A precise background theory of computational mechanics is formed. Saint-Ven
ant's principle is discussed in chain model by means of this precise theory
. The classical continued fraction is developed into operator continued fra
ction to be the constrictive formulation of the chain model. The decay of e
ffect of a self-equilibrated system of forces in chain model is decided by
the convergence of operator continued fraction, so the reasonable part of S
aint-Venant's principle is described as the convergence of operator continu
ed fraction. III case of divergence the effect of a self-equilibrated syste
m of forces may be non-zero at even infinite distant sections, so Saint-Ven
ant's principle is not a common principle.