isn't this one of those things where you want to be using some numbers?
I can't imagine that you want anecdotal info here on sizing.
Like how much tension do you have withstand? Depends on your tower. Probably
25,000 lbs per leg or so.
So how do you resist that with a glued in bolt: well the concrete has to not
fail and the bolt has to not fail and the bolt has to not pull out of the hole
So will the concrete fail? Well depends on how the pour was done. What strength
concrete? And how reinforced?
You probably want to ignore the first 6-8" just to get where you don't have to
worry about surface strength issues.
looking at that simpson table, they show some of the critical issues: distance
from edge, embed distance and spacing distance.
http://www.simpsonanchors.com/catalog/adhesives/et/loads_rebar_conctens.html
Now depending on your base, you might not have a lot of edge distance...and
depending on the new tower, you're limited for bolt spacing.
Given your situation, the simpson tables probably don't cover it exactly.
You're probably going to go with 1 1/8" bolt or so. So the real question is how
deep do you have to go.
Given all the variables, you can see how 24" deep might make sense. But you can
also see how someone can argue for something less, or more...But no matter what
depth you pick, the goodness of it depends on some numbers and how you feel
about margin.
Like a simple answer might be: If an original tower specified length XYZ anchor
bolts, there's nothing about a drilled-in install, that would say "less is
enough". Unless you thought the original was overspecified. It's still all
about
resisting the pullout force. So you could look at current commercial towers
that
use anchor bolts and match their length. But you probably don't want to drill
that deep, so you really want to know what's the minimum you can get away with.
No one is going to give you an example of a tower that fell down, so your
question really is about how to analyze the requirement...
-kevin
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