Experimental Study Of The Progressive Collapse Performance Of Unbonded Prestressed Concrete Frame

Recent years, unbonded prestressed concrete (UPC) structure had been widely applied in more and more engineering field due to its excellent performance and economical cost. Since there is no directly contact with concrete surrounding, strand keep an unbonded state and deform coordinately in horizontal and vertical direction when structure is under external loads. After the initial local failure, the load-carrying mechanism and failure mode of UPC structure would be different from reinforced concrete (RC) structure. However, little work had been conducted to investigate the behavior of UPC structure to mitigate progressive collapse. Thus, it is necessary to carried out a series of research on this purpose.Selection of reinforcement ratio, prestress and span-depth ratio as basic parameters, research group designed and fabricated six UPC beam-column sub-structures and two RC beam-column sub-structures. The main objective of this study is to understand the progressive collapse performance and load-carrying mechanism of sub-structures though an experimental study based on the nonlinear static push-down test method. Meanwhile, the thesis also revealed the effects of basic parameters on the structural behavior of sub-structures, which are subjected to prestressing load and uniformly distributed load. Main conclusions could be drawn as follows:1. When structure subjected to uniform distributed load (UDL), in normal RC frame, the failure of BENE will be aggravated while the damage of beam ends near to middle joint (BENMs) will be mild. Thus, the failure will first occur at the beam ends near to end stub (BENEs) due to rebar fracture, which will dramatically increase the middle joint displacement. Moreover, RC frames may develop catenary action with curved type at large deformation stage.2. Unbonded prestressing will alleviate the damage at BENEs and aggravate the damage at BENMs, so that the reinforcement fracture will first occur at joint interface. The middle joint displacement of UPC sub-structures will not increase dramatically after rebar fracture. Furthermore, UPC frames may develop catenary action with straight type at large deformation stage. During progressive collapse, the strand will be continuously stretched, thus the unbonded prestressing and its vertical component keep increased with external loading, resulting in improvements of the load-carrying capacity and the deformation capacity of UPC frame structures.3. The increase of cross-sectional area of reinforcement will significantly enhance the load-carrying capacity and the deformation capacity of UPC structures, reducing the likelihood of structures in collapse. The arrangement of prestressing strand may have negative effects on the damage at BENMs of UPC sub-structures subjected to a loss of center column scenario. Thus, the increased stretching control stress may locally aggravate the damage of BENMs, but it has slightly positive effects on the overall load-carrying capacity of UPC sub-structures. Span-depth ratio will affect the stiffness of sub-structures, higher span-depth ratio will have negative effects on the overall progressive collapse performance of sub-structures.4. Since there is prestressing strand help to develop catenary action in UPC sub-structures, the load-carrying capacity at catenary action stage is obvious higher than that at plastic stage, resulting in larger value of DIF of UPC sub-structures, which may even larger than the DIF value of 2.0 recommended by GSA. Moreover, the unbonded prestressing effects on dynamic progressive collapse resistance of frame structures are not as obvious as that on static resistance.