Failure Mechanism Analysis Of Bottom Slab For Long Span PC Box Girder Bridge

Long span PC box girder bridge has good structural performance and could meet the requirements of modern construction.It has been widely used in the span of 100~300m.With the further development of the structure of the single box and large span PC box girder bridge,some problems have been exposed continuously,such as cracks and deflection of girders.In order to solve the above diseases,it is often necessary to apply large tonnage of longitudinal prestress to the curve slab of variable cross-section box girder bridges.In this case,the bottom curve in high hole and stress state.In recent years,there have been lots of bottom slab cracking in construction or closure of bottom slab before grouting after pulling a period of time,and it has seriously restricted the popularization and application of this kind of bridge.In order to solve such problems,researchers have put in a great deal of resources to study the causes and countermeasures of the cracking of the slab,and obtain corresponding research results.The relevant research results mostly stay in the qualitative analysis of a single engineering accident level.However,the mechanism and countermeasures of crack are not systematic and quantitativestudy.Therefore,the relevant research results are not practical application value.Based on the refined finite element analysis,the failure mechanism is studied,box girder bottom slab design parameters and structure parameters are researched quantitatively.On the basis of related research results,it put forward corresponding to the bottom slab anti-breakage measures.The main work of this paper is as follows:(1)Based on collected and analysis of box girder bottom damage accidents and research data at home and abroad,the causes of failure for the bottom slab are summarized and analyzed.Shorages of dgesign method for box girder bridge are also summarized(2)Fine numerical analysis for typical failure of a large span PC box girder bridge slab is carried out,and it could illuminate the mechanism of box bottom slab cracks and different ordinary steel bars.(3)The direct cause of the bottom slab failure for a large span PC box girder bridge is equivalent radial force induced by closure prestressing strands.Therefore,the expression of the equivalent radial force induced by the tensile stress is deduced.According to the positioning and line fitting error,it put forward correction coefficient of equivalent radial force,the former factor is dominant.(4)To obtain the sensitivity parameters of crack,the parameters for box girder bottom slab are performed from the view of the design and construction,such as the bottom slab curve index,width and thickness,ducts spacing and protective layer thickness.The research results can be regard as a basis for optimuming requirements of box girder of design parameters and construction error control.(5)According to the results of parameters for box girder bottom slab anti-crack,and combine with design of code for "design of highway reinforced concrete and prestressed concrete bridges and culverts JTG D62-2004" and “ Technical specification foe construction of highway bridge and culverts JTG T F50-2011 ”,and some reasonable suggestions and measures of design and calculation preventing the bottom slab from breaking are proposed.The innovation of this dissertation are as follows:(1)The main span bottom slab failure of long span PC box girder is caused by equivalent radial force,and it is mainly composed of two parts: the equivalent radial force produced by the bottom closure strand and the additional radial force produced by the pipeline positioning error.Based on elastic mechanics method,the equivalent radial force expression is deduced,and verification by the test value and the finite element numerical.It could be prove the validity and the feasibility of equivalent radial force expression.(2)The classical equivalent radial force formula is constant,while the actual radial force is not certain values.There are some errors between the classical equivalent radial force and actual radial force,it is mainly caused by the location error of the bottom closure strand and of the bottom closure bundle.The error calculation charts are established and error correction coefficients expressions are derived.It is pointed out that the the location error correction coefficient plays a major role in the error correction.(3)The design of long span PC box girder bridge should be considered from the following aspects: As far as possible,it could choose larger the linear curve index;it is propose that pipeline positioning error is not greater than 5mm;the positioning bar spacing is 50 cm or 100 cm,the closure of main span section of 2~3 segments and the others;while location bar spacing is 25 cm,the location of near the 1m range for two segments intersect.