Flexural and Shear Behavior of Donut Type Voided Slabs

Abstract

The increase in the interest of residential environments as reflected in the strengthened regulation to the noise between floors and the lengthened span of buildings have brought about the increase in slab thickness in an attempt to mitigate slab deflection, noise and vibration. However, the application of thicker slabs weighing more causes some problems. The thicker slabs cause the increase in the size of vertical members such as columns, walls and bases besides the increase in self-weight of slab. The increase in the size of vertical members results in the increase in the overall weight of a building and the amount of material. The increase in building weight may cause the increase of earthquake load for dissipation of energy and deteriorates the building’s economic efficiency. A flat slab system is particularly affected by these problems because flat slab is usually thicker than slab used in a frame construction; these problems influence thicker slabs more severely. Because of these disadvantages, slab systems consistently have developed to reduce the self-weight such as waffle slab system, prestressed slab system, and hollow core slab system. However, the waffle slab system and prestressed slab system still suffer from floor noise problem because these systems have a relatively thin thickness of slab. In addition, it require very complicated form work. Hollow core slab system can reduce the self-weight of slab efficiently without any problem of floor noise and form work but it can be applied to 1-way slab only. Thus, it is difficult to apply hollow core slab to buildings. In the year 2000, a voided slab system was suggested to solve these problems of conventional slab systems. The first voided slab system used spherical or oval plastic balls as void-shapers to eliminate directivity of the slab and reduce concrete amount and slab weight. The main idea of voided slab system is that void-shapers are placed between the top and bottom reinforcing bars before concrete casting in a slab. This slab system can reduce the size of vertical members by lightening the self-weight of slabs without the deterioration in flexural capacity of slab significantly because the voids are placed at slab web which exerts little influence upon flexural capacity of the slab. Thus, the flexural capacity of the voided slab is as good as those of generic reinforced concrete solid slab of the same thickness. In addition, the amount of concrete, reinforcement bars and temporary resources such as supporting posts can be reduced because the voided slab reduce self-weight of slab. Because of the advantages of the voided slab system, usage and development of voided slab has been increasing gradually all over the world. According to previous studies, the performance of voided slab systems such as structural capacity, economic feasibility, and construct ability varied by the void shapes and details. Therefore, several types of voided slab system have been developed to improve their performance, and the donut type voided slab system, subject in this study, also has been developed as part of the research for improving the performance of voided slab system. Because of varying structural capacity of voided slab due to the void shape and details, most of studies on voided slab system were focused on evaluation of structural capacities in flexure and shear by their own void types and details for the application and design voided slab. The first problem is that the structural behavior and capacity of donut type voided slab in flexure and shear are not investigated clearly even if it behaves in 1-way action. For the application of donut type voided slab, it should be required to evaluate several structural capacities of slab such as flexural strength and stiffness, ductility, deflection, load redistribution, and shear strength including punching shear strength. Therefore, the first step should be the investigation of the structural capacity of donut type voided slab under 1-way and 2-way action. The second problem is that it is not yet verified what the influence factors are and how these influence factors affected on structural capacity of donut type voided slab. For design of donut type voided slab, it should be required to evaluate the effect of influence factors on the structural behavior and capacity. Therefore, the second step should be the determination and evaluation of influence factors which affect on structural behavior and capacity of donut type voided slab. The third problem is that there are no guideline and recommendation for design donut type voided slab. Some design method of existing voided slab was suggested by previous researches. However, for design of donut type voided slab, the appropriate design method should be developed based on the structural capacity and failure mechanism because structural behavior of voided slab varies by the void shape and details. Therefore, the modification of current design method and development of design method carefully performed with consistency of design assumptions for donut type voided slab. To solve these problems, Donut type voided slab system was developed based on FE analysis. In addition, experimental and analytical study were conducted for donut type voided slab system. Structural test was conducted for determining the influence factors and evaluating structural capacity of donut type voided slab. Finite element analysis was conducted for compensating the results of experimental study. Based on the result of experimental and analytical study, the design method of donut type voided slab was developed. This study focuses on developing newly voided slab system with donut type voids which has superior performance, evaluating the structural performance of donut type voided slab and deriving the influence factors under different loading and boundary condition. Finite element analysis was conducted to develop the donut type voided slab system as optimal one and FE(finite element) analysis results were investigated to determine what the influence factors are and how these influence factors affect on structural performance of donut type voided slab. In addition, the details of donut type voided slab system such as the material and fixing method of void-shaper, construction method, and etc. were developed through the pilot test and FE analysis. Details of voided slab such as shape, material and fixing method of void-shaper are very important factors for structural behavior of voided slab. Especially, void shape is main factor which changes the structural behavior of voided slab. The influence of these details of voided slab should be determined and evaluated to ensure the structural capacity of donut type voided slab. Therefore, preceding research analysis was conducted to deduce the influence factors on structural behavior of donut type voided slab. Flexural behavior of donut type voided slab should be investigated and evaluated because slab member is generally governed by flexure. Therefore, flexural test was conducted for investigating the flexural capacity and failure mechanism of donut type voided slab under 1-way and 2-way action. Flexural behavior and failure mechanism of donut type voided slab was investigated with 1-way flexural test in terms of strength, stiffness, ductility, and cracking behavior. And applicability of donut type voided slab for 2-way slab was verified with 2-way flexural test in terms of global behavior and load redistribution. Shear test was conducted for investigating the shear strength and failure mechanism of donut type voided slab under 1-way and 2-way action. Shear strength and failure mechanism of donut type voided slab was investigated with 1-way and 2-way shear test in terms of shear strength and shear crack angle. Especially in 2-way slab, punching shear failure could occur at the slab-column joint and punching shear failure of voided slab was generally affected by the arrangement of voids around columns according to previous study. Therefore, applicability of donut type voided slab for 2-way slab was verified with punching shear test in terms of punching shear strength and failure mechanism with various arrangement of voids around columns. For the design of donut type voided slab, the appropriate design method should be developed based on the structural capacity and failure mechanism. In order to suggest the design method, the applicability of current design methods was investigated whether it is feasible or not. And flexural and shear strength evaluation method for donut type voided slab were developed based on FE analysis and test result.