Numerical Analysis On Properties Of Long-short-pile Composite Foundation Under High-rise Buildings

Long-short-pile composite foundation is used more and more extensively in civil engineering because of its remarkable economic and social benefits. At present research on work mechanism of long-short-pile composite foundation is not deeply, so long-short-pile composite foundation application in high-rise buildings is less. In this paper finite element analysis is applied on the whole high-rise building by ANSYS software, conclusions are drawn as follow:(1) Through contrasting analysis is made between site measured value and finite element analysis settlement value, It is verified that analyzing interaction of high-rise buildings, raft and composite foundation by finite element analysis method with ANSYS software is feasible. And obtain parameters involved in examples by repeated calculation.(2) Analyze the variation of axial force, pile skin friction, top-pile stress, soil stress, pile-soil stress ratio, pile-soil load-sharing, settlement of pile and soil with the upper structure floor of high-rise building increasing. The result show that, as upper structure stiffness increasing, axial force and pile skin friction, stress of soil under foundation and under pile increase nonlinearly. Under the same construction condition,stress values of soil under raft edge are bigger than soil under middle of the raft. pile-soil stress ratio maintain within a certain range as upper structure stiffness adding. Long pile load-sharing is around 50%, soil load-sharing is around 38%-40%, short pile load-sharing is around 12%. (3) Analyze the variation of Mechanical characteristics of long-short-pile composite as variation of cushion modulus. The result show that as cushion modulus increasing, stress of the top of long pile is concentrating, but stress of the top of long pile and soil stress is reducing; pile skin negative friction region is also reducing; long pile-soil load-sharing is increasing, But short pile and soil are opposite. Increasing cushion modulus conducive to reduce the average vertical displacement of foundation.(4) Analyze the variation of Mechanical characteristics of long-short-pile composite as variation of short pile length. Changing short pile length affects axial force of long pile and pile skin friction little. As length of short pile increasing, axial force of short pile increases; long pile and short pile-soil stress ratio Maintains within a certain range; long pile-soil load-sharing reduces, maintain within 46%～50%; soil load-sharing Increases slightly, it is around 38%～40%.short pile load-sharing increases too, it is around 12%～14%.