<font color="blue">bi-fold doors - like airplane hangers. </font>
We had these on both ends of one of our hangars. Bad thing about them was if you opened one end towards a strong wind, and the safety latch wasn't engaged on the other door, it would pop open the other door and then we had a 70' wide X 20' high door flapping in the wind. I guess what I really didn't like was that when it was opening, it is hinged across the entire top and it only had two points where it was attached on the rest of the door: one wheel on each lower corner. I just didn't like them and recall lots of repairs being made on them. We had far fewer problems with doors that were hung from the tracks and slid open to the sides of the hangars or the overhead type doors like standard garage doors. We even had one hangar the had three overhead garage doors across the front. The two center tracks were aluminum and had a chain that you could pull. That released a latch and allowed the track to be removed. It slipped into a socket that was embedded in the concrete at the bottom. Bad thing was there were no switch lockouts, so someone could send the doors down without the center tracks in... made a big mess /forums/images/graemlins/shocked.gif
I will say that the bifold door, when secured with the safety latch, made a good seal and kept the weather out very well... better than the overhead and sliding doors. Very easy to heat in the winter.
One more thing... consider putting tubing in the floor for heating the slab. Even if you don't heat it now, in 10-15 years, you might want a heated shop. Put in several zones, so you can heat just a portion of it, if you want. But it won't cost that much to put in the tubing now, VS later. A heated slab is the most comfortable thing to work on. You won't regret it.