The SR520 North West Approach replacement project will consist of 40 spans of isolated, elevated roadway. The design includes approximately 100 triple friction pendulum isolator bearings atop columns supporting the superstructure. A detailed ADINA model was generated, which used a nonlinear user-derived element for the isolation bearings, coupled in the two principal horizontal directions. During an earthquake, the column top motion causes the isolator base to rotate. The user-derived isolator element considers this rotational effect, which results in substantially more bearing displacement than if the rotation is neglected. If the column top rotation is not considered, the design displacement can be under-estimated, resulting in a bearing that is too small for the design seismic conditions. Other issues considered in the isolator element are velocity dependent friction, and vertical damping.