Beam Element Properties

The beam element dimensions used in the footing finite element model are the dimensions of the actual footing. Where a column is supported by the footing, the beam element thickness for that extent of the footing that is coincident with the column is taken as two times the actual footing thickness, which results in an 8-fold increase in the footing stiffness at these locations. This is done to account for the stiffening effects of columns and walls on the footing and is used to better model the change in stiffness between the portions of the footing supporting vertical members and the remainder of the slab. This increase in stiffness also acts to reduce the stress concentration under the supported members due to the supported member loads and moments.

Note: Only axial loads, shears, and moments along the footing’s major direction will be considered in the design (the continuous footing major axis is defined parallel to the line drawn from the first column on the footing to the last column on the footing). The one-dimensional model does not account for shear and moment in the minor direction of the footing.

The effective moment of inertia of a given beam element, accounting for the effect of cracked sections, is assumed by the program to be I_E = 0.35 I_g, which is used under the guidance of ACI 318-14, Table 6.6.3.1.1 (a).