The Experimental Study And Theoretical Analysis For Load-transferring Mechanism Of Reinforced Concrete Thick Pile Caps

Nowadays, with their great advantages such as high bearing capacity, excellent anti-earthquake property and being able to reduce the uneven settlement of groundsill, pile foundations have been more and more widely used in the foundation of high-rise buildings, plan workshops, bridges, docks, man-sized machine-electricity equipment and especial constructs. As an important load-transferring part of pile foundations, piles cap is a connecting link between the preceding and the following .Therefore, making a thorough research on the mechanic performance of piles cap possesses important theoretic and engineering value.The experimental researches and theoretic analyses presented in this paper are a more deep probe into pile caps , following experimental researches on three-pile caps, four-pile and five-pile caps in Wuhan University of Technology. There are someaspects as below on the problems of this subject:1). The calculating methods of some countries on piles cap are summarized in this paper, their respective deficiencies will be pointed out. At present, the general study tendency about design method of piles cap is changing from the bending theory to applied space truss model theory gradually. But it is short of enough experimental and theoretic basis on making reasonable load-transferring model of piles cap. The experimental study on the issue will be carried out, 5 piles cap specimens, in which 2 specimens are four-pile caps and 3 specimens are five-pile caps, are tested in the contrastive experiments.2). According to observation and analysis of the entire course of the crack development in the experiment, it shows that the ductility of thick piles cap is certain poor, after-transfiguration capability is not enough, and so plastic hinge lines cannot be formed. When failure happens to thick piles cap, the thick piles cap is not divided simply into pieces along plastic hinge line, and its shear failure surface is a spatial curved surface. It is proved that thick piles cap doesn't have the characteristics of common bending structure. Therefore, it is not stress distribution in certain even section but integrate stress flow inside the cap that should be considered.3). On the comparatively analysis of experimental data of four-pile caps and five-pile caps, the results of finite element calculation, the load-transferring law of five-pile caps and the shape of laying-pile influencing on load-transferring between column and piles are researched. The reasonable laying-pile shape and calculating method will be presented.4). Observing cracks underside pile caps and measuring the steel bar strain on the bottom, the reinforcement shape of pile caps on the bottom will be studied and analyzed.5). The experimental results show that the effects of many factors such as concrete strength, ratio of distance and thickness, effective thickness of piles caps, reinforcement ratio and form of piles cap on bearing capacity of steel fiber concrete piles cap are analyzed.6). The space truss model theory argues that the stress flow transfers along with the truss composed of piles and column. The studies on it these days stay at qualitative and finite element analyses. It is short of the actual transferring route of the stress flow inside pile caps. On the comparatively analyses of the inner strain of pile caps by measure in the dissertation, it shows the stress flow doesn't transfers along with the truss. On basis of the actual transferring route of the stress flow inside pile caps measured in experiment and theory analysis ,the transfer model of equilateral thick pile caps- 'Dome Model with Tension Bar ' is presented. This conclusion is of great important theoretical meaning to develop reasonable calculating method of piles cap for design.Finally, some problems, which remain to be studied, are put forward on the basis of concluding the researching works in the paper.