Thank you for your response. I understand better the point.
Nevertheless, what I am exactly looking for is the numerical method used by Nastran for reducing the stiffness. In the manual, this is not very clear, they are talking about underintegrating some terms. Nevertheless, this is not true. When exporting a elementary matrix, I see that Nastran remove 10% of the diagonal term and some terms are removed by 5% and others 2.5%. I would like to know what Nastran does exactly. If it exists a paper about the algorithm used by Nastran, this would help for my present analysis.
Sincerely yours
Evangeline