Load-transferring Mechanism Research Of Railway Thick Piles Cap

Due to its high bearing capacity and ability of adapting to uneven settlement of foundation, the piles cap have become a pattern which is widely used in the railway bridge. The traditional design methods of piles cap in domestic and foreign countries are mostly based on the control section method derived from 'beam system', by which we can do internal force and reinforcement calculation. With further research on the cap, limitation on control section theory has becomes more obvious.In this paper, it picks up the five caps, six caps in Lan-Yu line and No.2 line of Xi-Ge line as a pilot, as to study the transfer mechanism of piles cap (distance and thickness ratio w/h<1)under conventional load. The main works are as follows:(1)Considering the interaction between the soil and piles, the FEM model which can be obtained by using ANSYS program is used to analyzes the stress distribution tendency under actual vertical load in four conditions(caps placement completed,pier placement completed, after the system conversion and the completion of setting the Simply Supported Beams)of five and six piles cap. The results showed that:only under the loads of cap weight, the minimum principal stress distribution trend of flow is not obvious inside the cap.But when the vertical load is large enough, the minimum principal stress contour shows that a obvious stress current trends is got, which is from the edge of pier nearby to the top of the piles. And the maximum principal stress contour shows that the bottom region of cap is the tensile stress zone.The Mechanical characteristics of its structure is similar to the tie and strut in 'space truss system', while the junction area of tensile stress and compressive stress flow area can be equivalent to the node in 'space truss system'.(2)By using the method of 'beam system' and 'space truss system', the stress of reinforced steel at the bottom of cap is obtained, compared to the measured data and the results of FEM model. The results show that: the reinforced stress at the bottom of cap grows as the value of vertical load upon the cap increased, however, in a non-linear pattern. The steel stress at the bottom of cap varies as the location different. The steel stress in the middle of adjacent piles in the transverse direction of bridge is smaller than the steel stress along the direction of bridge. By comparing all kinds of design methods, the result got from the method of 'space truss system' is much closer. Therefore, further validation of the use of 'space truss system'to explain the transmission mechanism of piles cap is more reasonable.(3)By using ANSYS finite element software, the topology design of Five Piles cap is done. The results showed that:in the condition of the vertical load 16034.3 kN and with the bottom of piles consolidated, the best transmission structure of five Piles cap is similar to the 'dome arching body'. The new structure which is redesigned according to the result of optimal force transmission structure can effectively reduce the use of the material. In this paper, the new structure gets a 48.2% reduction in the volume than the original structure and improves the utilization of mechanical properties.(4) Finally, full review the main conclusions of the paper, and propose some problems that to be improved and added.