SOG Shear Reinforcing Under Wall 5

[waytsh]

I am hoping someone can point me in the right direcion. I am looking for recommended shear reinforcing details for a slab-on-grade with a heavily loaded wall. Going thicker with the slab is not a practical option at this point and was wondering if there are any references out there that specifically address this situation. I can find lot with suspended slabs but nothing that seems specific to a slab on grade. Is this not a recommended practice? Particularly if the slab needs to remain water tight? I am currntly entertaining the idea of a "U" or "Z" sirrup but the spacing is going to need to be pretty tight.

I appreciate any thoughts you may have.

Thanks,

waytsh

 

[jike]

Put in a thickened footing or separate footing under the wall.

Refer to "Designing Floor Slabs on Grade" by Ringo and Anderson.

 

[NKT]

Consider strips of shear studs like those used on elevated slabs supported at columns. If you turn your problem upside down, the loading is very similar: a distributed load on the slab (soil reaction) and a point support (wall load).

A more desirable solution would be to install a thicker footing designed for the wall, but it sounds like the slab base, reinforcing, and wall forms are already in place (?).

 

[JAE]

Don't forget that ACI 318, section 15.7 states that the minimum footing thickness is based on a minimum depth to rebar, d = 6".

So with 3" cover you have about a 10" slab minimum.

 

[waytsh]

My slab is currently a uniform 15" thick. It does not make sense to me but the owner and contractor insist that it is going to be much cheaper for them to install the shear stirrups instead of just thickening the slab. I have considered the shear studs and may go with them if there is not a good reason for avoiding shear reinforcing. Maybe the owner and contractor will then realize just how expensive this shear reinforcing is going to be.

 

[dik]

The wall is bearing on the slab on grade and is supported by granular fill? Can you place a concrete pour under the SOG in the vicinity of the wall? Should be inexpensive just a matter of excavating, compacting and placing concrete... what is the consequence of the slab cracking? Is it worth the risk? even with large amounts of steel, the slab can still crack.

Can the wall width be increased? a small pedestal? Is the problem beam shear? or punching shear? Can you use shear friction? with large bars? a recent project has 35M@6"

 

[jike]

A 15" thick slab ought to be able to support alot of load. How much load are we talking about? Joint locations?

I assume it already has top and bottom reinforcing. How much reinforcing? Are you sure you need shear reinforcing?

What is the soil modulus?

 

[waytsh]

dik, thanks for the suggestions. The wall is precast so unfortunately I can not go any thicker than what it already is. The problem is punching shear. I had thought about shear friction since I will have a substantial amount of flexural reinforcing under the wall. I didn’t think I could “double dip” on the reinforcing if it is already being used to resist flexure…or are you suggesting additional reinforcing specific to that purpose?

Maybe I better mention my analysis approach as well in case that is flawed. I have analyzed the slab on STAADPro because of the complexity of the loading and the wall system. For my worst case load combination I am getting a shear stress under my wall of 108.2 psi. Which translates to about 19.5 k/ft. which I am then checking against ACI 318 Eq. 11-3 or 11-5. For my model I used a uniform thickness of 15” but in reality the slab is only 12” thick under the wall because of the keyway. So my d is approximately 8.5. My concrete is 4,000 psi so I am getting an allowable shear strength of 9.68 k/ft for a one foot wide section.

Is this a rational approach or am I oversimplifying?

Thanks,

waytsh

 

[waytsh]

sorry jike, you posted as i was writing and I didn't notice before submitting. The unfactored load from that particular wall is DL = 10.5 k/ft and LL = 13.9 k/ft. The wall is 14" thick. I will have top and bottom mats in the slab under the wall. It is going to be #5 bar @ 8" o.c. but I may need to change the bottom mat to something like #9 bar @ 4" o.c. to meet the flexural requirements. i haven't determined what I am going to do there yet.

 

[waytsh]

soil modulus = 90 pci

 

[jike]

Sounds like you did a beam on elastic foundation analysis.

Is there any way to increase the concrete strength?
or
Is there any way to eliminate the keyway?

 

[waytsh]

I can increase the concrete strength but i don't think it is going to get me to where I need to be.

I need to keep the keyway since it is integral to locking the base of the precast panel in place and the grout and sealent help make it a nice watertight connection. Any connection that would sit above the slab I do not think would perform as well.

Thank you for the suggestions.

 

[miecz]

I think shear friction is the solution. However, I wouldn't use the reinforcing near the slab face to provide shear friction. I don't believe bars near the face of a slab falls under "where it is approporiate to consider shear transfer across a given plane" verbiage of ACI 11.7.1. I'm sure others disagree. I would add additional shear friction bars at the mid-depth of the slab. You're not talking about a lot of reinforcing, as you only need to reinforce for the shear in excess of the slab capacity.

 

[csd72]

Design it as a strut and tie.

Provide enough reinforcement in the bottom to resist the triangulated forces. Then, like a truss, there is no shear only axial forces.

PS: a 15" slab! are they wheeling the titanic around on this?

 

[dik]

As for shear friction, additional... it's more efficient. Because reinforcing for shear friction is generally developed in tension... it should be additional As and not included in the flexural component... unless you have added steel for compression and the bar is in the right face.

miecz:
If the load is from the top, then top bars are proper for shear friction... it's a matter of tearing the top bars through the slab for failure to occur.

Dik

 

[dik]

Forgot to add... you might consider that all the load is transferred by the outer 1/4 parts of the wall... this will bump the shear up a tad.

The reasoning being that the slab will flex under load and if you had dowels in the middle portion of the wall, they would likely be in tension. Your FE model likely reflects this...

Depending on your lateral load at the base of the wall, a 2x4 key should be sufficient and will have little effect on the shear or flexural capacity... you may be able to use the full slab depth for calculating d.

Dik

 

[miecz]

dik

You're right, the top reinforcing is proper for shear friction, in this case.

 

[waytsh]

Good news. As it turns out the owner and contractor are now willing to thicken the slab. However, I am still interested in discussing this further for future projects.

My top bars in this location are only serving to meet the temp. and shrink. requirements of ACI 350 and are not flexural resisting steel. So if I am understanding everyones opinion correctlty it sounds like I could use the top bars as my shear friction bars since they are not providing flexural strength.

Would you be of the opinion that this is as good as thickening the slab? If so I could save the owner some time and money since they will need to bring excavating equipment back out to the site to trench out the thickened area under the wall.

I am also curious what you think about the way I evaluated the STAAD data. Does this seem reasonable? Is there a better approach?

 

[miecz]

On second thought, I'm going to back off on the use of top bars. I agree that they are effective for shear friction reinforcing at the failure plane in line with the face of the wall. However, there are other potential failure planes at some small distance away from the wall where the top bar may spall the concrete. The shear at these locations is still quite high and difficult to determine. I would just play it safe here and add reinforcing at the middle of the slab. I would extend it away from the wall to the point where the concrete can handle the shear on it's own, but no less than the development length.

 

[hokie66]

As is usual with shear friction, no two people agree on exactly how it works. To me, it is just desperation. The slab should be either be thickened to take the shear or proper shear reinforcement used. By the way, this is a line load, so the shear is beam shear, not punching shear.