Analysis And Prevention Measures Of Web Cracking In Construction Stage Of Long Span Continuous Rigid Frame Bridge

With the increase of the span,the box girder structure of Prestressed Concrete Continuous Rigid Frame Bridge tends to be wide box and thin-walled.In the hanging basket cantilever construction stage,most of the inclined cracks with a high degree of linear fitting with the prestressed tendon pipe under the web,and the deepest cracks can reach near the prestressed bellows.The cracking of the beam accelerates the oxidation corrosion and expansion of the reinforcement,increases the deflection of the bridge structure,and has a very adverse impact on its serviceability and durability.At present,the research on the cracking of box girder web along the pipe in the construction stage of long-span continuous rigid frame bridge is not clear enough,which can not provide effective reference for the subsequent bridge construction.In this paper,the inclined crack of the prestressed bundle pipe along the downbend of the box girder web during the construction stage of long-span continuous rigid frame bridge is taken as the research object,and the influencing factors of the cracking of the web along the pipe are analyzed theoretically.Combined with the local finite element model of the box girder of Pinglu Grand Canal Bridge,the sensitivity of the theoretical influencing factors of the web cracking along the pipeline in the construction stage is studied.The main research work is as follows:⑴ Based on the two-dimensional plane problem solving method of elasticity,the calculation formula of transverse tensile stress caused by concentrated load is derived,and the stress diffusion effect of large tonnage preload of longitudinal downward bending beam in Web anchorage zone is determined.The influence mechanism of pipe deflection,box girder space effect and transverse stress effect,web thickness,pipe concrete strength grade and stirrup distribution on Web inclined crack along pipe are analyzed theoretically.⑵ The local finite element model of 7 # characteristic segment at 1 / 8span is established to simulate the cantilever construction state of box girder.The influence mechanism of transverse tensile stress caused by web bending prestressed tendon on the main tensile stress of concrete along the pipe is studied.The influence law of the theoretical sensitive factors of web along the inclined crack was determined,and it was found that the precompression stress of web under bending bundle,the vertical precompression stress and the thickness of web had a great influence on the main tensile stress of web concrete.⑶ The finite element model of 0# ～ 3# section was established,and the influence of the deviation position and direction of the prestressed bundle pipe on the main tensile stress of the concrete near the deviation area of the pipe is analyzed,the most unfavorable coupling condition of the construction location of the prestressed bundle of webs was determined;The finite element model of0# ～ 6# section was established,and the distribution law of vertical normal stress of web along beam section was compared and analyzed when the vertical prestressed reinforcement was used in immediate tensioning,integral tensioning and hysteretic tensioning respectively.It is suggested that the vertical prestressing tendons of web should be subsection tensioned,and the less the number of lag tensioned beams,the better.⑷ In view of the influence factors of webs appearing inclined cracks along the pipeline in the construction stage analyzed in this paper,the prevention and control measures of webs cracking along the pipeline in the construction stage are put forward in combination with the design and construction of Pinglu Canal Bridge.