Study On Structural Design And Ultimate Bearing Capacity Of Weak Rigid Connection Structure Between T Beam And Box Girder

The splicing between main line T-beam bridge and ramp box girder bridge mostly adopts the mode A,in which both the superstructure and the substructure are not connected.The mode A can minimize the impact of the splicing between new and old bridges,which is verified in the resulted internal forces.Regarding the mode A,the difference of concrete shrinkage and creep as well as the adverse effect of uneven settlement of foundation can be neglected,and the construction is simple.However,the mode A still brings about many problems to the bridges.For example,the bridge deck pavement at the joint may be seriously damaged,and large deformation difference between the flanges of old and new bridges can be found,and so on.These problems would exert great impact on the serviceability,durability and even bearing capacity of bridge structure,as well as traffic safety,driving experience and road capacity,and may greatly increase the operation and maintenance costs of bridge in the future.In order to solve the above problems,through the combination of finite element analysis and test,studies of the mechanical behaviour of an alternate solution as called mode B has been put forward,in which the superstructures of main line T-beam bridge and ramp box girder bridge are connected while the substructures are not connected.The main work and results are listed as follows:(1)Based on the finite element model for whole bridge,the mechanical behaviour of the spliced bridges’ superstructure with three splicing schemes including weak rigid connection,rigid connection with partial diaphragm or rigid connection with full diaphragms,were compared.Furthermore,the applicability of current codes for calculating the internal forces of such spliced flange plate was analyzed.Impact of vehicle load,concrete shrinkage and creep difference,as well as foundation settlement difference,were analyzed.The results show that the flange splicing is beneficial to reducing the impact of vehicle load,but it is not significant if the diaphragm is further installed,and that the stress of diaphragm near the bridge bearing caused by the action of foundation settlement is very harmful.Therefore,further installing diaphragm for rigid connection should not be recommended.The three splicing schemes will increase the transverse tension of main girder of old bridge,and the weak rigid connection scheme has a problem of local cracking in concrete of flange plate.The three splicing schemes have little response to the effect of concrete shrinkage and creep.Because the torsional constraint stiffness of main girder’s flanges of new and old bridges is quite different,the bending moment at the root segment of box girder’s flange calculated by the current code,that is Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts,is smaller than the value calculated by finite element method,which means unsafe.As a result,the negative bending moment at the root segment of flange should be checked in combination with finite element analysis and code reference when a bridge splicing design is carried out.(2)Due to the construction convenience of weak rigid connection and the fact that diaphragm plate has little effect on further improving stress state of old bridge’s main beam,the static behaviour of the spliced flange plate of weak rigid connection are explored through ultimate bearing capacity test for vehicle load.The test and finite element analysis show that the spliced flange plate under vehicle load would be damaged due to excessive tensile stress in the oblique section,and a critical oblique crack would emerge at each side from the load plate to the support.(3)By the finite element model verified by the test,the serviceability limit state and the ultimate limit state of weak rigid connection under vehicle load and uneven settlement of foundation are investigated in depth,and the influence of space between new and old bridges is studied.The results show that the distribution and the width of crack were limited for weak rigid connection under vehicle load.Secondly,diagonal reinforcement should be embedded in the splicing flange plate to prevent tensile oblique cracking.Finally,reducing the space between new and old bridges has little effect on improving the mechanical behaviour of the splicing flange plate.