Sounds like BridgeMan is on the right track. Most of your rafter tie load may be tensile. If that is the case and you prefer the look of the tie rods, they will probably work well. Let us know what the engineer has to say.
Is the current insulation laying on the top of the ceiling tile? If so, are you planning to insulate between the rafters and drywall the bottom of the rafters? Just additonal dead load to think about which may contribute to a compression load on your rafter ties. With 30' rafter ties, a 12/12 roof pitch and assuming an 8' ceiling height that makes it about 23' to the roof peak from the floor? That is going to be quite spectacular when you get done. I don't think I would want to try to heat it though.