Concrete Walls

[VictorAK]

Does anyone know how to determine the maximum allowable eccentricity for a uniformaly distributed load on a 10" wide, 8 foot high foundation wall?
The walls were poured 3" off square and the result is a parallelogram shaped foundation wall.

 

[msquared48]

I am confused here - is the eccentricity (3" off) in the height of the wall, or in the thickness?

Mike McCann
MMC Engineering

 

[VictorAK]

The eccentricity is in the thickness. At worst case, the centre of the sill plate (2x6 flat) and wall above would be 3" outside of the centre line of the concrete wall.

 

[msquared48]

Is the 3" to the inside or outside face (soil face) of the wall?

Mike McCann
MMC Engineering

 

[VictorAK]

On one side its to the outside face (soil face) while on the other its to the inside face of the wall. I think the eccentricity on the outside face is more critical (?).

 

[msquared48]

If you could post a diagram showing the vertical reinforcing placement and clearances with the relative soil heignts, it will help with the comments.

Is this a free standing retaining wall, or a basement wall?

In general, for a basement wall condition, with reference to the soil face, any eccentricity toward the soil will add to the bending moment and away from the wall will decrease it.

For a free standing retaining wall, eccentricity towaed the soil will decrease the bending in the wall, whereas the opposote will add to it.

Sounds to me that you may not have an easy solution here.

Mike McCann
MMC Engineering

 

[VictorAK]

I've attached a typical section through the wall. The wall is reinforced with 15M @ 8" at the bottom of the wall approximately 36" from the bottom. The top section is unreinforced. The height of the wall is approximately 7'-4" and backfill height 6'-6". The strength of the concrete is 25 MPa.

 

[msquared48]

With no floor joists perpendicular to the wall at the top of the concrete, this is acting as a non-yielding wall.

Outside of the eccentricity, which I consider a minor problem, I have one major concern here:

The fact that it is detailed as a yielding wall, but the footing appears too small and wall improperly reinforced for the soil load. With a 6'+ soil retention depth, this should be an engineered wall. Is the dead load from the structure above sufficient that the soil side of the wall does not go into tension? The footing needs to be large enough with any overturning so as not to overstress the soil at the toe.

I have no feel for the load or rebar values you have mentioned. I am in the states.

Mike McCann
MMC Engineering

 

[VictorAK]

Hi Mike,
Thank you for your reply.
A little background into this case.
In Ontario, Canada, designers are permitted to specify wall sizes if they follow the values given in the Ontario Building Code, table 9.15.4.1. This table shows that a 20 MPa (2900 psi) strength wall, 9 7/8" thick can support ground cover to a maximum height of 7'7", provided the top of the wall is laterally supported (which this one will be).
The code futher shows that the minimum footing width for 2 storeys, exterior wall is 17 3/4" (table 9.15.3.3).
So, had the contractor made all the angles square, this wall would be considered acceptable under the Ontario Building Code.
As far as rebar and concrete strength, 15M bar is no. 5 bar and 25 MPa is approximately 3500 psi.
I'm trying to calculate the moment capacity at the top of the wall where there is no reinforcing and to determine if the wall will have sufficient capacity for the eccentricity.

 

[msquared48]

So, what you need to calculate is the cracking moment of the unreinforced section. Considering the embediment requirements of the reinforcing trhat is present, you may want to see what the cracking moment is at the base of the wall.

I have an ACI publication "Notes on ACI 318-83" put out by the Portland Cement Association. It has guidance for doing the calcs for this situation. There should be a similar publication for the most recent ACI code that you might be able to get and use.

Mike McCann
MMC Engineering

 

[kslee1000]

You have a mess here, because your concrete wall is curved (if I read correctly). Without lateral support on top, the wall is a cantilever, it deflects away from soil side, therefore you were correct in saying "the eccentric load on the non-soil side is more critical than the soil side". I would prefer to tear the wall down and rebuild it. However, you may try to add weighing block (counter weight) to the soil side (if space permits) to make the wall push-into the soil to counter the effect from the eccentricities. You should be able to determine the size of the block by performing simple stability analysis. If this scheme is viable, connect it to the wall by (drill & grout) dowels. This wall might show random cracks after all, depending the stresses level, which are very difficult to determined.

 

[VictorAK]

Thank you for the note kslee1000,
However, the contractor will provide lateral support at the top with joists.
What is complicating the project is we need to backfill asap since proper shoring was not provided.
I would like to tear down and rebuild as well, but that would significantly delay the project.

 

[msquared48]

Regardless of what the calculations show, do not backfill this wall prior to the installation of the joists and associated plywood diaphragm.

Mike McCann
MMC Engineering

 

[kslee1000]

Which side is the joists to be installed? Will the joists installed prior to backfill, or vice versa? Are the joists considered as lateral supports in the design?

 

[VictorAK]

Joists are to be installed first with plywood on the non soil side and are considered lateral supports in the design.

 

[kslee1000]

Now the matter is down to perform a structural analysis on a wall with pin-pin (top-bottom) condition using at rest earth pressure, and the eccentric load on the non-soil side. If you are not comfortable to do it by your own, you will need to get a structural eng. to help out.

Before you do the above, check with the code official to see if you can get hold on the background materials/assumptions used in developing the code recommendations. (It might have included certain amount of eccentricity from the structure.)

 

[hokie66]

VictorAK,

You have concluded that the wall is adequate to resist the lateral pressure of the earth, provided the top is braced. While some of us may disagree, that is not your question.

Building the wall above on top of the floor framing (platform construction) should remove any concern about eccentricity of loading on the concrete wall.

 

[VictorAK]

Thank you hokie66,
I would still like to calculate the moment at the top of the wall and compare to the moment capacity of the wall at the top. I'm thinking of treating as a simply supported beam, with a gradually increasing udl for the soil and a point moment at the top where the eccentricity is.
The problem is an assumption for percentage steel in the calcs.

 

[hokie66]

Yes, the soil load is triangular, and the wall should be treated as simply spanning. I think you can ignore the eccentricity from above, as that will be taken out by the floor framing. In the top part, p is of course zero. In the bottom part p=As/bd, where d is to the outside face. But realistically, the steel does nothing for you, as the maximum moment will be just about at the end of the bar.

 

[kslee1000]

Add surcharges to your soil diagram - from compaction, construction loads during/after backfill, normal live load. You shall include all loads that produce moment, even small, since your upper wall was not reinforced. Check compression in the joist as load has increased due to eccentricity. Check foundation base shear, make sure there is adequate shear friction to resist the reaction. If there is ground water, you might have big time problem.