Research On Design Of Long Span Prestressed Concrete U-Section Continuous Beam For Urban Rail Transit

Simply supported U-shaped beams have been widely used in rail transit in China.In addition,there are quite a number of long-span structures in the elevated lines of rail transit.If the conventional prestressed concrete continuous box girder structure is still used,the shape of U-beam and box girder transition is not harmonious,and the landscape effect is not good.Prestressed concrete continuous U-beam can solve the transition problem between long-span rail transit structure and simply supported U-beam,which greatly improves the universality and versatility of U-beam.For prestressed concrete continuous U-beam,many domestic rail transit lines have adopted it(with the main span of 40～75m),which is technically feasible and has achieved good results,and has been gradually popularized.But at present,the research on prestressed concrete continuous U-beam is still not systematic,and its force transmission mechanism and stress characteristics are not fully clear.Therefore,it is necessary to study the prestressed concrete continuous U-beam to provide a new idea for the gradually widely developed elevated structure design of urban rail transit engineering,and lay a comprehensive foundation for the wide promotion and application of U-beam technology technical basis.The main contents of this thesis include:(1)This thesis introduces the structural size and tendon layout of U-section continuous beam of Nanjing Jurong intercity rail transit(33 + 55 + 33)m,and analyzes and summarizes the key points of U-section continuous beam design from the aspects of section size design,tendon layout design and temperature calculation value of U-section continuous beam.Finally,according to the on-site construction cooperation experience,the problems existing in the construction of U-section continuous beam are analyzed.(2)Using Midas civil bridge calculation software,the single beam model and spatial bar grid model of(33 + 55 + 33)m U-section continuous beam bridge are established.The internal forces under various load conditions and condition combinations are calculated and analyzed,including the calculation results of bearing reaction and internal forces under dead load,train live load,bearing settlement,gradient temperature condition and the combination of main force and main force plus additional force.The two calculation methods are compared.Through comparison,it is found that there are great differences in the distribution of reaction forces of three supports at the beam end between the beam lattice model and the single beam model.Through analysis,the beam lattice model can better simulate the distribution of reaction forces of three supports at the beam end.The beam lattice model and the single beam model are close in terms of internal force and deformation of the main beam.There is little difference between the normal stress of the beam lattice model and that of the single beam model.The maximum shear stress of the beam lattice model is significantly greater than that of the single beam model.Mainly because the continuous beam with U-shaped section is an open section,the middle web bears most of the shear stress.(3)The reaction force,stress and displacement of the support of curved beam and straight beam are compared and analyzed by using the general finite element software ANSYS.Under the four working conditions,the structural stress and vertical deformation have little difference,and the bridge alignment can be considered as a straight line.At the same time,the overall stress and displacement of the straight-line U-section continuous beam are analyzed.Under the action of dead load,the normal stress of the web on the section is basically linearly distributed along the height direction,which meets the plane section assumption.The bar system model can be established according to the elementary beam theory for calculation.The transverse normal stress of track bed slab will be reversed at the upper and lower edges of the middle fulcrum and mid span,and the shear at the middle fulcrum is mainly borne by the box web.Both positive and negative shear lag effects exist in U-section continuous beam under concentrated load and uniformly distributed load.With the increase of load,the shear lag effect increases gradually.In the design process of U-section continuous beam,for the section of middle fulcrum,special attention should be paid to the influence of shear lag effect and strengthen the stress safety reserve at the middle fulcrum.For the side span and middle span,the shear lag effect is not obvious,and the number of steel tendons can be appropriately optimized.(4)In this thesis beam element,grillage and solid element of(33 + 55 + 33)m U-shaped continuous beam of Nanjing Jurong rail transit are studied and analyzed.The analysis results show that for the U-shaped continuous beam,the beam element model is used to simplify the calculation,and the main calculation results of the section can meet the requirements of engineering design accuracy.For the case of multiple supports,it is recommended to use the grid model or solid model to calculate the reaction force of the multiple supports during the design.For the maximum shear stress value calculated by the single beam model,since the influence of the shear lag effect is not considered,it will be smaller than the actual maximum shear stress value.It is recommended to take measures such as increasing the thickness of the Axillary angle and appropriately increasing the thickness of the middle web to improve safety reserves.(5)The U-shaped continuous beam is a cast-in-place thin-walled structure.The thickness of side web and middle web should be appropriately increased(the minimum thickness of web is recommended to be 400mm),which is conducive to increasing the transverse stiffness of the section,and also conducive to the layout of strands and construction control.The following principles should be paid attention to when allocating prestressing tendons of U-shaped continuous beam:(1)because the upper flange of the U-shaped part of the web is narrow,it is necessary to avoid setting short tendons on the top plate as far as possible,so as not to weaken the strength of the beam by slotting anchorage.(2)Because the thickness of the side web is small,only one row of tendons should be configured and the size of prestressing tendons should be reduced as far as possible.(2)When the side web is short of anchorage space due to its small thickness,part of the web tendons can be bent to the bottom plate for anchorage,but it should be as close to the web as possible and the thickness of the bottom plate should meet the anchorage requirements.(3)Because the mid web plate is thick,and prestressing tendons can be vertically bent to the bottom of the beam,the alignment is consistent with the stress form of the beam body,so the mid web plate should be fully used for tendons layout.(4)When the web prestressing tendon is horizontally bent to the bottom plate,the horizontal bending section and vertical bending section shall be staggered to ensure sufficient thickness of protective layer.