Research On Mechanical Performance And Design Methods Of Prestressed Composite Concrete Beam With Encased H-Steel

Prestressed concrete composite beam with encased H-steel (PCCBEH)refers to a kind of new beam, in which the presrtressing steel is rationally arranged and tensioned between the top flange and the bottom flange of H-steel, and the scheduled equivalent load is formed which can reduced the net load effect acted on the critical section. Compared with the common encased H-steel concrete composite beam, the dimension of the cross section of PCCBEH can be distinctly decreased with the same load and the same span; the span can be distinctly increased with the same load and the same cross section. This beam can be used in the new project, and it can also be used in the out-jacketing frame project for adding stories. So, it is of practical significance to do some research on this PCCBEH.For the common encased steel concrete composite beam, the research on the stiffness, the crack and the carrying capacity had been reported, and the construction standard can be used to guide the design and construction. But there is no report on the influence on of prestress and prestressing steel on the distribution of crack as well as their influence on stiffness. PCCBEH has its own characteristic on the mechanical performance. 5 PCCBEHs were designed and fabricated with the encased H-steel, non-prestressing steel and the prestressing degree as the variable and the steel ratio is 6.65%~7.24%. These 5 beams were tested with the two-point load test. The whole process from the beginning of loading to failure was investigated. Test results indicated: cracking of the beam affected little on the curve of load-deflection at the midspan. The yield point of the tensile flange of the H-steel and the tensile longitudinal non-prestressing steel bar become the turning point of the curve. Form the beginning of loading to reaching the strength ultimate limit state of the beam, the curve of load-deflection at the midspan was double broken line. The beam still had a certain stiffness and carrying capacity when it failed. The residual crack width and the residual deflection was relative small after unloading. PCCBEH had good ability on recovery of deflection.The sufficient redistribution of plastic internal force can occur in continuous steel beams, and the moment modification coefficient of the common concrete continuous beam has been listed in literature. There already have some reports on the calculation method of moment modification for prestressed concrete continuous beams. For PCCBEH, the idea for calculating the moment modification coefficient is put forward: the object can be modified is the deference between the elastic calculated moment caused by the load and the moment subjected by H-steel at the critical section at the intermediate support when reaching the ultimate limit state. The variable is the relative plastic rotation that is the ratio of rotation of the plastic hinge to the effective depth of the section. 3 continuous PCCBEHs were designed and fabricated with 3 key factors. They were: the prestressing degree of the critical section at the intermediate support, the relative depth of the critical section at the intermediate support and the carrying capacity of the critical section at the midspan. 3 beams were test with a single load on each midspan. The whole process from the beginning of loading to failure was investigated. The calculation formula for the length of the equivalent plastic hinge was got. It decreased with the increase of the prestressing degree. The ultimate load of 9 simulated continuous beams was obtained by using the moment-curvature nonlinear whole process analysis method, so the sample space is enriched. Based on the model test results and the simulated nonlinear analysis results, the calculation formula of the moment modification coefficient is established with the relative plastic rotation of the critical section at the intermediate support is the variable.Design proposals are presented for PCCBEH including selection of materials, selection of sections, calculation of internal force, selection and configuration of steel, prestressing steel, non-prestressing steel bar, calculation of the oblique section, control and check of crack, control and check of deflection, construction method and so on.