Computational systems analyses are described for bridge systems that employ quasi-isolation by using a set of fixed bearings in addition to isolation bearings such as elastomeric bearings with an elastomer-concrete sliding interface or elastomeric bearings with a PTFE (Teflon) to stainless steel sliding interface. The system uses stiffened L-shaped retainer brackets that limit transverse displacement of elastomeric bearings and the bridge is intended to respond predictably, reliably, and elastically under service loading (including small seismic events), but for larger seismic events, certain bridge bearing components are intended to "fuse" and experience nonlinear behaviors that can allow for passive quasi-isolation of the bridge superstructure. To facilitate the computational modeling, experimental findings from ongoing research are being used to formulate phenomenological element models that simulate nonlinear fusing behaviors in the bridge bearings and auxiliary components. Furthermore the substructure pier elements, the bridge foundations and the abutment backwalls were modeled as they can exhibit nonlinear behaviors under large loadings. A parametric study is being carried out to investigate the effectiveness of the isolation system for various bridge structures and bearing combinations, and the findings will provide guidance for confirmation and further development of quasi-isolation design strategies.