Study On Secondary Internal Forces Caused By Piles Cutting-off In Basement Story-adding Of Existing Buildings

Piles underpinning is a core technology in basement story-adding project and piles cuttingoff is a key step in underpinning process.The load of upper structure will be transferred to newly constructed column in a certain period of time within the process.The piles cutting-off process may cause dynamic effects,meanwhile,because the piles cutting-off are usually carried out one by one,the influence of different cutting sequence on secondary internal force is different for caps supported by three or above piles.In order to ensure the safety of the piles cutting-off construction,it is necessary to study correlate content such as piles-cutting sequences,internal force increment,method of piles cutting-off and dynamic effect.The main work and achievements of the paper are as follows:(1)Considering actual engineering cases,a finite element model of pile cap propped by two piles under a column is built via ANSYS software.Through analyzing the influence of different stiffness of beam and cap on the secondary internal forces of the lower column,the results show that the secondary forces of the new column decreases with the increase of stiffness of beam and cap,which means beam and slab play the role of sharing the external load.The secondary internal forces of piles-cutting are different when cap's position is different.The constrain of convex angle's cap is the weakest,corresponding to the maximum secondary internal forces,which means the new secondary internal force of piles-cutting column is relative bigger when the beam plate's constraint is relative weak.(2)Simplified mechanical models of pile cap propped by two to four piles under a column are established,and analytical formulas of secondary internal force for piles cutting-off are derived,which are compared with the result of finite element calculation.The research results show that: for the pile cap propped by two piles under a column or the pile cap propped by three piles,the secondary internal force of piles cutting-off is not only related with the external load,but also related with the pile cutting order.The error between results calculated by the analytical formulas and finite element method are within 5% in general,and the analytical formulas is more applicable because they do not involve such process as meshing and boundary conditions setting.(3)Dynamic effect study takes the pile cap propped by two piles under a column as an example.A scale model of pile cap propped by two piles under a column is designed and built to simulate the influence factor such as method of piles cutting-off,height of gap between top end of lower column and pile cap as well as external loads on dynamic coefficient.The experimental results show that dynamic coefficient caused by hammering off method is bigger than that caused by hydraulic jack unloading method;the influence of height of gap between top end of lower column and pile cap on dynamic coefficient can not be ignored.Even if the height of the gap is the same,the dynamic coefficients will be slightly different under different loads.The general variation pattern of dynamic coefficients are consistent,which means with the increase of the height of the gap,the dynamic coefficient increases.In practical project,the gap between the cap and the column should be filled as tight as possible before piles cutting-off process so that the effect of the gap on the dynamic coefficient can be reduced.