Dad built one on the home farm in 1970 still standing today in use as a grain store with drive on floor. On our farm we have a similar one that has housed pigs and machinery since 1976 . Both have telegraph poles concreted in to a depth of at least 3 foot. Now I am a bit worried so I better check them.Dad built a pole barn in the early 60's-dug holes and "planted the uprights (untreated tree trunks) as you would a gatepost
One evening we noticed the whole barn was leaning towards the road (and the power lines) Three strong ropes and three tractors to pull the whole thing upright.
The tractors stayed in place for a couple of days while we redug and put sleepers in the brace the original posts. That was the mid 70's, so your posts should last about 15 years.
They don't usually fasten sheets of tin to them to catch the wind though.Western Power dont concrete any in, here they've used a jcb to dig out the hole, never seen one blow over yet.
Nor do bt . They use a long hydraulic auger.
Just dig deep 4ft 》 they are long enough.
P.s dont forget to put the big end in first