740 ILCS 100/3

The pro rata share of each tortfeasor shall be determined in accordance with his relative culpability. However, no person shall be required to contribute to one seeking contribution an amount greater than his pro rata share unless the obligation of one or more of the joint tortfeasors is uncollectable. In that event, the remaining tortfeasors shall share the unpaid portions of the uncollectable obligation in accordance with their pro rata liability. If equity requires, the collective liability of some as a group shall constitute a single share.