Prior works [i.e., Lacayo and Allen, MSSP, 2019] have used discrete four-parameter Iwan elements to capture the localized energy dissipation and loss of stiffness that is experienced near bolted interfaces. Those works have validated that approach by inserting these joint models into a structural model and then verifying that it accurately captures how the natural frequency and damping of one mode of the structure vary with vibration amplitude. Those studies were limited to only a single level of preload in the joint. If this were to be used as an empirical means of modeling structures, one would need to know how the Iwan parameters of the joints vary with preload and to verify that the method is effective in a wider range of scenarios. This work explores this issue in more detail, seeking to ascertain whether a library of Iwan elements can be identified that capture a structure with two bolts when those bolts have a range of different preloads. Measurements were acquired from the S4 beam [Singh et al., IMAC, 2018] at various preloads and with a few permutations of the preloads. A finite-element model was then created with rigid-bar spiders that reduce the joint to a single pair of nodes, and Iwan elements were inserted between those to model the slip in the joint. The model was tuned to capture each set of measurements, to understand how the Iwan elements vary with bolt preload. The results presented show how the Iwan parameters evolve as preload is increased and also how the frequency and damping versus amplitude evolve over a wide range of preload and response amplitude.