How much does 3-sided support of granite countertop material increase the distance that's safe to span?

I am designing a counter for an outdoor kitchen, with a counter made of 3cm granite with block masonry support and stone veneers to decorate. An approximation of the planned layout seen from above appears here (ignore the 5'1" dimension, it's not set in stone, pun intended):

The long narrow element running along the bottom of the drawing is a higher front wall serving as a guest counter which will have ~12" wide granite atop. The part above - two end walls of 8" block, center "box" for sink and fridge, and two open sections between the box and end walls) was originally to be a box running the whole 18', but we decided we'd like to have open storage bays under the cook's counter for more flexible storage options (winter storage of stools, cook's cart, extra garbage bucket, whatever.) My concern is this: original design would have no doubt provided adequate support for the granite top, but the open areas shown would span 5' or better as currently drawn. I'm pretty sure this is well beyond the usual recommended span for 3cm granite (I see recurring mention of 3 ft. as the upper limit.) I'm wondering, if I add support in the back of the storage wells such that the 5-foot-ish spans are supported on 3 sides (maybe 8" back wall of block like the ends), whether that might be adequate to support a span of 5-5-1/2 feet?

Not to be picky, but the answer I'm looking for is not so much, "oh yeah, that'll be fine, trust me" but more like some reference to a source of information that might allow me to calculate my answer (formulae, tables of granite properties, etc.) I'm an engineer, but the wrong kind (studied EE decades ago, worked as a programmer for the last 30 before retiring), so I could do the math if I knew the rules, just don't know the rules. Thanks in advance!

• But will the granite have an internal flaw on the free edge? Commented Oct 17, 2021 at 18:58
• Sorry SMike, don't know what that means. Commented Oct 17, 2021 at 21:01
• Did you run this by the people who are supplying the granite? Commented Oct 18, 2021 at 1:56
• The supplier and installer gave assurances that weren't very assuring, of the vague type I mentioned, and would not guarantee that span themselves, so I'm still seeking answers if I can get them. I may have to post in a mechanical engineering or architectural forum to get the kind of info I seek. But thanks, Greg. Commented Oct 19, 2021 at 13:25