Study On The Seismic Performance Of GFRP And Steel Double-reinforced Columns Considering The Degradation Of Tensile Strength Of GFRP Bars

To solve the degradation of strength and seismic performances induced by steel corrosion in the offshore pier columns,the design and preparation of pier columns involved replacing or partially replacing steel rebars with fiber-reinforced polymer(FRP)bars have been concerned.However,the above two forms of configurations face the drawback of insufficient energy consumption capacity or the problem of steel corrosion under harsh environments,respectively.The use of FRP and steel double-reinforced configuration is one of the potential solutions to the above problems.For a double-reinforced configuration,the inner-layer steel rebars satisfy the require of the thick of concrete cover,and exert the energy-consuming advantages;the outerlayer FRP rebars achieve its high strength and durability properties.Glass fiber-reinforced polymer(GFRP)bars have become one of the most widely used FRP bars due to the economic advantage.However,the research on the durability of GFRP bars and the value of its reduction factor of durability still needs to be further studied.To improve the design theory of FRP and steel double-reinforced configuration and promote its application in civil engineering,it is necessary to evaluate the effect of tensile strength,elastic modulus,and ratio of two types rebars(FRP/steel rebars)on the performance of pier columns,and then give a configuration design method that takes into account the durability and seismic performance.In this paper,a combination study including theoretical analysis,experimental research and numerical simulation was adopted.The main research contents and research results of this paper are as follows:(1)For three different service environments,namely fresh water,seawater,humidity environment,the degradation,prediction formula,relationship of the tensile strength of GFRP bars were studied through laboratory acceleration tests,and the concept of environment transfer was proposed.The results show that the influence of fresh water and humidity environment on the tensile strength of GFRP bars was obvious;the influence of seawater and humidity environment on the tensile strength of GFRP bars was related to the thickness of concrete cover;The concrete with high water to cement ratio had larger influence on the tensile strength of GFRP bars;The prediction formulas related to the factor of water to cement ratio and depth of concrete cover,seawater,and humidity were given based on the test results,the calculation of environment transfer was provided as well.(2)Based on the results of axial-load test of double-reinforced columns,the restraint efficiency of FRP and steel double stirrups was established,the stress-strain relationship of the confined concrete and the calculation formula of peak stress,peak strain,and failure strain were given.Then the design method of double-reinforced configurations was established based on the principle of strain compatibility and equilibrium.(3)A finite analysis model was established for a continuous beam bridge,and the effect of tensile strength and elastic modulus of FRP rebars,and the ratio of the replacement of FRP rebars on the stiffness,post-stiffness,maximum and residual displacement of double-reinforced columns was studied compared to steel RC columns.The results show that the steel rebars reinforced in double-reinforced columns can meet the durability design requirements for the extreme environments;The seismic performance of FRP and steel double-reinforced columns was better than that of RC columns,especially in terms of ductility,post-stiffness,and residual displacement;When the tensile strength of FRP rebars dropped to 55% of their ultimate tensile strength,the ductility decreased obviously;After the tensile strength of FRP rebars reached to40% of their ultimate tensile strength,the change of maximum displacement was clearly;The decrease of ultimate tensile strength of FRP rebars increased the residual displacement of double-reinforced columns,which was smaller than that of RC columns.It is recommended to use FRP bars with low elastic modulus and high tensile strength,such as GFRP bars,to avoid failure of structures resulting from the rupture of FRP bars.(4)The fragility analysis of pier columns under seismic excitation was carried out.The durability design index and its recommended values of FRP rebars were given by the relationship between the degradation of tensile strength of FRP rebars and the result of fragility analysis.The results show that the vulnerability of double-reinforced columns can be reduced by up to 30.2% compared to steel RC column for the state of collapse damage;The ratio of the replacement of FRP rebars had a great influence on the fragility results,when the ratio of the replacement of FRP rebars was 66%,a maximum decrease of 30.2% in the fragility for the double-reinforced columns was found compared to RC columns;The ultimate strain of FRP rebars was recommended as their durability design index,the values of this index were between0.013 and 0.016 related to the ratio of the replacement of FRP rebars;Finally,a theory considering the durability and seismic performance was proposed for the double-reinforced configuration design.