During the hot rolling process of heavy plates, asymmetries in the roll gap due to different circumferential velocities, different work roll radii or vertical temperature gradients lead to a bending of the outgoing material. This so-called ski-effect brings along a degradation of the plate quality with respect to the flatness properties and may lead to problems in the further processing steps. Thus, it is aimed at designing a strategy to minimize the ski or even better to avoid the occurrence of the ski-effect. This work is devoted to the development of a mathematical model that can be used for online execution in process control as a basis of a ski control concept. Although most models in the literature are based on numerical methods (e.g. finite elements), we will present a semi-analytical approach utilizing the upper bound theorem for ideal rigid-plastic materials. Starting from a detailed model, simplifications are made to decrease the execution time. The results thus obtained are compared both with numerical data from finite element simulations and measurement data taken in a rolling mill by CCD-camera measurements.