M_g(indice,indice) =...
M_g(indice,indice) + (H' * H * thickness * dw_b * rho_c);

M_g(indice+tot_node,indice+tot_node) =...
M_g(indice+tot_node,indice+tot_node) + (H' * H * thickness * I * dw_b * rho_c);

M_g(indice+2*tot_node,indice+2*tot_node) =...
M_g(indice+2*tot_node,indice+2*tot_node) + (H' * H * thickness * I * dw_b * rho_c);

M_g(indice+3*tot_node,indice+3*tot_node) =...
M_g(indice+3*tot_node,indice+3*tot_node) + (H' * H * thickness * dw_b * rho_c);

M_g(indice+4*tot_node,indice+4*tot_node) =...
M_g(indice+4*tot_node,indice+4*tot_node) + (H' * H * thickness * dw_b * rho_c);

M_g: global mass matrix
H: shape function matrix
thickness: total thickness of plate
I: thickness^3/12
rho_c: density of the plate
dw_b: detjacob * gaussweights

eig(stiffness(activeDof, activeDof), M_g(activeDof, activeDof)