Rebar into rock 3

[milkshakelake]

How much should I embed rebar into rock for strip footings? The rock is assumed to be weathered rock but no rock cores were taken for cost reasons; it could very well be hard rock. The strip footings take a small amount of gravity load, with most of the load coming on columns (~400-500 kips typically). Base shear is about 200 kips but with the number of bars, it comes out to a few hundred pounds shear per rebar.

My current (non)design is (2)#8 @ 12" with 48" embedment and epoxy, which is overkill. Would like to reduce it. I don't think any rebar is required at all, even if rock has low friction coefficient, but I see others use it so I don't want to be blamed for not following industry standard. The only problem is, my competitors don't specify an embedment, and I don't like leaving something like that up for debate.

 

[dik]

Hi Lo... I was joking about weakening the rock; I wasn't serious. There is, however, that potential.


I was joking about that, too, but they should be eliminated... but think it's a good idea and can use it as a possible financial argument to pay for the cost of the to do the geotekkie thing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[80PercentTruth]

I'd go with 12d for the dowel embedment and be done with it--there is no sense in fretting about an unrealistic concern. At that depth you will not hurt the rock and if one was concerned with the holes, they will be filled with high strength epoxy adhesive following the dowel installation. If you are so inclined you could get more information about the bedrock (for your sanity), but at this embedment I don't think you'd have an issue.

 

[BAretired]

@mikshakelake,

Sorry, in hindsight I see that my earlier comments were insensitive and unkind and I apologize for them.

BA

 

[dik]

A BPS for contrite! I still have to learn that!

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Trippelsewe]

I've always understood that fixing into rock is very much dependent on defects in the rock, eg planes, clay lenses, etc. I'm no expert but geotechs I've spoken to are hesitant to provide definite guidance on this without having inspected the excavation, even bore holes will only tell you so much.

 

[dik]

...a really good reason for not fixing to it if it's not required.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Lomarandil]

Well, I'm certainly no expert on rock.. so take my opinion there with plenty of salt. Still dealing wih the trauma of the last time a project had "weathered rock" -- which ended up having an RQD in the single digits and deteriorated (further) rapidly as soon as it was exposed to air/moisture/vibration. That was a nightmare.

----
just call me Lo.

 

[bones206]

If there's any soil behind that retaining wall, I suspect the embedded bars are in tension.

@dik It's a basement wall, not retaining wall; I shouldn't have cut off the top. I assume pin-pin so zero moment.

I agree with dik, that in reality this is a fixed base condition. So if there were any out of plane loading on the wall it would yak on those dowels and who knows how the weathered rock would fare in that scenario. I'd be a little concerned in an earthquake scenario where that wall might be oscillating out-of-plane. It would be multiple cycles of yank yank yank on that weathered rock, whereas in a true pin condition, the strip footings could rock with the oscillation rather than trying to resist it with a fixed base. From a seismic perspective it might behave better with short dowels located inboard at the heel of the footing, rather than straight up into the wall. Or just not include them at all.