Innovative structural solutions have been achieved in the fields of structural and mechanical engineering by combining different materials to produce more economical and efficient elements. The interaction between different materials has been achieved in various manners and this paper is concerned with those situations in which layers of different materials are interconnected by means of flexible mechanical devices. Of particular interest to this paper are composite members with partial shear interaction which have been studied for several decades. One of the first papers dealing with the partial interaction analysis of two-layered composites was the one by Newmark et al. [
] who focussed their attention on steel-concrete composite beams. Due to the wide acceptance of this work, its formulation is simply referred to in the literature as Newmark model. This paper extends the applicability of this model to study the time-dependent behaviour of multi-layered composite beams with partial shear interaction formed by
layers. A generic displacement-based finite element formulation is presented for the derivation of
-layered beams and is then applied to the case of a three-layered element representing the particular case of a composite steel-concrete beam stiffened by a longitudinal steel plate in which the partial interaction occurs between the slab and the steel joist as well as between the joist and the stiffening plate. The accuracy of the proposed procedure is validated against closed form solutions for the two limiting cases in which both shear connection stiffnesses tend to infinity, representing the full interaction condition; and also where only one connection stiffness is infinitely high, thus degenerating to the conventional two-layered composite partial interaction behaviour. Applications are then presented to investigate the effects of the two interface connection stiffnesses on the structural response of the stiffened composite beam. For this purpose, different lengths are considered for the longitudinal plate and the time-dependent behaviour of the concrete is modelled by means of the step-by-step method.