Safety Margin Analysis Of Hydraulic Bridge With Medium And Short Sapn Based On Reliability Theory

In recent years,with the aggravation of global climate change and the frequent occurrence of floods and tsunamis around the world,the hydraulic failure cases of river bridges and coastal bridges occur frequently.The current bridge design methods still lack of the consideration on the flood resistance and hydraulic failure risks.The scientific and reasonable hydraulic failure risk assessment and design method on bridges is of great significance to both theory and application field.Due to the uncertainty of flood intensity which is affected by multiple factors such as climate,topography,the reliability theory is commonly used to evaluate the flood resistance of bridges in previous research.However,the limit state functions used is mainly based on the buried depth of foundation,which cannot be explained by structural mechanics models.The current hydraulic bridge design codes only regulate the layout of bridge openings,deck elevation and foundation depth,which is difficult to meet the engineering needs.The main contents of this paper are as follows:(1)According to the random characteristics of hydraulic safety margin of bridges,the hydraulic reliability index is proposed to quantitatively assess the hydraulic safety margin from the perspective of structure failure probability,providing the reference for the evaluation of hydraulic bridge failure risk.(2)Depends on the analysis of hydraulic failure modes of two types of typical medium and small span bridges(arch bridges with rigid foundation and simply supported beam bridges with pile foundation),the expression of limit state function for each type of bridges is established.Furthermore,the practical algorithms to build Structure-Flow-Soil coupled numerical model by adding soil spring and node force mapped from scour hole and flow force is proposed to obtain the structural response under flood,which provides the flood effect in the expression of limit state function.A refined evaluation scheme for hydraulic safety margin of existing bridges is established based on response surface method,and the application on the analysis of Heyuan Dongjiang Bridge failure case verifies the applicability and feasibility of the method.(3)To meet the need of pre-warning on hydraulic failure risks of simply supported beam bridges with pile foundation,this paper developed a graded warning system based on hydraulic safety margin and measured scour depth.The application case shows that the hydraulic failure risk can be directly told by measured scour depth,which also provided a reference for the f pre-warning classification interval.(4)In order to modify the hydraulic design regulation of bridges in current design codes,this paper establishes the relationship between bridges design modes and hydraulic failure modes,proposed the partial coefficient hydraulic design expressions for two types of typical medium and small span bridges.To avoid the large calculation cost of coupled numerical model,the simplified flood effect calculation mode is established by structural analysis,which is applied in the partial coefficient hydraulic design expressions.Finally,the reasonable partial coefficients in hydraulic design expression are proposed by optimum analysis,which conforms to the current design code system.