Experimental Study On Bearing Capacity On Casting Of Wrapped Column Of Underpinning Cap Of Underpinning For Existing Buildings

At present,rational,orderly and efficient development of underground space of existing buildings can expand the area of infrastructure construction and improve the land use rate,which is one of the main ways to alleviate "urban syndrome".It is an arduous and urgent task to underpinning for existing buildings.For the underpinning for existing buildings,earthwork excavation and underground storey-adding technology can only be carried out after underpinning of frame columns.Therefore,it is very reasonable and necessary to study the test of the underpinning joint of the wrapped column of the existing building cap.This thesis summarizes the characteristics of underpinning technology in three different projects of the Tunnel crossing existing buildings,underpinning for existing frame buildings and the buildings horizontal movement.and then,compares two underpinning methods of pile foundations for underpinning for existing buildings.This thesis studies the construction method of underpinning cap which underpinning piles on both sides of the upper vertical component and transforms the original foundation into a new underpinning cap stress and deformation mechanism and puts forward its design method.In order to study the effects of different impact-span ratios(?=0.4 ? 0.6 ? 0.75),reinforcement modes at the bottom of underpinning cap(B 10@180?B 16@140),orientation of longitudinal reinforcement(parallel to the length of column edge,parallel to the diagonal direction of column)and casting methods of underpinning cap(integral pouring of column and underpinning cap,pouring of wrapped column of underpinning cap)on bearing performance,Eleven sets of comparative experiments were designed.The ultimate bearing capacity ofunderpinning cap,crack development process,stress characteristics of joint surface and failure mode of underpinning cap are tested,and the stress characteristics of components are analyzed.On this basis,the calculation formula of bearing capacity is put forward.The results show that:(1)the reinforcement and other measures on the joint surface can ensure that the joint surface between the underpinning cap and the pillar does not slip,and the relative displacement between the pillar and the underpinning cap can be greatly reduced.(2)Comparing with grouting and inserting reinforcement glue into the joint surface,drilling reinforcement parallel to the diagonal direction of the column on the column will not only reduce the amount of reinforcement,but also provide tensile force to the upper concrete,improve the shear and compressive capacity of the cap,and thus increase the ultimate bearing capacity.(3)The design method of underpinning joints with wrapped columns in the underground caps of existing buildings is proposed and compared with the test results.The results show that the theoretical calculation formula of the strut-and-tie model is in good agreement with the test results.