The work presented in this paper represents the first step of a research activity aimed at the prediction of the plastic rotation supply of partial-strength bolted connections, starting from the knowledge of their geometrical and mechanical properties, by means of the component approach. As the main sources of plastic deformation of such connections are due to the components commonly modeled by means of an equivalent T-stub, the prediction of the plastic deformation capacity of bolted T-stubs is of primary importance within the framework of the component approach. Therefore, a theoretical model for predicting the plastic deformation capacity of bolted T-stubs is presented in this paper. The collapse mechanism topologies of bolted T-stubs are analyzed, and the corresponding formulations for predicting the ultimate value of the plastic displacement are given. Although the primary aim of the model is the prediction of the plastic deformation capacity, it can also be used for an approximate evaluation of the whole force-displacement curve. To evaluate the accuracy of the proposed model, an experimental program has been carried out. The experimental results and the comparison with the theoretical predictions are presented in a companion paper, providing a satisfactory agreement between theoretical results and experimental evidence.