A tie rod comprising locking elements to lock the tie rod to respective end supports (60,60′), the locking elements comprising, at each end support, two pairs of spacers (42, 43, 42′, 43′), each pair having surfaces joined to each other in the shape of an annular portion of a spherical cap, and at least two tightening nuts (41); wherein, at each end support, there are provided, symmetrically with respect to the support, two pairs of spacers, wherein the pair of joined surfaces of the first pair of spacers (42,43) has a radius equal to the radius of the pair of joined surfaces of the second pair of spacers (42′, 43′), said pairs of joined surfaces being arranged on different spherical surfaces, and wherein the two tightening nuts (41) are tightened externally on the first pair of spacers (42,43).