The present contribution proposes a two-step upscaling procedure to determine the resistance of a plain masonry wall. This procedure is based on the yield design theory, allowing for an efficient and rigorous way for evaluating the failure load of the structure. The first step consists in determining the in- and out-of-plane macroscopic strength domain of the masonry assembly based on the kinematic approach of yield design, following the semi-analytical approach proposed by Donval et al. [1]. The main original feature of this method relies on the formulation of the latter criterion as a second order cone programming problem. In such a way, it does not require to be evaluated for each and every loading path, nor to be approximated for the second step of the procedure. The second step consists in the numerical modelling of the entire masonry wall as a Kirchhoff-Love plate, where the local strength is characterised by the above determined macroscopic criterion. The whole structure is discretized through appropriate plate elements allowing to take into account the membrane-bending interaction, as proposed by Bleyer and de Buhan [2]. Then, the kinematic approach of yield design is formulated as a conic optimisation problem and solved through appropriate optimisation codes. Finally, the potentialities of the approach are illustrated on a complex problem: the determination of the stability of a masonry wall exposed to fire. In this case, the proposed procedure allows to take into account both the initial heterogeneity of masonry and the heterogeneity due to the degradation of the material properties at elevated temperatures. References: [1] E. Donval, D. T. Pham, G. Hassen, P. de Buhan, and M. Vigroux, ‘3D analytical and numerical upper-bound homogenization approaches to the in-plane strength domain of a running-bond masonry wall’, Submitted to the International Journal for Numerical and Analytical Methods in Geomechanics, 2023. [2] J. Bleyer and P. de Buhan, ‘A numerical approach to the yield strength of shell structures’, European Journal of Mechanics - A/Solids, vol. 59, pp. 178–194, Sep. 2016, doi: 10.1016/j.euromechsol.2016.03.002.