Basement Retaining Wall - Buttress Addition

[JohnnyB_]

Hello -

I am designing a basement retaining wall that is 60' long by 13' tall. The home owner is adamant on providing two buttresses in the basement of the retaining wall. My question is, with addition of the buttresses, will a two-way slab action be created?

Thanks in advance.

 

[PEinc]

It seems to me that the wall stem needs to be designed to span horizontally between the buttresses and between the basement corners and the nearest buttress. Thirteen feet seems very high for a residential basement. I would expect a 13' basement wall would be thicker than normal and have more reinforcing steel.

http://www.PeirceEngineering.com

 

[JohnnyB_]

It is not your average house. The stem wall is 12" thick per my current design which is attached.

 

[jerseyshore]

Looks like no floor bracing the top of your wall, is that correct? If so why not?

With a cantilever wall like that you can definitely design the wall to span horizontally.

 

[JohnnyB_]

There is floor bracing I just missed it on that drawing I posted. Thanks for pointing it out!

 

[jerseyshore]

I know a lot of people assume whichever is the shorter direction is going to be how the wall will span. I don't think you can go wrong assuming no buttresses and then adding them in for some extra support.

 

[jayrod12]

Be mindful that a long wall like that will develop shrinkage cracking. Might not be a terrible idea to have a control joint to break the span up. Or a proportional increase in horizontal reinforcing to maintain crack widths.

 

[JohnnyB_]

Thank you all for the feedback. I will make sure to take that into account.

 

[BAretired]

Make sure your floor construction is adequate to resist the "at rest" soil pressure at the top of the wall, particularly where joists are parallel to the wall. Add wall/floor detail for both perpendicular and parallel joists. Buttresses deliver concentrated reactions, resisted by floor construction.

I believe that 12" walls, by code, require two layers of rebar, irrespective of the bending moment. With buttresses, the wall behaves as two way slab. Also, there will be a negative bending moment at the wall/footing junction, so you will be needing vertical dowels at the base of the wall.

 

[Just Some Nerd]

Might not be a terrible idea to have a control joint to break the span up

I'd watch out for how the waterproofing on the back side of the wall is going to work if introducing a control joint

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Why yes, I do in fact have no idea what I'm talking about

 

[JohnnyB_]

Thank you all for the feedback.

 

[jayrod12]

I'd watch out for how the waterproofing on the back side of the wall is going to work if introducing a control joint

Agreed, however if you don't put the control joint, you're going to get one anyway. Concrete has a funny way of putting control joints in random locations if you don't plan for them ahead of time. There's a reason that the NBCC limits straight basement walls to 40 feet without a jog before they require an increase in reinforcing.

 

[JohnnyB_]

jayrod12, would you calculate the increase in reinforcement the same way it is done for shrinkage reinforcement in a SOG?

 

[jayrod12]

Yeah I feel that's applicable. The concept is essentially the same right? really long thin concrete? For a 12" thick slab-on-grade, I'd normally be spacing sawcuts no further than 20ish feet. with enough reinforcing you can go to jointless. But 0.2% isn't going to get you there.

 

[JohnnyB_]

Thanks for the feedback.

 

[JLNJ]

Your sketch shows a big footing, which implies a fixed base, yet the reinforcement you show is on the wrong side to develop any moment at the base.

As its depicted, the rebar indicates a wall simply spanning from footing to floor diaphragm. The detail just doesn't look right.

What is the intent?

 

[JohnnyB_]

JLNJ - Here is the updated detail.

 

[XR250]

I can't tell from your drawing whether you were trying to use the floor system to brace the wall or is it designed as a true cantilevered retaining wall? Either way it doesn't look correct.

 

[JohnnyB_]

XR250 - I have multiple calculations that show it to meet all criteria. The floor system is bracing the wall, it is a basement retaining wall. When you say "...it doesn't look correct." what are you referring to. Elaborate please.

 

[BAretired]

I am not clear on whether pilasters are still being contemplated. If so, perhaps the wall thickness can be reduced. Pilaster reinforcing must be shown as well as modifications to the floor framing to accommodate pilaster reactions.