The risk from assumptions should be yours only. If you don't need permit, why go ask the town inspector? Verbal or written it becomes official for the town. Saving $500 is not worth engineering on your own.
8' span is known and practiced standard for double 2x10 beams/girders. That is for one floor and roof, however. Whoever suggested 8' is trying to make life easier for you. 2x8 spans 12' but it is about at its limit. For unused ceiling it is more than enough.
Yes- I fully understand that risks & assumptions belong to the project originator. That's why I'm SEEKING advice & not winging it as they say. It's not a case of saving $500. I'm not sure where that came from as I don't recall mentioning $500, but maybe I gave you that impression somewhere.
I am not "Engineering" on my own. I'm not an engineer by profession. I don't diagnosis & self medicate either.
The plans I have for this barn come from a Professional architect. The plan is designed to cover codes & situations in just about every state. Original plan calls for 6x6 posts space at SIX feet apart on the 24' dimension. The girders called for are double 2x10's notched & bolted to the posts. Really overkill here in the South, but in the North in some states, local code requires it.
The 2012 IRC uniform code calls for a span of no greater than 8 feet between posts for AUX. residential buildings with double 2x10' girders.
I want to build with posts 12' on centers. Here is where the code becomes difficult to interpret. Code R106 references to code R301 which essentially requires you to show your theory & calculations & engineering when you span over 8 feet. The code also states that the local building authority can either enforce the code (IRC) or modify it in any way they want to modify it along with any submissions or arguments the owner builder may submit. This is more ore less on how the IRC works for ANY building requirements. AS a DIY'er who is trying to build a quality structure & to also build within the code requirements, it becomes difficult to understand what is legal & what satisfies local authorities.
Have you ever built anything that requires permitting? I've built a total of three homes in the past thirty years & all built & occupied by me & family.
I'm not exactly a stranger to this code stuff. Over the years it has become better with the adaptation of IRC 2012 by many of the states & local code officials. However it still leaves a lot of room regarding things like Barns, Pole buildings, outbuildings or special use residential buildings.
I agree with you regarding 8' spans with 2x10. My plan is/was to set 6x6 posts 12' feet apart & run double 2x10's on the exterior wall side & on the inside another 2x10 directly opposite the exterior double 2x10's. All notched & bolted to the posts. The top plate of the wall would be 2x8 doubled.
Based on the code of 8' between spans- I could be asked by an inspector to prove out to him/her that I can safely use 2x10's as described to carry the additional weight between 12' posts. I assume to do that I would need calculations on weight load of roof-dead & live. I would need to know raw lumber deflections for 2x10 spanning 12'. I would then need to show the math that proves out that the building design will work & not collapse under its own weight. I am not an engineer. Can you do the necessary engineering & calculation that an inspector may ask for? And keep in mind the inspector is under no obligation to grant an approval no matter what I present to the inspector. What happens then? Thanks for your interest in my problem. I'm trying to do it right.