An Experimental Study Of Cast-In-Situ RC Two-Way Hollow Slab On Beams

Cast-in-situ RC two-way hollow slab is a new kind of floor structure with circular holes made by tubes running in one direction left in slab after casting. Benefited by the technology of high strength pipes, it obtains the advantages of large span, lightness, convenience in construction, and various adaptation, can be applied in various kinds of civil engineering structures and has a promising future in application.In this paper, the research is mainly focused on the mechanical characteristics of beam-supported RC hollow slab subjected to vertical uniform loads, including the development of the cracks, the deflection behaviors, and the load bearing capacity. In the aspect of experiment, test of a simple-spanned(3m×3m) beam-supported RC hollow slab model is carried out. The appearance and the development of the cracks are observed and the stiffness, deformation,load bearing capacity is studied. It can be concluded that the hollow slab is quite similar with equally-thick solid slab in mechanical characteristics and the difference of moment distribution in two directions is small.In the aspect of theoretical analysis, beam-supported RC two-way hollow slab subjected to uniform load is discussed in its three main load-bearing stages: the precracking stage(elastic stage), the postcracking service load stage and the failure stage. The analysis includes elastic mechanical characteristics, short-term deflections, and the ultimate load bearing capacity accounting for membrane forces.To give a scientific accordance to the calculation of stiffness, deflection and ultimate load bearing capacity, finite element analysis method is used and the study is especially focused on the compare of mechanical characteristics in two different directions. By using the equivalent stiffness method the structure is converted to a kind of orthotropic shell element .It can be concluded that the behavior of the two-way voided concretes lab is determined by the ratio of the stiffness of the two directions. As a whole, the mechanical characteristics are quite similar with that of the solid slab, the conclusion of which is also proved by the experiment.As for the short-term deflection calculation of RC two-way hollow slab, an approximate method using formula in 'Code for Design of Concrete Structures' is presented, and the results give satisfactory agreement with the test records. It can be concluded that the method presented is reasonable and applicable in calculating the short-term deflection of the RC two-way hollow slab. At the same time, ACI code method is used to evaluate the short-term deflection behaviors of the slab, and the results are also compared with the test. The results indicate that the deviation of ACI code method is also acceptable.To give a more accurate estimation of the ultimate load bearing capacity of the slab, yield-line theory accounting for membrane forces is used. The theory presented by Nielsen gives a relatively simple method to solve the problem of membrane force, and the results verify that the yield-line theory accounting for membrane forces is the real upper-bound solution, and is much more accurate in slabs with strong horizontal restraints. It can be concluded that load bearing capacity of the hollow slab is quite similar with that of the equally thick solid slab and the safety level is satisfactory.