Despite the intensive experimental and numerical research focused on the behavior of steel and lightweight concrete composites, there are still significant uncertainties related to the materials and shear connection behavior. This paper analyzes one composite beam that was previously loaded up to failure in a laboratory experimental test. Firstly, a nonlinear numerical model is developed on the structural analysis software ATENA®. Then, a sensitivity analysis is performed to identify the most important parameters for the structural performance of these elements. A full probabilistic analysis is executed in order to determine the beam resistance distribution. An updated procedure, based in a Bayesian inference technique, is applied to adjust the obtained resistance model, considering the results of material lab tests. Obtained results point out the advantages of using additional characterization tests, as well as, the importance of considering randomness on the study of such a structure.