Punching Shear Research On Reinforced Concrete Slabs

The slab-column structure with flat plate,as a favorite structure system in engineering, was applied for large amount of housing recently, because of its simple structure, direct force transmission and clear height increasing of floor.The punching shear strength of the reinforced concrete slab-column structures had been widely probed in China and abroad,lots of theories were developed,such as limitted theory,plastic theory,truss theory and yielding-line theory. However, generally speaking, none of these theories were accepted uniformly by the researchers in this field. Experiental expressions were taken by the design codes of most countries.In other words ,the code expressions are short of theory support.Based on the previous study of others,12 reinforced concrete slabs (include 3 one-way slabs ,and 9 square slabs )were designed to be punching damaged .In the test , the curve of power-displacement ,strains of the reinforcement rebar and the concrete ,the strength when the first crack appeard and the limitted strength were got .Based on the testing data of both mine and others, elements which influence the punching strength of the squre slabs were discussed in this paper:1.The punching strength increased with the the concrete strength ,and scared with (f_c')^1/2 and f_t;2.The punching strength increased with the reinforcement ratio when the ratio was less than 3% ,the increasement became less evidance when the ratio was more than 3%;3.The punching strength increased with the thickness of the slab, which is approximately linear relationship with each other.4.The primeter of the cone increased with the column size ,so the strength increased with the column size ,but the increasement was not so evident with the span-thickNess ratio.Based on the previous study,it was found that the damage of one-way slabs is similar with the beam.The punching shear of the square slabs is different from the beam .For the reason of two way bending ,the concrete is pressed in three-dimension.With the increase of the strength ,the shear-pull area splitted firstly ,the punching cone is basicly formed ,the force is assumed by the reinforcement ,the concrete of the shear-press area and the aggregate interlock of the crack area.When the inclined crack developed to the limitted state,the concrete near the reinforcement rebar ripped suddenly ,the concrete stress of the shear-press area reached the limitted strength ,then the slab damaged .The strength suddenly dropped ,the concrete of the shear-press area crushed and the inclined cone punched. Based on the research of others ,a machanism for punching shear which based on the rigidity-flow theory and the limitted equation theory was proposed in this paper ,using this model the shear stress and the press stress of the cone were got ,it shows that press stress is minus and it was less than 10% of the shear stress ;and the angle of the cone was arctan(0.8λ) in theory.The crtitial section of punching shear is proposed to be 0.4λh0 to the column edge.Finaly ,testing data of 248 slabs were collectted in this paper ,comparing with the testing data and the criterion of TJ10-74,GB10-89,GB50010-2002 of China,ACI 318-95 of the USA and CEB-FIP MC90 of Europe ,it shows that the strength of the thick slabs was less estimated by the CEB-FIP MC90 ,and over estimated by TJ10-74,GB50010-2002 and ACI 318-95 ,GB10-89 and the expression proposed in this paper has good consistenty with the testing data.