Hello all!!!

If i have a 6x6 flexibility matrix of pipe bend which is the matrix for one end elbow while the other is fixed..What will be my 12x12 flexibility matrix where both ends are free and to be inverted to become the stiffness matrix, which will then be incorporated to the overall structure stiffness matrix?...Thanks in advance for your time...A specific reference or just a hint on how the process is derived is highly appreciated...Cheers...