Am I the only one who thinks rebar spacing matters way more than concrete thickness in bunker roofs?

I keep seeing builds where people pour 8 inches of concrete but space their rebar like 18 inches apart. That's wrong. The real strength comes from the rebar grid, not just how thick the slab is. I've seen tests at a demo site in Kansas where 6 inch slabs with 8 inch rebar spacing held up way better than 10 inch slabs with wide spacing. What spacing do you guys run for a standard fallout shelter roof, 6 inches or 10 inches?

3 comments

3 Comments

ray_williams4d ago

Oh boy, you hit on something I learned the hard way. Back when I helped a buddy shore up his basement workshop, we followed some old plans that had the rebar spaced out like a foot apart. That roof started sagging within two years, hairline cracks all over. We ended up jackhammering the whole thing out and redoing it with 6 inch spacing on a 7 inch slab, and it's been solid as a rock for a decade now. I think a lot of folks just grab whatever rebar they have lying around and don't think about the grid spacing, but it makes all the difference.

cooper.drew4d ago

Not even lying, my buddy in Wichita did almost the exact same thing. He poured a 9 inch slab for his storm cellar but spaced the rebar out like 14 inches thinking more concrete would carry the load. Three years later he had a 3 foot long crack running right down the middle and water was seeping through. We busted it out and went with 6 inch spacing on a 7 inch slab like you said and that thing hasn't budged since. That grid is everything

umar494d ago

Heard a structural engineer give a talk once where he showed photos of two slabs after a blast test. The thick slab with wide spacing broke apart, but the thinner one with tight spacing barely cracked.