Wow! You think small. Google 'home made arbors' and begin reading. The structural strength of conduit is not enough to withstand the mass of most vines. You will need something much, much stronger. Also, because of the size you might google 'bridge building' to get ideas. Good luck.

"I think small" LOL! If conduit is not strong enough, how about rebar or if the verticle conduit members are slid over the rebar which is concreted into the ground? As I said I could grow lighter weight annual vines like morning glories if the weight of grapes or wisteria is too much for this.....

What you think you need is beyond the unloaded structural strength of the materials you are thinking of. That means it won't be able to hold itself up in the center span ... without vines.

"the center section needs to be about 14 feet tall and wide enough that a bus can turn in that area without hitting it"

Start by figuring out how broad that span has to be ... then find materials that can make that span without sagging. What is the turning radius of the bus?

If you want it to be able to make a u-turn under the high spot of the arbor ... that's a structural riveted or welded steel beam arbor. Or a hulking glue-lam beam.

I dont need it so the bus can make a U turn, just a right hand turn and on a normal street that takes 2 lanes width. To make a right hand turn when driving a bus it is best to start from the 2nd lane over, so I guess the center section should have about 20' without verticles or horizontals (other then the overhead), the rest of the verticles can be anywhere. To make it top heavy until the vines fill in, I could hang wood blocks or something else to give it weight........

OK ... block it out with pylons and double check the turn radius. It sucks to build it and have the radius be too tight.

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp suggests you would need 2x12s on 24' centers for an end-supported 20-foot span with a load.

For supporting all this on 4 piers 20 feet apart, you are into some serious engineering.

Thanks LG! Hmm, one day we want to put in a 3 car garage in that area which would be tall enough to house the bus. Maybe I will just hold off on doing the center area since I cant do it cheaply. So, what about doing the metal pipe arbors on either side of that area for a walkway? It seems to me that here in the dry desert the metal would actually be better and last longer then either pvc or even wood.