We study some spring mass models for a structure having a unilateral spring of small rigidity $\epsilon$. We obtain and justify an asymptotic expansion with the method of strained coordinates with new tools to handle such defects, including a non negligible cumulative effect over a long time: $T_\eps \sim \eps^{-1}$ as usual; or, for a new critical case, we can only expect: $T_\eps \sim \eps^{-1/2}$. We check numerically these results and present a purely numerical algorithm to compute ``Non linear Normal Modes'' (NNM); this algorithm provides results close to the asymptotic expansions but enables to compute NNM even when $\epsilon$ becomes larger.