Modeling Base Plate Considering Plate Bending and Real Dowel Force Distribution (Nonlinear)

Technical Article

When designing column bases, high performance anchors are often used for an anchorage. This article describes different models and their evaluation.

Modeling Options

For dowel force distribution, the position and stiffness of a base plate are governing. Furthermore, the modeling of a dowel support is also important for the realistic force distribution.

In Model A, a base plate has been created as a 'rigid' surface. In this case, possible plate bending is not considered. In Model B, a base plate has been created using thickness and material with realistic stiffness. The openings of dowel holes are closed by rigid surfaces with linear nodal supports in the center of those rigid surfaces.

Model B is much more accurate. However, the linear nodal supports, which also absorb compression, and the filling of the dowel holes with rigid surfaces still leads to a distorted plate deformation that is too small.

Model C has been derived from Model B. Only the rigid surfaces have been removed and the dowel hole edges of the dowel have nonlinear line supports. The nonlinearity is defined in the way that the supports are not effective when subjected to compression. In contrast, tension should be absorbed. The base plate support is also modeled nonlinearly. In this case, the compression should be transferred to the soil as a bedding fails when lifting forces occur.

Figure 01 - Model A, B, C

Result Evaluation with Regard to Modeling

Model A: Due to the ‘infinite’ stiffness, deformation of the structure is underestimated. A nonlinear bedding of the plate cannot be affected as there are no deformations in the plate. Similarly, the rigid base plate makes it so that there is no realistic dowel force distribution.

Model B: This model is much more accurate, but the linear nodal supports absorbing the compression, and the dowel holes filled by rigid surfaces always lead to a distorted deformation, which is too small.

Model C: The real stiffness combined with the nonlinear surface elastic foundation and nonlinear line supports allows for plate bending, including bending in the tension area, and supporting effects near the plate edge. Therefore, there are also significantly higher dowel forces than in Model A and B.

Figure 02 - Base Plate Deformation and Display of Support Reactions

Determining and Evaluating Dowel Forces

To evaluate the support forces, the result diagram of line supports is used. This can be smoothed linearly or constantly to quickly get the over the dowel force. The numerical values of the anchoring forces clearly show the importance of realistic modeling.

In Model A, the dowel forces are seriously underrated. This is the worst case and should not be applied in practice. In Model B, the described support design causes too big support forces. This solution is conservative and usually not the most efficient. Model C shows realistic distribution of support forces, which is affected by plate bending and effective surface elastic foundation. This model provides optimized and efficient results for the dowel selection. Furthermore, the determined surface compression of the base plate can be used to evaluate the required concrete strength.

Figure 03 - Determination and Comparison of Dowel Forces

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