Research On Cyclic Behavior Of FRP-Strengthened Concrete-Filled Steel Tubular Columns After Fire

With the increased using of concrete-filled steel tubular column in civil engineering such as building structures and bridge structures, it is more and more possible for them to encounter all kinds of calamities, especially fire and earthquake. Therefore, how to retrofit concrete-filled steel tubes becomes more and more important in these days. Comparing with reinforce concrete structure and steel structure, concrete-filled steel tubes have better performance in fire and better feasibility of being retrofitted after fire. Fiber reinforced polymer (FRP) composites have many advantages such as low weight, convenience in uses,excellent mechanical strength, immunity to corrsion. In this paper, cyclic behavior of postfire concrete-filled steel tubular columns strengthened by FRP composites are researched. The main work is as follows: 1,The advantages of concrete-filled steel tubes and fiber reinforced polymers are reviewed. Then the objectives and content of this thesis are described. 2,Ten circular and ten square concrete-filled steel tublar columns are tested under constant axial load and cyclically increasing flexural load. Three normal, three post fire and four strengthened using FRP after fire are included in both circle and square series. The main parameters in the experiment are axial compressive ratio, FRP layer, sectional type, etc. The influences of these parameters on strength, ductility, and energy dissipation are investigated. 3,Based on the determinated stress-strain relationships of steel and concrete after high temperature under monotonic and cyclic conditions, as well as stress-strain relationship of FRP, the moment(M)-curvature(φ) curves and load(P)-deflection(?) curves are calculated in numerical method. The calculating results are in good agreement with the experimental ones. 4,Based on the calculating results, the influences of FRP layers, steel ratio, steel strength, concrete strength, axial compression ratio, the highest temperature exposed on the lateral load(P) versus lateral defection(?) relations and moment(M) versus curvature(φ) relations are discussed. 5,Simplified models for calculation of the moment (M) versus curvature(φ) relations and the lateral load (P) versus lateral displacement (?) relations are suggested. In general,the calculating results of the simplified model agree with those of numerical results.