Research On The Flexural Performance Of Concrete Filled Steel Tube String Structure Stiffened RC Beam

Construction of adding stories around existing building has become one of the hot issues in the field of civil engineering.To avoid the structure loads are directly applied on the roof of existing building,and ensuring that the existing building can be used normally in the process of construction,a new kind of self-supporting composite beam—concrete filled steel tube(CFST)string structure stiffened RC Beam is proposed.In the construction procedure,the internal CFST string structure is set up firstly.The steel reinforcement and the concrete template are placed and supported by the CFST string structure.Finally,the external concrete is poured.To understand the flexural performance of CFST string structure stiffened RC Beam,5 specimens are made and tested.The main works and research conclusions are follows:(1)Based on the idea of self-supporting,5 CFST string structure stiffened RC beam are designed and manufactured.It shows that the proposed CFST string structure stiffened RC beam can achieve the objective of self-supporting in the process of construction.(2)The flexural capacity tests of 5 specimens have been conducted.Testing parameters includes the reinforcement ratio,the ratio of CFST's area to the beam cross-section area and the effective tensile stress of unbounded prestressing tendons.In the tests,the parameter such as the deflection,strain and the stress of unbonded prestressed tendons are measured.The cracks development and failure models of specimens have been observed.Base on test data,the moment-deflection relationship,the stress of unbonded prestressed tendons and the strain distribution of concret are analyzed.Tests show that:?Both the M-??p curves and M-? curves have three segment:uncracked stage,cracked working stage and yield stage,stress increment(??p)of unbonded prestressed tendons and the deflection(?)of mid-span are in linear relationship;?After the external concrete crushed,the beams can keep stable in the test,and yield platforms are longer than ordinary reinforced concrete,which indicates that the beams have a good ductility;?Plane assumption can be used in the calculation of flexural bearing capacity.(3)By means of linear regression of the test data,the calculation formula for the ultimate stress of unbonded prestressed tendons was derived,which is expressed by the synthetical reinforcement ratio.The calculation results agree well with the test results.(4)By the whole process analysis of the cross-section stress,the flexural capacity of CFST string structure stiffened RC beam was derived based on the plane assumptions,and the simplified calculation formulas were also proposed.The results of simplified calculation formulas agree well with the test results.By equivalent stiffness area conversion,the calculation formulas of flexural rigidities were proposed base on the regression analysis of test data.The calculation results agree well with the test results.