In jointed structures, both stiffness and damping properties are affected by the way that the contact interfaces behave. The overall behaviour of the contact interface is the result of different parameters; among them are the contact surface quality and interface pressure distribution. Small changes in these parameters result in a considerable change in the physics of the contact interface and hence introduce variability in structural dynamics properties. The variability in the contact interface can happen among nominally identical structures. These features raise the need for robust models of the joint contact interface to predict their performance in the structure. In this paper, experimental modal testing of a set of nominally identical Brake-Reuss beam structures is employed to investigate the effect of preload on the variability in the natural frequencies and damping ratios for different modes of the structure. The contact interface is modelled using a representative stochastic model to account for the variabilities in the contact interface parameters. Finally, the distribution of the joint model stiffness parameters is identified using the variability in dynamic properties.