Research On Anti-sliding Safety And The Corresponding Key Influencing Factors Between Main Cable And Saddle In Long-span Multi-span Suspension Bridge

Multispan suspension bridge has been widely used in crossing wide water area because of its superior advantages such as strong crossing ability,high economic benefits,and convenient construction.However,multispan suspension bridge has limited structural stiffness due to the lack of effective intermediate longitudinal constraints.This is a key factor that affects the crossing capability and structure safety of the bridge.The use of rigid middle tower in multispan suspension bridge is an effective method to improve the overall structural stiffness.Nevertheless,increasing stiffness of the middle tower not only increases the unbalanced cable tension of the middle tower,but also significantly reduces the anti-sliding safety between main cable and saddle of bridge.The "Mid-tower effect" refers to the dilemma that the stiffness of middle tower is difficult to determine in order to satisfy the contradictory requirements of structural stiffness and anti-sliding safety at the same time.However,the existing defining way of anti-sliding safety factor proposed in the Specifications for Design of Highway Suspension Bridge(JTG/T D65-05-2015)possesses ambiguous physical meaning,and is conservative in calculation and unable to quantify the frictional resistance between main cable and saddle.This is one of the most important reasons for the narrow range of values of the middle tower stiffness.Therefore,it is necessary to study the frictional resistance and anti-sliding safety between main cable and saddle of the middle tower.(1)By analyzing the stress state of main cable and saddle under the sliding critical condition,the calculation of the frictional resistance between main cable and saddle can be simplified as a plane strain problem.The simplified two-dimensional finite element model of saddle was established by ABAQUS to obtain the normal stress distribution between main cable and saddle at an arbitrary cross section of saddle under the sliding critical condition.The frictional resistance between main cable and saddle was calculated through multiplying the normal stress by the coefficient of friction and assuming them at all cross sections.The proposed finite element method was validated against large-scale saddle model tests.The results indicated that the method can accurately calculate the frictional resistance between main cable and saddle with or without the vertical friction plate,and can obtain the stress distribution at an arbitrary cross section of saddle.Adding vertical friction plates can effectively improve the frictional resistance between main cable and saddle,which is an effective way to enhance the frictional resistance between main cable and saddle.(2)In view of the problem in calculating the anti-sliding safety factor in the Specifications for Design of Highway Suspension Bridge(JTG/T D65-05-2015),the ratio of the actual value of frictional resistance between main cable and saddle to the required value of frictional resistance was proposed as a new evaluation method of anti-sliding safety factor.The actual value of frictional resistance can be quantified by the proposed finite element method and the contribution of the side surface of saddle and vertical friction plates to frictional resistance can be taken into account.The new evaluation method of anti-sliding safety factor possesses clear physical meaning and its form is simple.It is convenient for engineering application,and can be used as a basis to determine the anti-sliding safety factor.(3)The cable tension of tight side under the sliding critical condition was considered as 1.1 times as the cable tension of tight side when a central span is fully loaded and the adjacent central span is not loaded with any live load,and the Euler's equation is introduced.Referring to the design parameters of four multispan suspension bridges in China,the nominal coefficient of friction was set as 0.26.According to the recommended range of rise-span ratio of main cable,the angle of the contact interfaces between main cable and saddle was assumed to be 0.644 rad.Based on the above two values,the limited of anti-sliding safety factor of the new evaluation method is 2.5.Considering the influence of nominal coefficient of friction and angle of the contact interfaces between main cable and saddle,the limit is universal and convenient for engineering application.It can be used as a basis for determining the limit of anti-sliding safety factor under the new evaluation.(4)Taking four multispan suspension bridges in China as the research objects,the stiffness of the middle tower,the height difference between the middle tower and the side tower,and the way of restriction between the middle tower and stiffening girder were analyzed respectively.The range of design parameters meeting the limit of anti-sliding safety factor is obtained by using the traditional method.Then,combined with the finite element method of frictional resistance between main cable and saddle,the new evaluation method of anti-sliding safety factor was used to obtain the range of design parameters under the limited considering the actual design parameters of the saddle and the saddle arranged with the maximum number of vertical friction plates.It can be seen from the analysis that the new evaluation method can effectively relax the restriction of anti-sliding safety factor on design conditions and expand the value range of design parameters.If the vertical friction plates are set in the saddle,all the design parameters meet the limit of anti-sliding safety factor,and the anti-slip safety factor is no longer the control condition for the design parameters.