For FEM analysis the AddMatrix method is very useful. In opposite to SetMatrix this method summarize elements.
You can now get the submatrix e..g
mySubMatrix= myMatrix.Submatrix(2,3,2,5) // = myMAtrix(2:5,2:7)
and with that submatrix you can get a sum of all the columns, by mutliplication with a 1 by n vector.:
e.g. to sum all elements of myMatrix
var v = new DenseVector(myMatrix.ColumnCount,1);
var w = new DenseVector(myMatrix.RowCount, 1);
SumMatrix= w * myMatrix * v ; //1 by 1 matrix , which is a sum of all elements.
//can do the same for the mySubMatrix
I hope that answers the question, not sure what else 'FEM' analysis requires..More info needed.