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.

 

[centondollar]

What are you worried about? Sliding? I don´t see any possibility for that, if the rock surface does indeed follow the foundation wall. How are you calculating the sliding force? I suspect that you will need quite a lot of rebar (which will act as dowels in pure shear) to provide resistance to sliding by such a mechanism.

 

[milkshakelake]

@centondollar I think the only reason would be for sliding, not uplift. If I don't technically need it, I could still put it in as "decoration" just to follow the industry standard here. The force is not a lot, I think a few hundred pounds per bar but I don't see any problem getting it to work even if it's 1 kip per bar.

 

[JStructsteel]

I would not say its industry standard, but I guess that depends what industry.

I would not rely on for tension. Its doing nothing for sliding as shown.

 

[80PercentTruth]

If you are only relying on the bars for 200lbf shear (no uplift), you don't need much embedment to transfer the load. As a general practice I like to provide embedment of at least 12d for all post installed applications. Keep it simple.

 

[dik]

If there's any soil behind that retaining wall, I suspect the embedded bars are in tension. 48" is excessive development into any competent rock. Hilti anchors often use 4" to 6", albeit for smaller diameter... maybe a foot or so.

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

 

[milkshakelake]

@JStructsteel Why would it not do anything for sliding? Seems like that's the only thing it would do.

@80PercentTruth Got it, thanks!

 

[milkshakelake]

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

 

[dik]

If so, why embed?

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

 

[milkshakelake]

@dik I guess I don't have a great reason for it. I just saw others do it, assumed that rock is slippery due to low friction coefficient.

 

[JStructsteel]

@ Milkshkelake, If the footing is poured into rock, there is no movement, thus no sliding. If it was on top of the rock only, I can see it doing something.

If not needed, dont put it in and defend your design and show the cost savings over 'well others put it in, so I will too'

 

[dik]

or if the rock had a slope... then cut it flat.

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

 

[dik]

two 1-1/4" dia holes x 4' @ 12" o/c would likely weaken the rock.

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

 

[dik]

I've had difficulty with that statement for over 50 years...

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

 

[BAretired]

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.

How much to embed rebar into rock? Assumed? For cost reasons? Wow! It could very well be not-so-hard rock. You are asking engineers from, who knows where...Canada, Germany, Lower Slobovia for an opinion about New York soil conditions?

Small amount of gravity load? Do you consider 400-500 kips a trivial load? What is the spacing of columns? If 20 or 25 feet, that would be about 20k/' on the strip footing, which does not appear to be more than about 2'-6" in width. The 200k base shear is for what length of wall? If 20 or 25', that would be 8 or 10k/', more than a few hundred pounds per rebar.

Posts filled with garbage do not deserve a considered response.




BA

 

[dik]

...and with a geotech report, you might be able to remove the dowels to the rock.

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

 

[KootK]

Tough room.

1) First, some general philosophy when one sees unexplained stuff in the work of regional cohorts. The two options being:

a) Don't do what the Romans do unless you can prove that it's necessary.

b) Do what the Romans do unless you can probe that it's unnecessary.

For me, it's all day long. In my opinion, going with [a] lies somewhere between hubris and a lack of respect for one's peers and elders.

2) It is my understanding that, in NYC bedrock, it can be quite difficult to create that trench for the footing. Rather, you end up with a footing that is not keyed into place for out of plane shear.

3) The use of this detail predates anyone's concern for in plane sliding by at least fifty years I suspect.

4) I feel that the dowels are mostly likely intended to resist out of plane wall shear at the base. This, where the keying into the bedrock is questionable and where one does not feel comfortable relying on the SOG. It's quite common in these NYC basements to see weird stuff like partial plan wine cellars etc that compromise the SOG for axial load.

5) I would be inclined to treat these dowels as true dowels rather than as shear friction reinforcing requiring any kind of "development". So 48" seems long to me too.

6) My first step would be consulting the project geotechnical engineer. Although, I can't imagine that you've not thought of that yourself already. So maybe this is something that SE's are doing of their own accord.

 

[Lomarandil]

I share dik's concern that drilling into potentially weathered rock of unknown competence could weaken the rock in the local vicinity of the drilling. Never worked in your geology, but I've had other sites where this would have been quite detrimental.

----
just call me Lo.

 

[milkshakelake]

@JStructsteel I was erring on the side of caution. If others do it, there is probably a reason. I guess I'm not 100% confident in my knowledge about rock, or it's a silly regional practice.


@BAretired I guess I didn't describe the loading clearly. I should clarify, the 400-500 kips gravity load is for isolated footings (generally 6'x6', which satisfies bearing pressure) that are between the strip footings. The strip footings are carrying 1 floor so it's not much gravity load. I don't think the gravity load is the concern here, I just described it in case it has some relevance I didn't think of. The base shear is loaded over two 60'ish basement walls and several shear walls. With 2 dowels at 12" OC, 200 kips / (60x2x2) = 833 lb shear, but a bit less because of the other shear walls.

I also wasn't asking specifically about how to deal with my rock conditions; I don't expect anyone to know about my region. I was looking for more like a general guideline for how to start thinking about this. I couldn't find any references online or past posts on this forum, and figured this would be a good place to start. I don't think that qualifies as a garbage post.


@dik True, I should get in touch with the geotechnical engineer. Might be able to eliminate them completely. Thanks for the advice.


@KootK [a] would be when I know a lot about something, therefore probably wouldn't ask about it in the first place. I agree that is the correct way in most cases until one gets smart enough to do [a].
Thanks for the tips. Everything you said makes sense. I thought it was for in-plane shear but hadn't thought about out of plane shear. Regarding 6, I'm not super confident in this particular geotechnical engineer (I have a history of getting sketchy intel from him), but I'll definitely reach out.

 

[milkshakelake]

@Lomarandil Thank you for the response. Would you also have the same concern if the embedment is something like 12" as dik suggested? The cone of influence would be less intersecting-y. Assuming it's needed in the first place, which is something I'll investigate based on the comments here.