Experimental And Calculation Method Study On Strengthening Of Steel-concrete Composite Beam Bridge With Prestressed CFRP Plate

At present,more and more steel-concrete composite beams are used in the superstructure of medium-span highway bridges and urban bridges.With the increase of bridge service life,traffic volume and vehicle design load,the reinforcement of composite structures will definitely become more important.Carbon fiber-reinforced polymer plate,a kind of structural reinforcement material with excellent comprehensive performance,provides a new idea for the reinforcement of steel-concrete composite beams.Combined with the revision and compilation project of “Specifications for Strengthening Design of Highway Bridges”?JTG/T J22?granted by the Ministry of Transport and “Postgraduate Research & Practice Innovation Program of Jiangsu Province”?KYLX16₀261?,this paper carries out the experimental and theoretical research on the strengthening of steel-concrete composite beams with prestressed CFRP plates,aiming at the problem of the insufficiency of flexural bearing capacity of the beam and the insufficiency of crack resistance of the concrete slab in the negative moment region.The main work and conclusions are as follows:1.An innovative type of prefabricated anchorage system of prestressed CFRP plate with engineering application value is designed.The key mechanical problems of each component of the anchorage system are analyzed,including the force of the anchors and clips during the prestressing stage,the shear capacity of the friction-type high-strength bolts,and the tensile strength and pull-out resistance of the threaded rod of limit block.Finite element modeling and non-linear analysis of the prestressed CFRP plate anchorage system are completed based on ABAQUS.Material selection and trial production of the prestressing and anchoring device are carried out on the basis of the numerical analysis results.Furthermore,the supporting construction technology of the prefabricated anchorage system is put forward.2.For I-shaped simply supported composite beams,failure tests of four strengthened specimens and one control specimen were completed under static load.The location of prestressing and anchoring is the positive moment region of composite beam,on the bottom flange of the steel beam.Experimental results showed that: the strain loss of the CFRP plates tended to decrease when the prestressing level was increased;the strengthening with prestressed CFRP plate effectively improved the flexural capacity of steel-concrete composite beams;three failure modes of prestressed CFRP plate were obtained in the ultimate stage,including transverse rupture,disintegrating rupture and mid-span peeling;the strain distribution along the strengthened mid-span cross-section conformed to plane-section assumption;the prestressed CFRP plate is not suitable for the functional reinforcement which is aimed to control the structural deformation;excessive prestressing force is not recommended for the post-tensioned fiber composites in order to ensure the ductility of the strengthened structures.3.For box continuous composite beams,failure tests of two strengthened specimens and one control specimen were completed under static load.The location of prestressing and anchoring is the negative moment region of composite beam,on the top surface of concrete slab at the central support.Experimental results showed that: the strengthening with prestressed CFRP plate effectively improved the flexural capacity of the central support section and mid-span section of continuous composite beams;the strengthening with prestressed CFRP plate substantially enhanced the crack resistance of the concrete slab in the negative moment region;the moment redistribution of continuous composite beam decreased after strengthening;the specimens exhibited the typical bending failure,and the prestressed CFRP plate in the negative moment region did not rupture at the ultimate state;the flexural rigidity and ultimate deformation increased when the continuous composite beams strengthened with prestressed CFRP plates.4.The elasto-plastic constitutive model of metal and damage-plastic constitutive model of concrete in ABAQUS are studied.The finite element models of the positive moment region of simply supported composite beams and the negative moment region of continuous composite beams strengthened with prestressed CFRP plates are established.The reliability of the simulation models is verified by comparing and analyzing the test data and numerical simulation results.Further verification of the strengthening effect is made for the composite bridges strengthened with prestressed CFRP plate.Parameter analysis is conducted for the flexural capacity,deflection development and ultimate deformation of continuous composite beams.The recommended value of the optimum prestressing level of the CFRP plate is given.5.Based on plastic theory,elasto-plastic theory and elastic theory,the calculation method of flexural capacity for steel-concrete composite beams strengthened with prestressed CFRP plates in the positive moment region is presented.Based on plastic theory,the calculation method of flexural capacity for steel-concrete composite beams strengthened with prestressed CFRP plates in the negative moment region is presented.The analytical formulas of elastic deflection and ultimate deflection of the simply supported composite beams are derived.The reduced rigidity method of composite beam deflection is studied considering the interface slip.The calculation formulas for the elastic deflection of continuous composite beams and the elastic deflection considering the change of sectional rigidity are derived.Based on the mechanical characteristics of steel-concrete composite continuous beams,the calculation formulas of the crack width of concrete slabs in the negative moment region is proposed.Furthermore,the checking formula for crack resistance of the strengthened central support section is deduced.6.The calculation methods of prestressing losses of steel-concrete composite beams strengthened with prestressed CFRP plates are proposed,including the losses due to the anchorage deformation and inner shortening,the seasonal temperature variation,the different batches of stretching and the long-term relaxation.The calculation formulas of the tensile stress in CFRP plates of composite beams during service stage is derived.The determination methods of the design value of tensile strength,the adjustment coefficient of centroid,the area reduction factor and the controlled stress for stretching of CFRP plates are presented,respectively.The reliability of the above calculation methods is verified by the engineering examples of a RC simply supported T-beam bridge and a three-span steel-concrete composite continuous beam bridge strengthened with prestressed CFRP plates.