Here is my rememberences of building an historical wood barn ladder at UCV.
The sides were made from a 6" dry cedar, this small round timber then was cut as accurately as possible through the centre.
The edges of both pieces were then removed leaving two halves that were round on the outside but flat on the inside and the edges of each flat.
The rungs were about 1.75" white oak but here is the catcher they were rounded and then tappered towards each end slightly, down to about 1.25"
The holes in the sides were tapered also to the taper of the rungs, so that the rungs when placed in their respective holes were really tight.
They were then wedged from the outside crossways to the grain in the ladder sides.
3 1\4" rods were placed from one side of the ladder the other one at the top, bottom and at the halfway mark,
These rods were placed right under a rung
3-- 1\4" carriage bolts were then placed on each side to ensure that the sides did not split laterally, The round of the heads of course placed towards the top surface.
Taperring of the holes in the sides was acomplished by using a bung hole auger from our collection at ucv, otherwise it would have to be done by hand.
This makes a very light but strong mobile ladder for many chores. It will get lighter as it ages
Hope this helps