Research On The Calculation Method And Failure Characteristics Of Shear Of Spread Footings

The shear calculation is a important procedure in design of spread foundation. This article summarized research situation and design method about questions of shearing at home and abroad, some examples are comparative analysis according to different codes(ACI318、EN1992、GB50007). Some problems which need to be studied further are put forward. The main content of this research include shear failure characteristics of extended foundation; impacts of foundation plane dimensions, reinforcement ratio and eccentric loads on shear bearing capacity and so on. Sixteen models of reinforced concrete foundation are designed. The punching shear failure characteristics of reinforced concrete foundation are achieved through model tests, various influential factors have been analyzed, also some advice of shear calculation method is put forward to provide some basis and reference for spread foundation punching shear design and specification revision.The main research results are as follows through the test and data analysis:(1) The shear failure characteristics of spread foundation are achieved through tests. It verify that punch failure will occur when the width of foundation is more than the column width pulsing double ho, when the foundation width is less than or equal to the column width pulsing double ho, the foundation without shear reinforcement will occur shear failure, the foundation with shear reinforcement will occur bending failure.(2) The shear rupture angle is about fifty-five degrees for shear failure foundation under centric load, when there are eccentric loads, the rupture angle of eccentric side is about sixty degrees, the reverse side is fifty degrees; the punching shear rupture angle of foundation is about fifty-one degrees under central load, the punching shear rupture angle of eccentric side is bigger and the reverse side is smaller for punching foundation under eccentric load, the rupture angles are sixty degrees and forty-eight degrees respectively.(3) The range of shearing strength of shear type foundation is at about0.98MPa to1.45MPa, which is between0.59ft and1.88ft under the condition of this test; the range of shearing strength of punching type foundation is at about1.27MPa to2.09MPa, which is between0.75ft and1.31ft. The shear strength level of development of concrete in shear type foundation is below that in punching type foundation. (4) The reinforcement ratio can increase shear capacity of foundation. Base on the condition of this test, it may be increased about15%～20%averagely.(5) The influence of plan dimensions of foundation plate on punching shear bearing capacity is analyzed. The semi-empirical formula of punching shear calculation considering influence of foundation side ratio is put forward; shear calculation formula is put forward according to the critical plane of ho/2.(6) The result of experiment show that cylinder foundation is replaced by square column foundation when punching shear calculate, if according to equal of shear area, their punching shear capacity is approach; if according to equal area of column cross section, the calculation result is inclined to safe.The main innovative results are as follows combined with the above research conclusion:(1) The punching shear failure characteristics of spread foundation are achieved through tests. It verify that punch failure will occur when the width of foundation is more than the column width pulsing double ho, when the foundation width is less than or equal to the column width pulsing double ho, the foundation without shear reinforcement will occur shear failure.(2) The rupture angle of side of eccentric direction is greater than the rupture angle of shear when centric load acts.(3) The range of shear type foundation shearing strength is at about0.98MPa to1.45MPa, which is between0.59ft and1.88ft base on the condition of this test; the range of shearing strength of punching type foundation is at about1.27MPa to2.09MPa, which is between0.75ft and1.31ft. The shear strength level of development of concrete in shear type foundation is below that in punching type foundation.(4) The influence of plan dimensions of foundation plate on punching shear bearing capacity is analyzed. The semi-empirical formula of punching shear calculation considering influence of foundation side ratio is put forward; shear calculation formula is put forward according to the critical plane of ho/2.