A multifibre finite beam element for geometric and material non linear analysis of space frame structures is presented, extending the well established planar multi-layer beam elements to the three-dimensional case. The element consists of two modelling levels, the local and the global one. The non-linear behaviour of the material composites, concrete and steel, is treated at a local modelling level (fibres), while global beam element quantities are used to render account of the geometrical non-linear behaviour (great deflections, finite rotations, small straining). At the local material level a triaxial elastoplastic model with damage is presented for concrete. For reinforcing steel fibres an uniaxial elastoplastic relation with linear hardening is applied. The necessary assumptions to determine local stress- and strain state are presented and their physical implications are discussed. The updated Lagrangian procedure is used to derive stiffness matrix and vector of inner forces for large displacement analysis of RIC-frame structures and (lateral) buckling analysis of RIC-girders, respectively. By way of example, some results of lateral buckling analysis of RIC-girders are presented and compared with results from laboratory tests. We show the field of application of the element, as well as its limits, as design tool for structural engineers.

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