Shear in Concrete Wall

[JrStructuralEng]

I am designing a concrete wall that backs into a hill for a condo. The concrete wall will hopefully be 10" to 12" thick.

Factored Moment --> 107kN*m (79,000 lbs*ft)
Factored Shear --> 255kN (~57,300 lbs)

Based on my calculations, I will require shear reinfocing (i.e. stirrups of some sort). My question is will horizontal reinforcing act as shear reinforcing or do I need stirrups? I've never seen shear reinforcing in a basement wall.

Help would be great!
Thanks,
JrEng

 

[Taro]

Is the shear in the plane of the wall or out-of-plane? For in-plane, reinforcing is commonly used to resist shear. For out-of-plane, shear reinforcing is not common. In this case, the thickness of the wall is usually increased as necessary.

 

[JLNJ]

For a 57.3 kip load (in plane)it seems like a short length of wall will work without shear reinforcing.

The moment is a bit curious. Is it as induced by the shear at the base of the wall?

 

[JrStructuralEng]

Thanks for the quick response. What I have essentially is a 3 span beam. The soil pressure exherted on the wall causes the shear. I'm not sure whether you would call that in-plane. I believe its called out-of-plan since it is perpendicular to the wall. Your thoughts? I should mention my above moment/shear was for a meter wide of wall.

the walls looks like this. I will attempt to draw

/| |<---Floor system
/ | |
Soil Pressure/->| | 3m span
14.1kN/m3 / | |
/ | |<---Floor system
/---->| |
/ | |
/ | | 3m span
/------->| |
/ | |
/ | |<---Floor System (max shear here)
/---------->| |
/ | | 1.5m span (crawlspace)
/-------------| |

Sorry for all the metric units. I modelled the wall with the bottom as a 'cantilever'. I was hoping not rely on much lateral resistance from the concrete pile foundation (there is no concrete slab in the crawl space to take out the lateral load)
Hope that clarifies things, more help would be GREATLY appreciated.

 

[JrStructuralEng]

whoops, that drawings got a little messed, but hopefully it makes sense, it should look closer to this

Grade Level
_____
/|<-----support
/ | 3m
/ |
/ |<-----support
/ | 3m
/ |
/ |<-----support
/ | 1.5m
(cant)

 

[JrStructuralEng]

I ran it again using the base as a support instead of cantilevering it. That significantly reduced my moment to 32,000 lb*ft (43kN*m) and max shear of 23kips(103kN) -->again per meter of wall. I suppose we could use a slab in the crawlspace to take out the lateral load at base. The engineered wood floor system is going to be taking a huge axial load as well which i am quite worried about. Factored axial load of 190kN/m or 13kips/ft.
Any thoughts?!

 

[JAE]

I would second what Taro says....for out-of-plane shear in walls and slab, shear reinforcing is not common. In most cases, by adding thickness or by increasing f'c you can up your concrete shear capacity and make it work.

A forest of shear ties through a wall is a mess and impedes the deposition of concrete.

 

[JrStructuralEng]

Thanks JAE! Very helpful. I forgot about the obvious strength increase.
Two more questions on that topic:

1) If they pour that wall in 2 or 3 pours, would it be best to place the cold joints at the supports to encourage simple span action? My reasoning is to get away with using only one mat of reinfocing (i.e. on the inside face) As opposed to using 2 mats of reinforcing to carry the negative moment at the supports.

Just a thought. Otherwise I will likely cold joint it at minimum moment location and try to achieve a continuous "beam/wall" using two mats of reinforcing.

2)Do you have any thoughts about me adding that kind of axial load into the floor system. I know this is a general question, but i'm worried about how much axial load the floor system will be required to resist. It's a LOT larger than any of my previous designs.

 

[msquared48]

In addition to thickening the wall, you could spec a higher f'c to increase the allowable shear, or, step the wall thickness as needed rather than increase it all the way down.

Mike McCann
McCann Engineering

 

[MarcbSE]

Once you get over 10" thick, code requires two mats of steel in the wall.

 

[JrStructuralEng]

Thanks, Where is that in code?

 

[JAE]

Two mat requirement for >10" walls is in Chapter 14.

 

[ljk80oze]

JREng

I would definitely verify the lateral stability of the structure to resist the 7.5m of earth thrust. I understand your reservations about putting too much lateral load into the piles - but this thrust has to be resisted somewhere.... I take it the rest of the building is suspended and on piles?

What is the construction of the building? Is there enough bracing to resist this load and can floor diaphragms distribute the loads to these points? I assume the bracing is rigid and the loads will not cause serviceability concerns?

As this wall will have to be constructed hard up against the building there may be potential to utilize existing bracing walls built into and perpendicular to the backwall. If a connection to the vertical bracing elements can be justified these walls will serve as 'buttresses' and enable two way action in the wall and subsequent reduction in BM and shear.

I might be a bit of track here but i thought id put my 2c in.

By the way, i specify cold joints as scabbled with bonding agents and the reinforcing continuous. As such from a bending perspective the wall acts effectively continuous. I specify cold joints in the middle third of the span as i am more concerned about the shear rather than BM at these locations. I would not specify a cold joint at a support.

Cheers

LJK

 

[haynewp]

I would also state the joints to be in the middle third where the shear is less.

 

[JrStructuralEng]

Thanks for all the replies! As far as the joint location of the joints in the middle third, I am assuming that means you would be less concerned about a cold joint in a region of high moment.

to answer your question LJK, its a wood framed 4 storey stepped condominium. I-joist construction with concrete foundation walls supported soley on concrete piles.

 

[haynewp]

See ACI 6.4.4. It is somewhat analogous to your case. You assume the concrete cracks at ultimate moment anyway, I would want the concrete continuous where the shear is the most.