Determining Masonry Arching Action Conditions

Our (old) code BS 5977 looks for 600mm. So you’re fine with 2ft in my view.

I seem to be able to check off all of the required conditions except maybe the one about having "sufficient masonry on each side of the opening to resist lateral thrust from the arching action".
However, it doesn't quantify what is considered sufficient.

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It's basically a case where you check the adjacent wall (as a shear wall) for the thrust (and vertical load) produced by arching action.

If it is not sufficient, the lintel would have to be designed support the full height and width of the 18ft brick over the opening, correct?

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Yep.

[red]EDIT[/red]: By the way, this is something we've kicked around before. I had a wall so heavily loaded....it got me looking very closely at things:

My rule of thumb is: be careful with rules of thumb.

NCMA is full of rules of thumb and I am a little wary of their TEK notes.

Think in engineering terms. What are the gravity forces above the lintel? Where do the forces go to ground? How do they get there? Are there control joints near the opening? What effect do the joints have? Are there concentrated forces to account for above the lintel? Is the lintel steel or CMU or precast? Is the wall reinforced? Can it work as a deep beam instead of an arch? If the wall has not cured, where do the wet concrete forces go (wet CMU can’t arch). If you get a crack here can you hang your hat on a “rule of thumb” you used?

I like to start with the load from 4 or 5 feet of wet grouted wall as my lintel load and then work my way up from there if conditions warrant.