The accuracy metric, η (eta), is a measure that compares the areas below the global load-displacement curve in 3D (reference) and 1D (See the Manual, Section 4.7.4.). Thus, the value of η depends on construction on the maximum displacement (v) reached along the curve. In the PISA publications, this displacement is fixed at two values: vul = D/10 for the ultimate or large displacements condition and vsd = D/10,000 for the small displacements condition. In the current implementation of PLAXIS MoDeTo, however, this displacement is calculated as the lower value reached by any of the two analyses. This causes the value of η to be input-load-dependent, which is inappropriate for a comparison between the curves.
A first approximation of the value of η can be displayed if the horizontal force, H, is modified so that the reached displacement is close to the target value, vsd or vul. Proper values of the accuracy metric for the target displacements can be calculated e.g. by exporting the tabulated 1D and 3D H-v curves to a spreadsheet.