G.17.2.5 Multilinear Analysis
When soil is to be modeled as spring supports, the varying resistance it offers to external loads can be modeled using a multilinear analysis.
An example is when the soil's behavior in tension differs from its behavior in compression. Stiffness-Displacement characteristics of soil can be represented by a multi-linear curve. Amplitude of this curve will represent the spring characteristic of the soil at different displacement values. The load cases in a multi-linear spring analysis must be separated by the CHANGE command and PERFORM ANALYSIS command. The SET NL command must be provided to specify the total number of primary load cases. There may not be any PDELTA, dynamic, or TENSION/ COMPRESSION member cases. The multi-linear spring command will initiate an iterative analysis which continues to convergence.