In this talk we study Fock-Goncharov generalized shearing parameters for representations of a punctured surface group in PGL(3,K) for an ultrametric field K, acting on the associated (real) Euclidean building.  The main motivation is to understand degenerations of real convex projective structures on surfaces.  We will explain how to interpret these parameters in the Euclidean building, using a geometric classification of triples of ideal chambers.  Under simple open conditions on the parameters, we construct a nice invariant subspace and an associated finite  A2-complex encoding the marked length spectrum. This allows to describe degenerations of convex projective strucures in an open cone of parameters.