Research On Overall Stability Behavior Of Concrete-filled Pentagonal Flange Beam

Concrete-filled steel tubes (CFTs) have the beneficial qualities of both concrete andsteel materials and are widely used as columns and beams. The core concrete can preventthe local buckling of the steel tube and increase the stability and strength of the memberas a system. On the other hand, the steel tube provides confining pressure to the concreteand makes the concrete under a triaxial state of stress. If the top flange of a conventionalsteel I-section beam is replaced by the CFT, a new type of steel-concrete composite beamcan be formed, which is called the concrete-filled steel flange beam (CFSFB). Comparedwith the flat plate flange steel-concrete composite beams, CFSFBs have advantages ofproviding higher bending capacity, higher stiffness and better stability. However, limitedexperimental and finite element (FE) research has been carried out. Based on the existingtest results, this dissertation focuses on the flexural behavior of concrete-filled steel flangebeams (CFPFBs) under concentrated loading. The main contents are as follows:Firstly, in the numerical study based on FE analysis, the concrete and steel materialsare modeled using the eight-node solid and four-node shell element respectively. Acomparison of the failure mode, ultimate capacity and load-displacement curves of theCFPFBs with and without stiffeners reveals that the FE models simulate very well theflexural behavior of the test specimens.Secondly, a comprehensive parametric study of CFPFBs under concentrated loadingis carried out with the verified FE models. Compared with the hollow pentagonal flange(compressive flange), the concrete-filled flange can enhance the flange stability, and thusincrease the torsional and flexural stiffness. The effects of five (5) non-dimensionalparameters, including the steel ratio of top flange (s), the depth-thickness ratio of web (f),the width-thickness of bottom flange (d),the span-depth ratio (k) and the pentagonal angle(θ), on the behavior of overall stability have been studied. Suitable suggestions are raisedto increase the ultimate capacity of CFPFBs. The elastic critical moment formula derivedfrom the energy approach can provide an approximate prediction for the CFPFB modelwith the failure mode of overall instability. Three (3) limit states can be used to define asthe lateral torsional buckling strength of CFPFBs by investigating the characteristic of theload-longitudinal strain curves.3limit states are the limit of instability, onset of instability and first yielding respectively.Lastly, based on the parametric results of four (4) non-dimensional parameters (s, f, d,k) and the results of192models, multi-variance nonlinear regression analysis isconducted to obtain the parametric equation of the ultimate capacity of CFPFBs underconcentrated loading. It is worth noting that the failure mode of all these models is overallinstability. The accuracy and reliability of the proposed parametric equation is assessedby the FE analysis results and the experimental results.