Influence Of Fire Of Bridge Deck On The Mechanical Properties Of Long-span Prestressed Concrete Box Girder Bridge

A great number of rivers is distributed widely in China,which determines the construction of transportation facilities,especially the construction of long-span bridges is in the ascendant.Vehicles passing on bridges,especially large trucks,oil tankers and other potential sources of fire,are prone to fire accidents,which bring great threat to the safety of bridge structure.However,the research on fire resistance of bridge structure is mostly focused on the component level,and there is little research on the fire resistance of long-span bridge structure,so it is necessary to carry out research on this kind of structures.In this paper,the long-span prestressed concrete box girder bridge is taken as the research object.By using numerical analysis and statistical method,a set of calculation theory for simulating the fire loss effect of PC box girder bridge with long span is summarized and put forward.The structural mechanical properties of this kind of long span bridges under different fire scenarios are studied.The main work and achievements of this paper are as follows:(1)The thermodynamics characteristics and structural damage analysis theory of long span PC box girder bridge materials are summarized in detail.According to the relative temperature difference deformation between prestressed steel bar and concrete inside the beam,the formula for calculating the loss of prestressing force in temperature difference is put forward.A set of calculation and analysis theory for simulating the fire loss effect of PC box girder bridge with long span is put forward systematically.(2)The damage depth of concrete,the strength of prestressed steel bar and the prestressing loss of temperature difference are calculated,and the influence of different fire sites on the mechanical properties of PC box girder bridge with long span is studied.The results show that the longer the length of fire is,the more obvious the structural mechanical response is under fire load,and the more disadvantageous to the structure.The effect of middle span fire on the overall deflection of the bridge is the most prominent.In actual engineering,the overall safety of the structure should be paid more attention when the middle span is under fire.The flexural bearing capacity of PC box girder bridge with long span is weakened most obviously in the direct fire area,and the flexural bearing capacity of other sections which are not directly affected by fire also has a certain degree of decline,but the influence is not significant.(3)The influence of different spans of long span PC box girder bridge deck on the structural mechanical properties before and after fire is studied.From the engineering example in this paper,we can know that in the same fire scene,the high temperature strength damage of prestressed steel bar with small span is serious relatively,and the loss of prestressed steel beam is large relatively.Under the same fire damage effect,the mechanical response of the structure with larger span is not significant.(4)The flexural bearing capacity of normal section of PC box girder bridge with different span is calculated after fire,and the difference of flexural bearing capacity of PC girder bridge with different span is compared.The results show that when actual bridges are under fire,it is suggested that priority should be given to the evaluation of structures with small span and small thickness of steel bundles.The research results in this paper can give reference for the identification of bridge and the quick evaluation of bearing capacity of the PC box girder bridge with long span.