WAAM (Wire Arc Additive Manufacturing) is a metal AM (Additive Manufacturing) technology that allows high deposition rates and the manufacturability of very large components, compared to other AM technologies. Distortions and residual stresses affecting the manufactured parts represent the main drawbacks of this AM technique. FE (Finite Element) modeling could represent an effective tool to tackle such issues, since it can be used to optimize process parameters, deposition paths and to test alternative mitigation strategies. Nevertheless, specific modeling strategies are needed to reduce the computational cost of the process simulation, such as reducing the number of elements used in discretizing the model. This paper presents an alternative technique to reduce the number of elements required to discretize the substrates of WAAM workpieces. The proposed technique is based on dividing the substrate in several zones, separately discretized and then connected by means of a double sided contact algorithm. This strategy allows to achieve a significant reduction of the number of elements required, without affecting their quality parameters. The geometry and dimension of the mesh zones are identified through a dedicated algorithm that allows to achieve an accurate temperature prediction with the minimum element number. The effectiveness of the proposed technique was tested by means of both numerical and experimental validation tests.