Design of column base plates - Rigidity and neutral axis

Characterizing the stiffness of column foot connections, which we'll call anchors, is a tricky subject due to the diversity of elements involved in their design: the plate, the anchor rods, the concrete compressed by the plate, the interaction between the soil and the structure, and the normal force taken up by the anchor.

Before the introduction of the Eurocodes, it was customary to classify anchors according to a binary distribution mode: either perfectly articulated, or perfectly embedded. The rules of the art, both empirical and conventional, allowed anchors to be divided de facto into one or other of these categories, according to their typology. Articulated anchors are considered to absorb zero or very low moment, while embedded (rigid) anchors are considered to absorb zero or low rotation. In particular, laminated column feet, consisting of a plate fixed with 2 anchor rods very close to the main axis of the column, are considered de facto articulated when the length of the plate is less than 300 mm.

In reality, most anchors have an intermediate behavior, and are referred to in the Eurocodes as semi-rigid anchors. It is essential to estimate the degree of semi-rigidity as accurately as possible: in fact, overestimating the stiffness of anchors can have a significant impact on the design of the structure, in particular by underestimating the deformations of the structure. Such an underestimation would lead to an invalid verification of the so-called SLS (Serviceability Limit State) criteria, as well as an underestimation of the associated second-order moments. The distribution of stresses in the structure's constituent elements (beams and columns) is also affected.

The first part of this article looks at the classification of these anchors in terms of stiffness. Two methods are proposed for this purpose: the component method presented in the Eurocodes, and a simplified predictive method. Two examples, in which all the calculations are developed in detail, enable us to compare the two approaches.

The second part of the article is devoted to calculating the distribution of tensile and compressive forces in the anchor. Emphasis is placed on the exact calculation of the position of the neutral axis, the axis of zero deformation separating the tensioned and compressed zones, when the plate can be considered sufficiently rigid. A generalized formulation of the equations governing the equilibrium of the system is presented, as well as a complementary approach allowing, for example, geometric "irregularities" to be integrated into the calculations.

In cases where the plate cannot be considered sufficiently rigid, it is advisable to use more complex calculation methods, such as finite element...