OK Allan has a dose of the nasties, your posts here and on the back shed are becoming legend.
Tower joins, well whatever you chose make sure they are tight, so as to not create a area of movement, and do use stay wire at each join to asist in eliminating such movement.
The best advice I can give is machined pieces that fit inside as couplings are the best, but they must be tight.
You have asked a lot of questions about towers and I for one honestly wonder where you are comming from, I will share my experiences with all comers who are sincere, but your questions here and on the back shed are so numerous I am left wondering, I sincerely hope I misjudge your coments/questions.
allan down under