Ultimate Load Bearing Capacity Analysis And Integral Safety Evaluation For Bifurcation

Steel penstock is one of the most important water conveyance structures in water conservancy project.When the penstock is used for supplying water to many hydraulic turbines,it is necessary to introduce the steel bifurcation.Once the steel bifurcation which is besides the powerhouse of hydropower station is destroyed,it will endanger the safety of powerhouse in hydropower station and people's lives in addition to causing huge equipment and property damage.In this paper,on the basis of elastic modulus reduction method(EMRM),research works on the analysis method of ultimate load bearing capacity and the integral safety evaluation of steel bifurcation.The main contents are as follows:(1)The elastic modulus reduction method for upper bound limit analysis of steel bifurcation is presented.The element bearing ratio(EBR)of steel bifurcation is defined in term of the generalized yield function of shell element and the reference EBR(REBR)is given as a threshold which identifies the highly loaded areas dynamically.Then,the strategy for elastic modulus adjustment based on the strain energy equivalence principle is established,which is used in linear elastic iterative analysis to simulate failure evolution until the plastic limit state is identified.Meanwhile,the formula of limit load multiplier is derived based on the upper bound theorem.Then the linear elastic iterative technique for the upper bound limit analysis of steel bifurcation is constructed on the basis of structural failure mode.This method overcomes the disadvantages of EMRM for lower bound limit analysis of steel bifurcation.(2)The reference volume method for limit analysis of steel bifurcation is presented.The reference volume,representing the failure elements,is introduced into the EMRM.A sorting strategy of failure elements for reference volume is given in term of the magnitude of the element bearing ratio.The selection principle of reference volume is established based on the characteristics of global failure and local failure of steel bifurcation.The identification method of reference volume is then determined through the sorting strategy and the selection principle.Then the reference volume method for limit analysis of steel bifurcation is constructed on the basis of the elastic modulus reduction method for upper bound limit analysis.This method can provide an efficient way for limit analysis of steel bifurcation.(3)A quick estimation method for limit analysis of steel bifurcation is proposed.Based on the first linear elastic analysis of the structural analysis model,the failure coefficient of each element in the first iterative step is given.Then on the basis of selection principle of reference failure coefficient for structures,the reference failure coefficient for steel bifurcation in the first iterative step is proposed.The reference failure coefficient is given as a threshold which identifies the failure elements dynamically.The strategy for elastic modulus adjustment based on the strain energy equivalence principle is established,which is used to simulate the internal force redistribution and failure evolution.Based on the second linear elastic analysis,the failure coefficient of each element and reference failure coefficient in the second iterative step are determined,and the failure elements are identified.Then,the estimated value of ultimate load bearing capacity for steel bifurcation is determined according to the reciprocal work theorem.The quick estimation method can be used to obtain the ultimate load bearing capacity of steel bifurcation through two linear elastic analysis.It overcomes the disadvantages of elasto-plastic incremental analysis which needs complex loading process and multiple iterative analysis.The quick estimation method can provide an efficient way for limit analysis of steel bifurcation in the preliminary design stage.(4)A method for integral safety evaluation of steel bifurcation is presented preliminarily.In view of the limitation of the safety evaluation method of steel penstock in specification in the Chinese current design code,the limit value of safety coefficient of steel bifurcation in specification is proposed based on the method of equivalent safety coefficient.Then,considering the ratio of limit value of integral safety coefficient to limit value of safety coefficient in specification for pressure vessel and the application situation of steel bifurcation in China,the limit value of integral safety coefficient for steel bifurcation is established.The reference volume method and estimation method are both used for determining the ultimate load bearing capacity and integral safety coefficient of steel bifurcation.According to the integral safety coefficient and its limit value,a method for integral safety evaluation of steel bifurcation is presented preliminarily.This safety evaluation method can provide a scientific basis for the innovation and improvement of the safety evaluation and design of steel bifurcation in water conservancy project.