Research On Fundamental Promblems Of Prestressed Concrete Composite Beam Encased Grout-Filled Square Hollow Section Trusses

With the transition of construction industry in our country from a period when new buildings are mainly constructed to another when constructing new buildings and reconstructing old buildings are paid equal attention to, the reconstruction technique using outer-jacketing structure to add stories around the existing building has shown brilliant prospect. In order to ensure that the original building works as usual and avoid construction load being transferred to the roof of the original building during construction, a design thought"self-supporting in-situ concrete floor structure during construction"is proposed. One of the essences of the thought is that: the prestressed concrete composite beams encased trusses made of steel angle are used as the frame beams in the new-added first floor of the outer-jacketing reconstruction project. The bottom form is suspended on the basis of the steel trusses, then side form is installed on bottom form. In this way, the weight of frame beams and the construction load could be supported by the steel trusses during casting concrete. The encased composite beam and the outer-jacketing column are connected to form an outer-jacketing frame which can bear the newly added loads during service period.The length of side of the angle steel in prestressed composite beam encased angle steel trusses is rather large, as bring disadvantages for layout of longitudinal reinforcements in column when composite beam connected with column. In order to avoid these problems, a sheme is proposed that trusses made of grouted square hollow section are used to replace trusses made of angle steel. This kind of truss has prominent superiorities: Side length of square steel tube is 50% smaller than that of angle steel as can facilitate layout of reinforcement bars in columns; The bottom chord of truss can worked as duct of prestressed tendons and can facilitate tension and anchorage of these prestressed tendons. The prestressed concrete composite beam encased trusses made of grouted square hollow section comes into being once trusses made of grouted square hollow section are used to replace trusses made of angle steel.The design of joints is committed step during the process of designing a truss, and the principle"strengthening joints and weakening members"should be satisfied. The research on SHS(square hollow section) truss joints lays emphasis on hollow section joints at present time. In order to increase capacity of joints made of square hollow section, the joint chord is grouted. Then shear failure in normal section and cave-in failure of chord member could be avoided. Though punching shear capacity of chord member doesn't increase obviously, the limit state determined by deformation could be put off. In order to make clear mechanical properties and failure modes of grout-filled SHS truss joints, experimental study of 8 N-shape and 15 T-shape joints are carried out, the influence of parameters such as ratio of side length of T or N joint members and magnitude & direction of axial force applied on chord of T-shape joints is analyzed.The non-linear finite element model is established to simulate grout-filled SHS truss joints. And shell element, three-dimension body element and contact element are used to simulate the SHS tube, hardened grout and the interface between tube and hardened grout respectively; the constitutive equations of steel tube and hardened grout were obtained by material test results, and the constitutive equations of contact interface are determined by Coulomb's friction law. Finite element analysis with extended parameters of test were carried out, the results obtained by calculation agree well with the test results. Based on the test results and calculation results, the formulas calculating ultimate bearing capacity of different kinds of joints are put forward: Ultimate bearing capacity of compressive T-shape and N-shape joints should be calculated in accordance with cave-in failure mode of chord member; Ultimate bearing capacity of N-shape Tension joints should be calculated in accordance with the following two kinds of failure modes: punching shear failure mode and effective width failure mode, and adopt the smaller value;Ultimate bearing capacity of chord member should be calculated in accordance with shear failure mode in normal section.As performance of the trusses are guaranteed in period of construction, in order to bring the composite beam into use in seismic zone, a low cycle test on 5 prestressed concrete composite beams encased the trusses was carried out through the quasi-static method. The parameters to be researched in the study include reinforcement indicesβ0,prestressing degreeλand ratio of vertical reinforcement in plastic zoneρsv. The failure mode, hysteretic behavior and energy dissipation capacity of specimens are examined through experiment. The test results indicate that influence ofβ0 on the hysteretic behavior of composite beams is obvious. Whenβ0<0.2,the displacement ductility of specimens reached 4 approximately; the hysteresis curves are plump and the specimens have a good energy dissipation capacity; whileλmainly exerts influence on area of hysteresis loops, and its influence on ductility is relatively smaller thanβ0 ; It can increase the ultimate compressive stain of concrete and enhance the ductility of the composite beam to increase the ratio of vertical reinforcement in plastic zone. Based on fiber-based model, a non-linear numerical simulation program is employed to simulate the hysteretic response of the five beam specimens. In general the calculation results from programm agree well with the experimental results. On the base of experimental study, a great deal of parameter analysis of such member was conducted, the effects ofβ0,λ,ρsv and load type on the behavior of the specimens are investigated in details with fiber-based model. On the base of experimental study and simulation analysis, the formula of secant stiffness at yield point is suggested, which can be used for checking deformation; Simplified restoring force model for the moment-curvature response and model for load-restoring deformation are suggested, which can be used in elastoplastic dynamic analysis of structures.