I'm writing my dissertation on seismic risk assessment of rc frame building (G+7 with plinth of 1.5 m) with fixed base and base isolation by performing incremental dynamic analysis

For base isolation, l used the axial loads coming from columns on fixed base to design lead rubber isolator and then I assigned the bearing at the 0.5m above fixed base by using links

I read somewhere that gaps are also designed in Lrb, if it is so then how to select the value and design it in etabs ?