4" Concrete Wall 1

[bookowski]

This is an exterior wall, a fence really. The architect wants 4" concrete. It's about 6ft tall, cantilevered. Based on strength this easily works (assuming proper footing, steel etc. etc.) This is obviously very tight - exterior so 1.5" cover both sides eats up 3". I'm thinking I'll use #3 bars, 1 curtain each way, and up the spacing as necessary to give myself all the room possible. Any issue with this (assuming the contractor is competent).

 

[KootK]

Been there: Link

Maybe precast or tilt up and grouted into a foundation pocket?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bookowski]

and those are 6" walls! I would be pretty skeptical about it but for the fact that it's essentially a fence. I am pretty sure that even if they totally botch it I'd still be ok for strength (wind/eq) so I'm wondering what my risk is. Seems like my most likely problem are that they pull the forms and I see half the rebar. I initially told him it wasn't likely to be buildable but am leaning towards being a bit less negative about it. He had a bunch of photos of carlo scarpa walls and was quite excited, hate to rain on his parade. At the end of the day why is it much worse than getting the concrete to outside of any wall rebar?

 

[KootK]

Go for it with SCC and report back? I'd reinforce heavy with horizontal bars to deal with restraint cracking unless you've got a spiffy vertical jointing scheme to deal with that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Just googleD Carlo Scarpa walls. Most look thicker than 4" to me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

There is so little space for a vibrator that compaction will be very tricky. Maybe some of this "self-compacting" concrete, which I haven't tried? KootK's suggestion of using precast into a pocket sounds best to me. I didn't know that Carlo Scarpa was particularly known for thin walls. A lot of his stuff was rough and brutal.

 

[bookowski]

Maybe they photoshopped the picture to pressure me.

I'll give it a shot.

 

[MacGruber22]

I don't think it is a big deal, as long as you have a competent and "hands on" inspector.

Specify a batch mix with 3/8" max. aggregate and specify a maximum placement height. They can build birdmouths at one or two wall elevations spaced at some distance and then cut them off after the wall has cured. Also, having a formwork vibrator (clamps on formwork) can make a huge difference and mitigate human vibration error.

This is not very different to partial depth wall and column face repairs that I perform often.

"It is imperative Cunth doesn't get his hands on those codes."

 

[Brad805]

We do many 4" precast fence panels, but they are not cantilevers. They have a standard concrete concrete column that has a tapered slot to accept the panels. Those are anchored to an appropriate footing or pile.

 

[hokie66]

With form vibrators, yes. Trying to do it internally, quite difficult.

 

[bookowski]

Thanks - form vibrators sound like a good idea.

The cracking moment, Mcr, is > Mu so any steel will suffice for strength. Looks like it's just about a competent contractor.

 

[XR250]

Maybe use heavy wire mesh for the reinforcement and only rebar vertically near the bottom. That may help with potential cover issues.

 

[MacGruber22]

XR250 - you beat me to it. A heavy mesh sounds very good. Reduces placement error and increases cover a bit.

"It is imperative Cunth doesn't get his hands on those codes."

 

[TehMightyEngineer]

As I work for a precaster now I can say that this would be much better quality if you could do it laying down on the flat shop floor vs trying to form it up. You can get the footing cast onto the wall as a second pour. As it's somewhat architectural the precast will have a much better finish and hold up better IMO. Wire mesh would be the way to go in my opinion.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[MacGruber22]

I agree with Teh, but often times we do not get to decide (CIP verses precast) and are forced to make the best out of a given situation. But, I would definitely relay to your architect that the easiest way to get the finish you want (without having to prep the surface later) is precast. Now, whether the owner finds that it is cheaper to use CIP and is happy to deal with the finishing afterwards, then you have made your recommendation and are prepared to deal with a solution for the CIP.



"It is imperative Cunth doesn't get his hands on those codes."

 

[racookpe1978]

Seems to ne you are much more likely to have the wall fail (lean over, fall down) due to the foundation dirt and fill slumping or twisting WITH the whole wall and foundation staying together, rather than the "wall" fall over by breaking in two or more sections.

Auto collision excepted of course. How tall? Any serious chance of people trying to climb it or jump it? (Fence for a concert or stage or stadium would be more likely to see climber for example than a junkyard wall or industrial privacy screen.

 

[MacGruber22]

Seems to me you are much more likely to have the wall fail (lean over, fall down) due to the foundation dirt and fill slumping or twisting

Yes? That is why you design a proper footing with a conservative bearing value if there is no geotechnical information. I am sure Boo knows what he is doing in that regard, or else he would have asked a more broad question.

"It is imperative Cunth doesn't get his hands on those codes."

 

[bookowski]

Agreed that precast would be nice but it's not an option. This is in a backyard of a rowhouse, they'd need to bring it in by helicopter. It's also part of a larger landscaping assembly - it's a wood fence, then turns into this 4" concrete fence for about 10ft, then turns into a concrete stair up to a conc balcony etc. I was just trimming the problem down to my main concern which was the 4" wall part and constructability.

rac - I will have proper footing etc., no worries on that part.

 

[TehMightyEngineer]

I find it hard to believe you can get a concrete truck out there but not a delivery truck. Still, I've design a house that was on an island and they hand-mixed the entire foundation so I won't say you're not wrong.

Oh well, cast-in-place then but you'll want to use plenty of skinny WWR with tight spacing on the wires to make sure you have as much cover as possible. As KootK suggested above a tilt-up wall may be the best way to go.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[bookowski]

I have never used precast so maybe I'm wrong. This isn't an issue of getting a truck to the site, but they are rowhouses - i.e. there is no access to the rear yard except through the hosue so everything has to be carried by hand. I suppose if they were small enough panels but I don't think it will fly on this project. I'll give the cip a go.