The 3 displacements δz, φx, φy of each node belonging to the diaphragm are independent of each other, while the rest δx, δy, φz are depended on the 3 displacements of point CT called Center of Elastic Torsion of the diaphragm. Displacements δxi, δyi, φzi at the point i of the horizontal diaphragm are expressed as: