Experimental Research On Progressive Collapse Behavior Of Reinforced Concrete Frame With BFRP Bars

In recent decades,with the progressive development of Engineering technology,the form of building structure is becoming more and more complex,and people are constantly improving the requirements of structural safety.The study of structural anti-progressive collapse performance is particularly important.The definition of progressive collapse by foreign scholars is: after accidental factors,the local damage of the structure causes the progressive damage of adjacent members,and the progressive development of the damage chain will eventually lead to the collapse of the structure or cause the damage which is not proportionate to the initial damage.At present,the research on progressive collapse of reinforced concrete frame structure is becoming more and more mature,and there are many achievements.It can be seen that the main research object in the field of structural anti-progressive collapse is still reinforced concrete structure,and the research on the anti-progressive collapse of BFRP reinforced concrete frame structure is relatively few.Because BFRP bars have the characteristics of high tensile strength,low density,linear elastic material and corrosion resistance,if the reinforcement of frame beams is replaced by BFRP bars,the concrete frame structure with BFRP bars will be formed.Due to the low stiffness of BFRP concrete members,the risk of progressive collapse of the remaining structure will be greatly increased after the collapse of load-bearing columns due to unexpected loads.Therefore,it is necessary to study the progressive collapse behavior of BFRP reinforced concrete frame structures.In this paper,through collapse test and collapse finite element simulation,the research on the progressive collapse resistance of BFRP reinforced concrete frame structure after failure of mid-column is carried out.The main research contents include:(1)Six 1/4 scale concrete frame beam-column structures with BFRP bars(single-storey and double-span structures)were designed,including one specimen with bonded prestressing tendons-BFRP tendons,one specimen with unbonded prestressing tendons-BFRP tendons and four specimens without prestressing BFRP tendons.The dynamic collapse behavior of the structure is studied by quickly removing the middle column.The results show that after the failure of the middle column,many cracks are formed at the end of the frame beam,resulting in obvious residual deformation.The maximum dynamic strain response of the column in the structure is about 1.32 times of that after vibration stabilization;and the damping ratio of the remaining structure is about 0.028.(2)After the dynamic collapse test,the vertical monotonic static load is applied on the top of the failed column.The collapse performance of BFRP reinforced concrete frame structure with span-height ratio,column axial compression ratio,prestress degree and beam reinforcement form is studied.The results show that the tensile failure of the beam-column junction at the side column position is the most serious,which is the weak section of the structure;the catenary working mechanism is the collapse of the structure.The main resistance mechanism in the failure stage;the load of the structure with BFRP fiber stirrups is 1.16 times of that of the ordinary stirrups;the replacement of steel stirrups in frame beams by BFRP stirrups has slightly improved the collapse resistance of the structure,but has little effect;the failure load of unbonded and bonded pre-stressed structures is 1.25 and 1.57 times of that of unbonded and bonded structures respectively,and the breakage of bonded pre-stressed structures.The bad load is 1.25 times that of unbonded prestressed structure.It can be seen that the collapse resistance of concrete frame structure with bonded prestressing BFRP tendons is stronger than that of unbonded prestressing concrete frame structure with unbonded prestressing and unbonded prestressing BFRP tendons.(3)By means of numerical modeling,12 non-linear finite element models are established,and the effects of column section size,prestressing position,effective prestressing force,reinforcement ratio at the bottom of beam section,concrete strength and reinforcement ratio of beam longitudinal reinforcement on the progressive collapse resistance of concrete frame structures with prestressing BFRP bars are analyzed.The results show that the failure load increases by 9.0% and 13.0% when the column section increases by 0.25 and 0.5 times,respectively;the failure load decreases by 46.0% when the prestressing tendons are laid on the upper and lower parts of the frame beams;the collapse resistance of the structure can be increased by 4.3% if the effective stress is increased by 0.1 times,otherwise the collapse resistance of the structure will be reduced by 2.5%;the reinforcement ratio at the bottom of the beam is 1.5% and 2.0%.The ratio of failure load to reinforcement ratio increased by 14.1% and 26.1% respectively,and the failure displacement increased by 3.1% and 5.6%.Replacing the upper BFRP bars with the same diameter bars can increase the collapse resistance of the structure by 53.0%,and mixing a common bar with the same diameter at the lower part of the frame beam can only increase the collapse resistance of the structure by 3.5%.The research work in this paper can provide reference for collapse resistance design of BFRP reinforced concrete frame structure.