| Fiber Reinforced Polymer(FRP)is a high-performance material with the advantages of high strength,low weight and good resistance to corrosion.New hybrid composite structures,incorporating conventional materials(e.g.,concrete and steel)and FRP,have been proposed to use the advantages of both the conventional materials and FRP.In recent years,the research on the new hybrid composite structure has become a hot topic worldwide in the research community of structural engineering.To date,the following five types of new hybrid composite columns have been researched:(1)FRP Confined Concrete-Filled Steel Tube(FCCFST),(2)FRP-Confined Steel-Reinforced Concrete Column(FCSRC),(3)FRP-Steel-Concrete Double-Tube Concrete Column(DTCC),(4)FRP-Steel-Concrete Double-Skin Tubular Column(DSTC),(5)Concrete-Filled Steel Tube with FRP-Confined Concrete Core(SCFC).A review of literatures reveals that most of the existing research has been on the hybrid new composite columns incorporating normal-strength concrete,whereas the studies on those incorporating high-strength concrete are limited,thus further research is needed.Against the above background,this thesis reports an experimental study on the behavior of hybrid FRP-high-strength-steel hybrid columns subjected to concentric compression,focusing on the research of FCSRC and DTCC.The following three parameters have been investigated in this research program: concrete strength(normal-strength and high-strength concretes),section configurations of the steel sections(steel tube,cross-section,I-section),and number of layers of FRP.A total of 43 columns were tested.In addition to FCSRC,DTCC,plain concrete columns(CC),contain concrete-filled FRP tubular(CFFT)columns,concrete-filled steel tubular(CFST)columns and DSTCs were also tested for comparison purpose.The objectives of this study are twofold:(1)to study the behavior of the hybrid column subject to concentric compression,and(2)to investigate the interaction between FRP,concrete and steel in the hybrid column.The experimental results showed that the concrete strength had an obvious effect on the compressive behavior of the hybrid columns.For columns using normal-strength concrete,the load-strain curves of the CFFT columns,FCSRCs,DTCCs,and DSTCs are usually bilinear,with an initial first elastic stage and hardening second-stage.For columns using high-strength concrete,the load-strain curves are usually tri-linear,with an initial elastic segment,followed by small descending segment,and a final ascending segment;in the case of DSTCs,there may exist two descending segments.The test results also showed that the load-carrying capacities of the above columns were usually increased with increasing number of FRP layers and concrete strength.When the concrete strength and FRP layers are the same,the load-carrying capacities of DTCCs are generally larger than the corresponding FCSRCs;among the FCSRCs,the columns with I-section steel usually have larger load-carrying capacities than those with cross-section(X-section)steel,given that the steel ration(in terms of area)are similar for the two steel section.The experimental results also showed that for FCSRCs and DTCCs together,the effect of steel sections on the strain distribution in GFRP tube is insignificant.The confining effects of GFRP tubes and concrete can effectively prevent or delay the buckling/local buckling of steel sections.Such a favorable effect is most significant for steel tube,followed by I-section steel and X-section steel in sequence.As a result,full composite action of GFRP tube,concrete,and steel can generally be achieved for those hybrid columns using normal-strength concrete(i.e.,their load-carrying capacities can be superimposed).However,for those hybrid columns using high-strength concrete,such full composite action nearly exists only when the GFRP tube provides enough confining to the high-strength concrete.The outcomes of this research program are of great research significance and important application merits. |
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