Foundation Wall 40 Feet Below Grade 4

[JrStructuralEng]

Please excuse my metric units (<--Canadian)

I am designing a foundation wall for a condo that is approximately 12m below grade at the base. The condo foundation is stepped so the lowest portion of the wall is simply supported and has a clear span of ~2.5m. Silty/Clay soil and the geotech determined equivilent fluid pressure distribution to be 15.7 kN/m3. There is no option of using a lighter free draining granular backfill material since this portion of the foundation wall is right on the property line.

Ok, so here is the questions/problems.

Question 1. Should I be using a dead load or live load safety factor? i.e. 1.25 or 1.5. I am assuming this is considered dead load.

Question 2. I am getting very very large shear from the lateral soil pressure. ~310kN per meter of wall at the base. Using the shear strength of concrete alone (30MPa) I would need a 26" thick wall! My question is does the reinforcing mats on the inside and outside face of wall help with shear in this plane? It seems like shear reinforcing should be perpendicular to the wall. I have never put shear reinforcing in a wall, or needed it, so I am open for ideas. I am hoping the mats help, although logically I don't see why they would.

Please help! any comments welcome. (By the way there is no option to use buttresses in this case)

 

[msquared48]

For question 1:

I would use the live load factor for the wall design, as the lateral soil load can change with time (water table elevation, seismically or vibration induced settling, etc.)

For question 2:

Surely the wall does not span the full 12 meters or 38 feet. There have got to be interstitial floor diaphragms, hopefully concrete PT or CIP to shift the lateral forces to the perpendicular concrete walls thru diaphragm action. A 12" wall with 9 or 10 foot story heights should be able to handle the lateral load from the soil with the floor diaphragms in place. Am I missing something here?

Mike McCann
McCann Engineering

 

[hokie66]

For question 1, I would use your dead load factor for the hydrostatic component of the "equivalent fluid pressure", and your live load factor for the buoyant soil pressure component. Thus, if I understand correctly that the geotech has defined your pressure as triangular with an "equivalent density" of 15.7 kN/m3, you would use an "ultimate density" of 1.25 x 10 + 1.5 x 5.7 = 21 kN/m3. But I would check with the geotech to make sure that he has not already factored something.

Therefore, the pressure at the bottom of the 12 metre wall would be 12 x 21 = 252 kPa, and your base shear calculation is about right. Therefore, the 660 thick wall sounds reasonable to me.

The connection at the wall to footing and wall to slab joints would bother me, and I would try to make the wall bear against the slab rather than depend on roughness, shear friction, etc. But that is a contentious issue, and there was a recent long discussion in the Foundations forum which you may wish to read.

 

[AggieYank]

Mike McCann, he mentioned the clear span of the wall was 2.5m.

JrStructuralEng, your shear per meter calc looks about right. I'd make sure that the load the geotech gave you includes added water pressure. I'd also use a live load factor on the soil load. I'm not familiar with the code that uses 1.25 and 1.5 for load factors, but I'd guess if you looked through it, you'd find a factor for soil loads, and that it'd be similar to 1.5.

Your wall depth based on shear looks to be about right, based on the 310 kN at the base. However, if you use the shear at a wall depth d from the base, you'll be able to reduce the wall thickness for shear significantly. I've not a big fan of using shear reinforcing in a wall. I like to reinforce for bending, and use the concrete strength only for shear.

You're not allowed suitable backfill for a wall that is 12 meters in the ground, so I wouldn't worry about having a "large" wall. I don't think buttressing would be much of an economical improvement, because as it is the wall only spans 2.5 m. To reduce that, you'd have buttresses every <2.5 m, with all the additional formwork and special reinforcing that goes with the buttresses.

 

[JrStructuralEng]

Thanks for the responses,

hokie, I did read that article. Are you suggesting the joints for the wall best placed at a slab support? (p.s. the wall is supported by piles with a slab on grade) Keep in mind this is a stepped foundation, so top of wall at most locations will tie into slab.

Aggie, you said "However, if you use the shear at a wall depth d from the base, you'll be able to reduce the wall thickness for shear significantly." I don't understand what you mean by this? Could you elaborate.

So...generally speaking it sounds like i'm on the right track, calculations wise. If I end up needing shear reinforcing, because a 26" thick wall isn't possible (i.e cuts too much into parking space) what type of shear reinforcing is used in walls, how does it typically look or installed? I've never seen it before in a wall or slab. I am assuming it would end up being little bars that run perpendicular to the two main mats. Any more thoughts or information on these topic is appreciated.

 

[kslee1000]

Try use wall mount drainage fabric to lower the hydrostatic pressure. Shear reinforcement for wall, if required, is similar to that for the slab, see ACI for help.

 

[AggieYank]

JrStructuralEng. For a concrete beam, shear failure occurs at a 45 degree angle (or is assumed to), with the line of shear failure running from the bottom corner up and in at an angle of 45 degrees to the top of the beam. Pull out a concrete design book, and you'll see a good picture of it. This means that you check the maximum shear at the point where the 45 degree line hits the top of the beam. Based on geometry, using a 45 degree angle, the distance from the support where you check the maximum shear is "d", where "d" is the depth of the beam / wall.

For a relatively short wall with very high loading, ignoring this can cause an over-conservative design. For a typically long beam, ignoring this doesn't make a big difference.

In your case, if you have a service pressure of 173 kN/m2 at the support, at a distance "d" = 0.6 m from the support (because d is the depth of your wall), the maximum service shear is:

V = pressure * (span / 2 - d) =
= 175 kN / m * (2.5m / 2 - 0.6m) =
= 113.75 kN

If you ignore this reduction, the service shear is:
V = pressure * (span / 2) =
= 218.75 kN

As you can see, you'll get a big reduction. As you reduce the wall thickness, make sure to redo the shear equation to account for a smaller "d".

 

[AggieYank]

I should add that I would still design the construction joint between the wall and the support for the full shear. I like to provide a shear key but design the joint using shear friction.

 

[msquared48]

Aggieyank and JrStructuralEng:

I guess I am confused with the problem as I still read that the foundation is 12 meters below grade. I do see the 2.5 meter dimemnsion too - about 10 feet - that is simply supported. I guess that that is the problem span. OK. I am still amazed though about a 26" thick wall. Still doesn't feel right.

Mike McCann
McCann Engineering

 

[OldPaperMaker]

JrStructuralEngr,
I don't understand how you can have a wall that is about 12M below ground (at the bottom) and have a simple span of 2.5M. Are there vertical walls that are spaced 2.5M appart?
Reguarding your question about load factors, I would use code required 1.6 for the Live Load from the soil on the stem and 1.2 for the Dead Load of the soil on the heel & toe (if the wall has one).
I don't believe that the US codes, like ACI, ASCE7 or the IBC have any load factors of 1.25 or 1.5. Since you are using metric units maybe the Canadian or European code have different load factors.
Did your geotech give you any recommendations for what kind of loads you should use for seismic load to the wall?

 

[JrStructuralEng]

Hey Mike, 26" didn't seem right to me either, thats why I started this post. However, its seems like others are finding something similar. By the way thanks for the post Aggie.

To clarify on that part your confused about. Picture a simply supported ~8.5ft (or 2.5m) foundation wall, where the base of the wall is 40 ft (or 12m) below grade. Now consider a lateral load on the wall of 100lbs/ft^3 (or 15.7kN/m^)

The foundation walls for the parkade are stepped. This is for a varitey of reasons, i.e. parking requirements and building size restrictions

Roof
-----|
4th |-----------Grade
-----|
3rd |
-----|
2nd |
-----|
Main| Top =100lbs/ft3(31.5ft)(1ft)=3.15kips/ft
---------------|
Park | <---wall in question
---------------|

Bottom = 100lbs/ft3*40ft*1ft= 4.0kips/ft

I get a specified shear of 17kips/ft at base or ~250kN/m

 

[JrStructuralEng]

OldPaperMaker your comment:

"I would use code required 1.6 for the Live Load from the soil on the stem and 1.2 for the Dead Load of the soil on the heel & toe (if the wall has one)."

I am using Canadian code. What code are you finding 1.6 in? And by stem i'm assuming you mean vertical portion of wall.

 

[OldPaperMaker]

JrStructuralEngr,
I am referring to ACI318-05.Equation 9-2 which is written as:
U=1.2D +1.6 (L+H)where L=live load & H=load due to lateral soil pressure.
This is the same load combination found in chapter 2 of ASCE7-05 & Section 1605 of the 2006 IBC.
These load factors were 1.4D & 1.7(L+H)in ACI 318-99. I don't remember the exact year that they changed to the current values.
Yes,I was referring to the vertical section of the wall when I used the term "stem". I was thinking like it was a cantilevered retaining wall.

 

[gwynn]

For load factors for soil, check the latest National Building Code of Canada, the Canadian Foundations Manual or even CSA S6-06 (the Canadian Bridge Code). Unless my memory fails me, all three give explicit values for load factors for both earth pressure and hydrostatic pressure (hopefully these loads are seperated in the geotech report).They may even be repeated in the concrete design manual.

Also, by Canadian Code, shear can no longer be taken at a distance equal to "d" from the support. The distance has been reduced to 0.9d or 0.72t (where t is the total thickness), whichever is greater. This is also the distance you are allowed to use to resist shear forces.

It may be worth chamfering the corners at the bottom of the wall. It's a safe assumption that there is clearance between the top of the floor and the bottom of any car bumper. If this is permitted you may be able to get sufficient width at the top and bottom to avoid shear reinforcing.

 

[msquared48]

JrStructuralEng:

I assume that you mean your lateral soil load is 100 psf/ft of wall height. That load seems awful high to me - even more than water. Does the wall see pressures from nearby buildings too? Normal pressures would be in the range of 40 to 60 psf/ft of wall height.

Mike McCann
McCann Engineering

 

[hokie66]

Mike,

Lateral loads from soil below the water table are always more than just equivalent loading from water, because you have both the hydrostatic pressure and the buoyant soil pressure.

 

[gwynn]

msquared

I have seen similar soil loads recently. Some areas in nearby deltas have soil like toothpaste. Get it wet and it flows like water, it's just heavier.

Anyway, back to the original poster. It seems at first glance that you are using the "simplified method" for shear resistance. If you are running the rebar full height and the wall is simply supported at the bottom you should be looking at the general method. The low strain will allow for higher concrete shear resistance.

 

[hokie66]

JrStructuralEng,

Sorry I didn't answer your question sooner. The wall is just a vertical slab, so the reactions of the wall are supported by the floor slabs in the same manner that gravity loads of floors are supported by columns or walls. You wouldn't support a slab on the very edge of a wall with just reinforcement to develop the shear, so I suggest the same philosophy should apply to the wall support by the slab. In your case, the horizontal wall reactions are much greater than the vertical slab reactions, so should take precedence.

 

[DRC1]

the 100 psf for lateral load seems extremely high. Just because the material is below the water table does not mean you would have water pressure against the wall in the 100 psf load. Clays have undrained strenght that gradually transition to underained strength. In undrained loading, there is no computation of the water presure. with drained loading, there is a seperate load from the water and the soil. Further for shallow foundations, say 6 meters, the pressure is generally hydrostatic. For deeper cuts, it may not be as high, depending on method of construction. you need to review pressures with your geotech.
Vertical rebar can and does carry shear.
Water is evil. keep it away from your foundation somehow, even if it is with a drainage panel. Besure there is a place to which the water can drain.

 

[kslee1000]

DRC1:

For subgrade (enclosed) structure, it is dangerous practice to mininize the effect of hydrostatic pressure without positive drainage system, no matter what type of soil encountered. Also, for basement wall, at rest pressure pressure shall be used.