Study On Shear Lag Effect Of Carbon Fiber Reinforced Concrete Box Beams

With high physical properties,CFRP as a kind of high performance materials have become the main material of strengthening concrete structure.Box girder is widely used in modern railway bridges,highway bridges and city overpasses because of its good whole space property and the good stability function.Affected by shear lag effect,the analysis and calculation of box girder is different from normal girder.Under the circumstance of reinforcing old bridge,it is necessary to calculate to know the area and the layer of carbon cloth after knowing the ultimate bending moment of girder.So,the dosage of carbon cloth which used to reinforce box girder must be discussed.According to the principle of minimum total potential energy,based on transforma tion-section method,if the compression property and the shear performance of box gi rder are not change,the expression of stress for the transformed section can be got.The article discusses the influence of the pre-stressed steel on shearing force lag of ca rbon fiber reinforced concrete box girder in the process of composite reinforcement of old bridge from the standpoint of mechanics.It turns out that the carbon fiber cloth plays an positive role on shearing force lag in the process of composite reinforcement of box girder.Through the analysis of stress conditions that the carbon fiber cloth reinforce concrete box girder under concentrated load,building a finite element model by Ansys.By comparison between unreinforced box girder and reinforced box girder and by comparison between theoretical calculations and finite element analysis under different working conditions,it has been found that the displacement occured in the midspan or the 1/4 floor and web is a little bigger than the displacement occured in roof and floor after reinforcing by carbon fiber cloth;the displacement occured in roof and floor of box girder is obviously less than unreinforced box girder.Before and after reinforcement,the stress of floor is minimum along the horizontal center and gradually enlarge far from center;the stress of roof near web local enlarges suddenly and stress concentration phenomenon occurs and gradually decrease far from web;it is obvious that the stress of roof decreased and the stress of midspan decreased about 25% than unreinforced.After reinforcement,the stress of roof also decreased and the theoretical derivation basically coincide with the result of model test.After reinforcement by CFRP,the stress of the concrete in the midspan of reinforced concrete is reduced about 21% than unreinforced,the stress of the concrete slab in the midspan is reduced about 14% than unreinforced.Considering the bond slip constitutive relation between carbon fiber and concrete,a finite element model is established by using spring element.We extracted the box girder with 1/4 span the top and bottom of the values of deflection and stress,and compared with the consolidation model between concrete and CFRP,found in considering the slip,whether 1/4 or cross cross,the top and bottom of the deflection and stress were increased to different extent.According to the balance of force and the principle of calculation of lood-bearing capacity,the article discussed the calculation method of CFRP amount when the section compression zone in box girder plate under the bending load of carbon fiber cloth reinforced concrete box girder.By comparison the amount of carbon fiber cloth between example and formulas given in the specification,it turns out that when reinforcing the box girder in flange on compression zone,it is practical to solve the mount of the carbon fiber cloth by using formula derived from this article.