TFDIR variable cannot be locked or defined in site level?

I find it frustrating that in Structural Version 08.11.05.25, I could not use TFDIR variable at all. It automatically gets defined from TF_DATASETS it seems. Is it a known issue? I even tried locking the %lock TRDIR but still no success. Any help would be great for me. I am testing it for other application such as Architecture, Mechanical, and Electrical and there is no problem redefining TFDIR variable. But simply doesn't work in that version of Structural.