This might not answer your question directly, but you should probably read sections 8.4a and 9.9 of AISC 341-05 Seismic Provisions for column splice requirements. The welds are demand critical, so check out section 7.3b as well.
I would design the plate back stop as a bracket plate. There is a quick and easy method given by Shakya and Vinnakota in their paper presented in the 3rd Quarter 2008 AISC Engineering Journal which is quite easy to follow and based on the current AISC Specifications for Structural Steel...