Analysis Of The Causes Of Cracks In The Web Of Fabricated Prestressed Concrete Continuous Small Box Girder

Prefabricated prestressed concrete continuous small box girder has been rapidly developed and widely used in modern traffic construction.However,there are many kinds of cracks in the existing prefabricated prestressed concrete continuous small box girder in China,especially the cracks in the box girder web,which affect the service performance and durability of the bridge It is very necessary to provide ideas for the prevention and repair of box girder web cracks.In this paper,the causes of Web cracks of prefabricated prestressed concrete continuous small box girder at home and abroad are analyzed and summarized,and the types,sensitivity factors and analysis methods of box girder cracks are preliminarily discussed.Then,the concrete distribution of Web cracks in the actual project is studied through the analysis and statistics of real bridge diseases After that,the forming mechanism of web crack of prefabricated prestressed concrete continuous small box girder is studied in detail by the finite element simulation numerical analysis method.The main work and conclusions are as follows:1)The crack types,distribution,strength,thickness and crack shape of the web of the small box girder in the actual bridge are analyzed.The statistical results show that there are more web cracks in the cracks of small box girder,including web vertical cracks,oblique cracks and longitudinal cracks,in which the proportion of Web vertical and oblique cracks is the largest;the measured strength of four of the seven measuring points of web is in poor condition;the relative design deviation of web thickness is more than ± 2cm,accounting for 75%.2)Midas civil is used to establish the overall finite element model of the supporting project,and analyze the influence of prestress,shrinkage and creep,bearing settlement,gradient temperature and other sensitive factors on the formation of box girder web cracks.The main conclusions are as follows: the tension of Web tendons has a great influence on the stress state of web,which directly affects the crack state of web in the construction stage;when the prestress loss is 30%,the box girder web is prone to vertical cracks;the concrete shrinkage and creep and gradient temperature will not directly lead to cracks in the box girder web,which are often accompanied by other factors;in the most unfavorable bearing settlement combination The shear stress produced by the web of the small box girder can directly lead to the cracking of the web of the small box girder at the joint end;the normal tensile stress of the normal section of the web plate at the support of the continuous beam will be significantly increased by the gradient temperature rise.3)Midas FEA is used to establish the local solid finite element model of the supporting project,and the influence of the change of web thickness,the directional deviation of prestressed pipe and the reduction of concrete strength on the web crack of small box girder is analyzed.The main conclusions are as follows: the reduction of web thickness of small box girder by 2cm and the length of variable section by 50 cm have the greatest influence on the first principal stress of web,and inclined cracks are easy to occur at the junction of continuous end web and roof and outer side web;the directional deviation of single prestressed pipe has little influence on the stress of steel beam and small box girder,but it is easy to change the stress component of beam end The decrease of concrete strength has little effect on the stress distribution of the box girder,but it will significantly reduce the tensile capacity of the box girder and make the girder more prone to cracking.