Punching shear behavior of eccentrically loaded footings

在过去的研究中，许多学者对中心荷载基础的冲剪性能进行了研究。试验结果作为现行钢筋混凝土基础冲切设计规定的基础，一般假定剪应力在控制周线上呈对称分布。然而，在实践中，中心加载的基座是一个例外，因为非预期的(例如缺陷)和预期的(例如起重机轨道)荷载偏心适用于结构。为了考虑荷载偏心对冲剪承载力的影响，各种冲剪设计规定定义了增加施加荷载的系数。对不同条款的安全水平作出最后声明是不可能的，因为到目前为止，只对偏心荷载基础进行了很少的冲压试验。对于点支承的板，剪切和弯曲应力的叠加导致柱面沿线的多轴应力状态。中心加载板沿着柱面形成一个压环，与单向板相比，这增加了抗剪强度。然而，随着荷载偏心率的增加，这种旋转对称的多轴应力状态在柱的非荷载侧不再完全可能形成，从而导致冲切能力降低。同时，偏心荷载对内部剪切裂缝的形成有很大的影响。本课题的主要目的是对偏心荷载下基础的冲剪性能进行基础性研究。通过对钢筋混凝土单向偏心和双向偏心的受力性能的试验和数值研究，研究了不同的偏心距和不同的支座几何形状下，钢筋混凝土柱面(压环长度)的多轴应力状态以及内部剪切裂缝的形成。主要结果被用来推导一个一致的模型，该模型允许从根本上描述载荷偏心与压缩环或内剪切裂纹形成的长度之间的关系。基于该模型，将现有的偏心受力基础冲剪设计规定推广到偏心受力情况。

The punching shear behavior of centrically loaded footings has been investigated by various researchers in the past. The results of the tests served as the basis for current punching shear design provisions for reinforced concrete footings, which generally assume a symmetrical distribution of shear stresses in the control perimeter. In practice however, centrically loaded footings represent an exception, since unintended (e.g. imperfections) and intended (e.g. crane tracks) load eccentricities apply to the structure. To consider the effects of load eccentricities on the punching shear capacity, the various punching shear design provisions define factors to increase the applied load. A final statement on the level of safety of the different provisions is not possible, since only few punching tests on eccentrically loaded footings have been conducted so far.For point-supported slabs, the superposition of shear and bending stresses results in a multi-axial stress state along the column face. Centrically loaded slabs form a compression ring along the column face, which increases the shear resistance compared to one-way slabs. With increasing load eccentricity however, the formation of this rotationally symmetric, multi-axial stress state is no longer entirely possible on the non-loaded sides of the column resulting in decreased punching shear capacities. Also, the formation of inner shear cracks is strongly influenced by the load eccentricity.The main goal of the requested research project is the fundamental investigation of the punching shear behavior of eccentrically loaded footings. By means of experimental and numerical investigations of the behavior of reinforced concrete footings with uniaxial and biaxial load eccentricities, the multi-axial stress state along the column face (length of compression ring) as well as the formation of inner shear cracks is investigated for different load eccentricities and footing geometries. The main findings are used to derive a consistent model, which allows for the fundamental description of the relation between load eccentricity and length of the compression ring or inner shear crack formation, respectively. Based on the model, existing punching shear design provisions for centrically loaded footings are extended for eccentric loading situations.