What design are you using with the bamboos?

A-frame. Also open to other design. The area is a long row, about 75' long, N-S orientation. Also plan to grow cukes and other vinning plants. But pole beans will take at least 1/2 of the space.

I use 8 foot T posts so the trellises are 6 feet or so tall. It would be hard for me to pick beans at 8 feet.

Some beans are much taller than 6'. Then the production will be very limited. I always keep a short ladder handy. I even use a tall ladder to pick fruits from trees....

I think it depends on the variety. The scarlet runner beans I had last year grew to at least 15 feet. My trellis was only 8 feet, so I just turned them around to go back down and back up. I'm guessing 20 feet would probably have the top useless for many varieties, and they would be very hard to reach. The question is probably how far up you are willing to go for harvest

I used bamboo poles for holding up my nylon bean trellis, so the top was 7 or 8' off the ground, but the bamboo is more like 12' tall. Well, every place there was a bamboo pole, pole beans climbed them to the top and beyond. We had a wind storm, and it made the whole fence lean. I used bamboo poles at right angles to prop it up but realized I could just let the poles flop some and reach way up them. I let the beans that I was growing for next year's seed mature at the tops them could lean the poles and reach them or get up on a ladder and reach them.

I wouldn't build them taller than you can comfortably reach. I grow rattlesnake pole beans (green beans) and they get 12' long or more. My trellises are only about 6-7 feet tall. The excess just kinda falls over and finds somewhere to continue growing horizontally. The plants don't mind.

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