The common method for inter river buckling is based on Euler column behavior and does not include a correction for Poisson’s ratio effect. There is a method defined in (NACA-TN-3785, 1957) that takes account of the Poisson’s ratio effect and generates allowables about 10% greater than alternative methods.
The ruling equation is an adaptation of the classic plate buckling equation:
The value of the end fixity coefficient is given by the following table:
A spreadsheet of this method, including plasticity reduction factors, is available here: