Experimental Study On Punching Shear Strength Of RC Slabs With The Effect Of Column Aspect Ratios And Flexural Reinforcement Ratios

Punching is one of the most important problems in the design of structural concrete, such as buildings, bridges, port constructions, containments in the nuclear power station etc., and generally punching failure is a sudden local brittle damage and could lead nearby components, even whole structures, to arise chain reactions. So it is necessary to avoid punching failure. Furthermore, the favorable effect of flexural reinforcement in punching design has been properly taken into account in European codes including Europe code(EN 1992), MC 90, Britain code and German code etc., but our national standard and American code (ACI) neglect it. And for buildings, especially parking structures, the rectangular columns with a greater aspect ratio are desired to design, but it is lack of correlative designing reference. Thus it is necessary to further study punching failure and punching mechanism.Based on collecting a lot of punching reference and test data, punching mechanism, major concrete design codes' punching calculation methods and effect factors were studied in this paper. And 12 reinforced concrete slabs were tested under vertical punching loads, slabs' dimension was 2200mm×2200mm, slabs' thickness was 200mm, strength classes(grades) of concrete were C39～C46.9 due to practical error, flexural reinforcement ratios and column aspect ratios were primary effect factors, flexural reinforcement ratios adopted 0.5% and 1.3%, column aspect ratios adopted 1∶1,1∶2 and 1∶4, and each different condition slab had one repetition. By using ANSYS program to analysis test slabs, the effect of flexural reinforcement ratios and column aspect ratios was studied. In the end proposed formulas were put forward and reliability analysis was studied, and it is useful for designing reinforced concrete slabs and revising correlation contents of our national standard.In this paper major innovative efforts and achievements are as follows:1. Based on test results of flexural reinforcement ratios' effect on punching shear strength, increasing flexural reinforcement ratio could enhance punching shear strength, test data of referred material indicate that punching shear strength is linearly increased withρ₁f_yk increasing, and when flexural reinforcement ratio is greater than 2.5%, increasing flexural reinforcement ratio has little effect of enhancing punching shear strength.2. Based on test results and referred data about column aspect ratios' effect on punching shear strength, average shear stress around critical section perimeter is linearly decreased with column aspect ratio increasing. By using ANSYS program to simulate reinforced concrete slabs' punching failure, it is found that shear stress of middle part of column's long side is decreased with column aspect ratio increasing, especially when column aspect ratio is 1∶4, shear stress of middle part of column's long side is close to zero after slab is cracked, it implies that due to column aspect ratio increasing, two-way punching shear failure is changed to one-way shear failure, so it is suggested that 1∶4 is adopted as critical aspect ratio of distinguishing between punching failure and shear failure.3. All test slabs' punching failures are brittle failures with a certain extent ductility, the punching failure of test slabs with column aspect ratio being 1∶1 and flexural reinforcement ratio being 0.5% is close to flexural failure, and the punching failure of test slabs with column aspect ratio being 1∶4 and flexural reinforcement ratio being 1.3% is close to brittle failure. Based on former work, test slabs' punching mechanism is described: test slabs are under vertical loads, firstly slabs bottom' concrete is cracked due to flexural stress and shear stress in tensile zone, these cracks extend to neutral plane with the load increasing, flexural reinforcements around the column occur to yield and flexural reinforcements' dowel and shearing are favorable for punching shear strength, along with cracks extending, concrete compression zone is decreased gradually, and cracks extend more rapidly to concrete compression zone, in accordance with different flexural reinforcement ratios, along with non-cylinder special oblique section, sudden local failures with a certain extent ductility are occurred in the end. And the average of punching crack inclinations is 67.7°in the test.4. Based on test data and referred data, the regression formula (6.14b) was put forward, and the proposed formulas (6.22) and (6.26) were put forward by using plasticity model and twin shear strength theory. The distance of critical section perimeter and loading area perimeter is h₀, and the effect of flexural reinforcementratios and column aspect ratios on punching shear strength was adopted in the proposed formulas. Three proposed formulas' calculated results were shown to be satisfactory with test results. Furthermore, reliability analysis was used for the proposed formulas, results showed that the proposed formulas well satisfied the targeted reliability index 3.7 in structural design code.