Freestanding Solid Concrete Wall 7

[RusMax]

Hello everyone!

I'm hoping some people can give me guidance/experience with a design such as this. I have to design a solid concrete wall that will be 7' above grade. What I'm coming up with so far is a 6' wide footing with a 6" thick stem wall. The top of the footing is 1' below grade. However, I'm starting to wonder if 6" thick stem wall with a single curtain of #4 rebar is too narrow. How would I check deflection in this case, as that would seem to be a governing factor for such a thin wall. If you have any other suggestions/cautions/etc., I would greatly appreciate anybody's input.

Thank you!

 

[BAretired]

Why do you have a 6'-0" wide footing? If such a wide footing is required, then 6" is not adequate for the wall thickness. Is it a retaining wall?

BA

 

[dik]

How deep is your foundation? The active/passive soil pressure will provide the restraint against rotation. The footing may only have to be 2' to 4' wide'. As BART notes, 6" may be on the thin side and you could have an 8" wall. What is the loading? Wind and/or seismic? Can you slope the faces? going from 8" wall to a 6" wall at the top?

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

 

[le99]

Why put a solid concrete wall on top of a 6" thick stem wall foundation? Why not set the wall directly on top of the strip footing? What is the thickness of the wall, and what is the intensity of lateral load the wall is subject to?

 

[RusMax]

Thank you, all, for your responses so far. I should have been more clear in my initial problem statement.

I'm in the seismic zone D2 and so my seismic lateral force controls. With the 6" thick wall, I had 431lbs/ft of lateral seismic load by provisions in ASCE 7-16, Chapter 15. Unfortunately, that was still with me disregarding the 1' of stem wall below grade in the seismic weight of the structure. If I was to count it (which I think I should), that lateral load is going to increase, as well as it's point of application (centre of mass). To ensure the eccentricity lands within the footing width, I arrived at a 6' wide footing.

le99, yes, what I meant was that my 6" thick wall (or as I called it "stem wall") rests on the footing and extends 7' above grade.

Thank you very much, in advance, for any further input. This really helps!

 

[le99]

No matter what you call, your total wall length is 7'+ D (depth below grade), which is to be used to design the foundation and the moment at the wall-footing interface. The 7' height is the effective height for wind load calculation though. 6" is quite thin for this wall, I doubt it can survive in seismic zone D, 8" may have a better chance. You should make sure there is adequate clear cover on both faces, and anticipate complications in concrete placement.

 

[XR250]

Wow - 431 plf! That is like 0.8G lateral load for a 75 psf wall. I don't do much seismic design but that seems like a lot.

 

[BigH]

A rule of thumb maybe - to get the gist . . .

https://wallace.design/retaining-wall-design/

 

[PEinc]

Also consider how the concrete will be placed in such a thin wall with reinforcing steel in the center. You will have only 2.5 inches between the form and the reinforcing steel and the concrete needs to pass down through the 8' minimum height of wall forms.

http://www.PeirceEngineering.com

 

[RusMax]

Thank you, all for tips and reference! XR250, yes, that's what I'm getting for seismic 0.82G and that does seem pretty high. I would love to use V = 0.30SDSWIe from 15.4.2 in ASCE 7-16, but the fundamental period of the structure doesn't allow me to use that equation, which brings me to 0.82G after doing analysis according to Chapter 12. Any suggestions on that?

Would you need 8" thick wall and two curtains of steel, in which case you will again run into complications with concrete placement or is 8" thick wall with one row of curtain steel enough?

How would you go by trying to calculate the deflection in such a wall?

Thank you!

 

[driftLimiter]

I would think an 8' wall with 1 mat of rebar should be sufficient for this. I am curious about your method to develop the seismic force. 0.82g seems really high, the wall isn't really a 'building like' structure. I would think Ch 13. has some equations and parameters for you to be able to justify a lesser seismic force. Just looking at Ch 12 anchorage for structural walls the seismic force should range between 0.4 and 0.8 Sds Ip that is an amplified seismic force with no accounting for ductility. Ch 13 application for nonstructural walls should be a reasonable approach for the seismic force on the wall.

Regarding the deflection of such a wall, you have the deflection of the wall, typically would be checked using cracked section properties and iteration of P-Delta effects ( slender concrete wall type approach). But you also have the soil spring effect that contributes to the deflection and would be based on the footing geometry and the soil subgrade modulus.

 

[RusMax]

driftLimiter - Thanks for your reply and your tips on deflection. Seems like this requires a bit more of work than I anticipated originally.

For my method of developing seismic force, I used Chapter 15 (SEISMIC DESIGN REQUIREMENTS FOR NONBUILDING STRUCTURES). I'm not sure that Chapter 13 would apply in this case, as the Scope listed in 13.1.1 would appear to direct me to Chapter 15. At that point, following provisions in Chapter 15, unfortunately, lead me to 0.82g for seismic force. Chapter 15 even has its own R value for Ground-supported cantilever walls or fences of R=1.25. So, yeah, not an ideal result. What do you think?

 

[driftLimiter]

Well I don't know the complete reasoning your using to apply 13.1.1 and consequently Ch 15, R = 1.25 seems reasonable, ground supported cantilever wall seems appropriate as well. The force result is what it is, as long as you double check your methodology, if you still arrive on the same load then I guess you go with it. Design for sufficient strength should be enough, when it comes to deflection your mostly concerned with P-Delta and stability imho.