I see potentially two questions:
1. Is it necessary to bond the transformer metal to building steel?
Since it is reasonably likely to become energized, the transformer case must be bonded to ground somehow. Whether that is via the primary EGC (either wire or raceway), building steel, or some other bond to the building GES will depend on your exact situation.
2. Is it necessary to bond some point on the transformer secondary to ground?
In general if there is a neutral wire on the secondary it needs to be grounded. If you do not meet the conditions, including a ground detector, for operating ungrounded, you must ground one of the secondary terminals. You generally do this by bonding to the same point of the GES where the transformer case is bonded, so see answer #1. Do not connect directly to the primary neutral.
