桩基厚承台的试验研究

桩基以其承载力高、能减少地基不均匀沉降等优点,在工程上得到广泛的应用。承台作为桩基的重要组成部分,起着承上启下的作用,在设计中应予重视。因此,对桩基承台的受力机理、传力模型及其承载力问题的研究具有重要的理论意义和工程参考价值。 传统桩基承台的内力计算都是建立在梁、板弯曲计算理论基础之上,对承台的受剪、受冲切、局部受压承载力的验算基本沿用一般梁、板等受弯构件的计算方法。这种设计方法只考虑承台某一指定截面上的受力,而没有考虑承台内部的完整力流（本文试验探索性测定了厚承台内部完整的力流分布）。随着国内外学者对承台研究的深入,人们发现平截面假定对承台尤其是厚承台不适用,于是提出了各种厚承台的传力模型,如拉压杆模型、空间桁架模型以及空腹式模型。各国关于承台设计方法的总体研究趋势,虽已逐渐由弯曲理论向桁架理论靠拢,但建立桩基承台合理的传力模式仍缺乏足够的试验基础和理论依据。 本课题是继武汉工业大学三桩、四桩承台之后进行的二桩、四桩、五桩钢筋混凝土承台试验研究。并将钢纤维混凝土这种新型的复合材料应用于钢筋混凝土承台,按l:5缩尺模型共制作47个承台试件,其中二桩承台30个、五桩承台15个和四桩承台2个。进行了静荷载试验和有限元非线性分析。三次所做的试验研究和理论分析均属于《纤维混凝土结构设计与施工规程》（CECS38:92）的修订课题,旨在利用钢纤维混凝土材料优良的抗拉、抗剪、抗弯和抗冲切性能,改善桩基承台的受力特性,拓展钢纤维混凝土的工程应用范围。 在分析比较国内外有关承台设计计算方法理论体系的基础上,建立了桩基厚承台非线性有限元计算力学模型,运用有限元分析软件,对桩基厚承台进行了从加荷、开裂、屈服直至破坏的全过程非线性有限元计算,分析了承台的裂缝扩展情况、应力分布规律和传力机理,为桩基承台传力模型的建立提供理论计算依据。根据基于空间桁架模型的试验研究分析结果与有限元分析结果可以得出,桩基厚承台的传力机理更符合空间桁架模型,即以承台底部桩顶处水平受力钢筋条带为拉杆,柱头至桩顶区域的混凝土为斜压杆的空间桁架。 分析研究桩基承台的破坏形态随着距厚比w/h0的变化趋势。当承台w/h0较大时,承台的极限承载力是以柱和角桩连线范围内混凝土或钢纤维混凝土的劈裂破坏为控制条件;当承台w/h0较小时,承台的极限承载力则是以柱和角桩连线范围内混凝土形成的斜压杆的剪切破坏或角桩的冲切破坏为控制条件;研究表明,厚承台破坏时,底部纵向钢筋并非全部均匀受力,而是集中在桩径范围内布置的钢筋受力大且
逆变焊机
pile foundations have been more and more widely used due to their great advantages such as high bearing capacity, and being able to reduce the uneven settlement of groundsill. As an important part of pile foundations, piles cap is a connecting link between the preceding and the following and should be taken into account in designing. Therefore, making a thorough research on the mechanic performance of piles cap possesses important theoretic and engineering value.The calculation of inner force of pile caps establish on the bending theories of beam and plane, the checking computation of load bearing capacity for pile caps generally used calculation method of beam and plane. This kind of design method has only considered the stress in a special section of pile caps, but has no a consideration of complete stress flow inside the pile caps (a lot of stress inner thick caps is measured in the paper). When domestic and international scholar study pile caps deeply, people find that the plane section hypothesis of average strain is not fit in with pile caps, especially with thick pile caps, hence bring up many kind of load-transferring model of pile caps, such as tie-arch model, spatial truss model and limosis model. At present, the general study tendency about design method of piles cap in some countries is changing from the bending theory to the applied truss theory gradually. But it is short of enough experimental and theoretic basis on making reasonable load-transferring model of piles cap.An experimental research on two-pile caps and four-pile caps and five-pile caps is presented in this paper on the basis of experimental researches on three-pile caps and four-pile caps in Wuhan University of Technology. Steel fiber concrete, as a new kind of composite material, is applied in reinforced concrete pile caps in the research. 42 piles cap specimens in 1/5 ratio, in which 30 specimens are two-pile caps and 12 are five-pile caps, are tested in the experiments, and finite element analysis is done using ANSYS. These experimental researches are embellished subjects for Specification for Design and Construction of Steel Fiber Reinforced Concrete Structures (CECS38: 92), and in order to provide some background information for Technical Specification of Fiber Reinforced Concrete as well. Applying the perfect tensile, bending and punching shear performance of steel fiber concrete to improve mechanic feature of piles cap, and opening up construction applying range of SFRC is the major purpose of this study.On the comparatively analysis of various design method to pile caps at home and abroad, studied the nonlinear finite element mechanics model of steel fiber reinforced concrete, applying finite element analysis software, made further research on load-transferring model of steel fiber reinforced concrete pile caps. The load-transferring mechanism of steel fiber reinforced concrete pile caps accords with spatial truss model is found, which made of tension-link and slanting-compression-link, on the base of experimental research and finite element analysis.Failure configuration with ratio of distance and thickness w/ho decreasing is researched in the experiment. When w/ho is larger, ultimate bearing capacity of files cap is determined by splitting tensile failure happened to concrete within the range of linking line between column and corner pile; when w/ho is smaller, ultimate bearing capacity of files cap is determined by shear failure of slanting compressive member of concrete formed within the linking line column and corner pile, or by punching failure of corner pile. Tests indicate that, the stress of longitudinal reinforcement on the bottom of thick piles cap is not total same, and the stress of reinforcement arranged within the pile diameter is little changed and bigger, and is yield firstly. Therefore, the transferring pattern of stress of piles cap accords with the transferring mechanism of spatial tied arch. This conclusion is of great important theoretical meaning to research deeply mechanics behavior of piles cap, especially thick piles cap, and develop reasonable calculating method of piles cap for design.The mechanic performances of thick piles cap are researched on the base of experiment and the finite element analysis in this paper. 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. Therefore, it is not stress distribution in certain even section but integrate stress flow inside the cap that should be considered.The mechanics performance of steel fiber reinforced concrete five-piles cap has been studied in this paper. In practical engineering applying, due to the geological condition or place’s limit, sometimes a pile should be set under the column of pile caps, but this setting has not been described clearly in correlative code. When engineers design this kind of pile caps, because of lacking reliable theories, they have to rely on their experience. On the base of experimental research, using analysis of finite element, the mechanics character and reaction to pile caps of the middle pile of steel fiber reinforceconcrete five-piles cap. The conclusion is that (1) For the thinner five-pile cap, with the same dimension of corner and middle pile, middle pile takes on more load. (2) With the increasing of pile caps’ height, the difference of load bearing about middle and corner pile is reducing; when pile caps’ height increases to some extend, there is nearly no difference between them. (3) With the adding of middle pile’s diameter, the difference of stress is reducing. At last seeking after the application of steel fiber reinforced concrete five-pile cap in practical.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.On the basis of experimental research and theoretical analysis, a simplified designing method and calculation formulas are developed in this paper. Some theoretical background information and reliable basis are put forward for Specification for technology of Fiber Reinforced Concrete Structures (CECS38: 2004).The reinforced mechanic performances of steel fiber in the pile cap are discussed in this paper. Adding steel fiber postpones the extending of crack of piles cap effectively, improves shear behavior and punching behavior on slanting section of concrete and increases the ductility and ultimate loading capacity of piles cap obviously. Meanwhile, the thickness of piles cap can be reduced by mean of adding steel fiber on the same demand of bearing capacity. This conclusion is of important value for reference in building engineering with piles foundation. So it has spacious foreground for steel fiber to be applied in piled cap.Finally, some problems, which remain to be studied, are put forward on the basis of concluding the researching works in the paper.