronger the shadowing effect,and the relative optimal value of the orientation of the front building appeared between 30°and 60°.The shadowing effect depends on the flow conditions and the confluence state of the branches,and the influence of the scenario parameters on the shadowing effect is not independent and needs to be considered comprehensively.In high-intensity flood scenarios,the shadowing effect not only had a limited protection effect but might also exacerbate the flood damage.(6)The structural response and damage process of the typical two-story rural building impacted by floods in 40 disaster scenarios were simulated and the damage mechanism was analyzed.The physical vulnerability PV was quantified,and a five-level classification scheme for flood damage levels of rural brick-concrete buildings was proposed.The results showed that structural measures may ensure alternative load paths for damage-induced overloads,and improve the robustness and resilience of the building.The impact overturning moment was the principal reason for the building damage.Attention should be paid to the influence of key physical phenomena in the impact process(such as wall damage)on the physical vulnerability.Regarding the building itself,the physical vulnerability depends on both the local structural strength(local view)and the structural resistance hierarchy(global view).