A method for determining the amplitude-dependent mode shapes and the c
orresponding modal dynamics of weakly nonlinear vibratory systems is d
escribed. The method is a combination of a Galerkin projection and inv
ariant manifold techniques and is applied to a class of distributed pa
rameter vibratory systems. In this paper the general theory for a clas
s of conservative systems is outlined and applied to determine the non
linear mode shapes and modal dynamics of a linear Euler-Bernoulli team
attached to a nonlinear elastic foundation.