Free Standing Wall - Foundation near property line best options (foundation below slab)

[hssbrian]

Hello everyone,

Im designing a simple free standing 14cm cmu 3m tall wall, I need to bounce some ideas with anyone that has an insight or opinion on this matter. This wall will serve as an enclosure for an existing slab on grade due to several reasons we were asked not to modify the slab on grade and just add the enclosure around it. Although I understand that there are several ways to solve one problem and several things, to consider like cost and construction time, I would like to focus on design.. I have attached the existing conditions with two options.

Existing condition: You will be able to see two different details for grade beams at the ends of the slab, this is because they were built in different times.

Option 1: Using isolated footings at 3m o.c. with columns and a bottom beam that will carry the load of the wall

Option 2: Using a strip L shaped footing to not mess with existing

I have made an effort to provide as much information as I can think of, but if you think something else would be relevant please just ask....

Maximum out-of plane load is 0.3kPa (includes seismic)

What do you guys think of option 1? I feel a bit hesistant to dig up below the grade beam but Ill admit is such a small span that realistically it might not be as influencial.
What do you guys think of option 2? I feel a bit hesistant do to it relying mostly on the toe of the footing, although I know this is typical in certain places, I would feel better having a heel restraining out of plane loads
Do you have any suggestions? any typical design? details? or checks you might think I made need to do?

all rebars shown are 12mm

note: i also added a sample of option 2

Thank you in advance....

 

[hssbrian]

see attachment for sample calc option 2

 

[hokie66]

A version of "Option 2" would be my choice. But some comments: 1) The section through the wall doesn't show a 3 metre high wall. If it is that high, I would use 190 block. 2) For the direction of loading indicated, the vertical bars are in the wrong face. 3) The wall would be subject to loading in both directions, I think. 4) 0.3 kPa is very light on for loading, even for wind in places that don't have much wind. 5) Why the mixture of all those different units in your calculations? I wouldn't even attempt to review them as presented in that fashion.

 

[hssbrian]

hokie66 regarding your comments:
1) I agree 190 would be my choice, locally though these are not available
2) good catch regarding the vertical bars, i did not add them on the sketch but it will be on each face
3) loading in both directions is correct but controlling will be as represented in the calcs, i believe
4) as it is a free standing wall per UBC1997 en ASCE for non structural I used for seismic 0.4*SDS*W*I/R per local code max wind of 45mph hence very small load
5) due to it being a project in a metric using country (myself being familiar with USCS) you will notice imputs mostly in metric while calc results in USCS for me to understand it.. these are not final calcs or for a report just quick checks btw

Please let me know if you have anymore comments, I appreciate your help
Thank you

 

[KootK]

Although I understand that there are several ways to solve one problem and several things, to consider like cost and construction time, I would like to focus on design..

I'm going to risk your wrath and disregard the statement above. It's almost meaningless to discuss something like this without considering cost and constructability.

My first stop would be to see if I could justify just building the wall on top of the existing slab thickening. It probably won't work but I'd investigate it to be sure as that would be most economical solution.

My second step would be to try to make something like the detail shown below work. The wall is only 10' after all.

One thing to recognize is that, for many soil types, even installing the L-shape footing is going to result in some undermining of the existing slab thickening. That's why I've suggested a bearing ledge of sorts below. That way, the new footing can prop up the slab edge to some degree.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

And if the numbers on the above detail still don't work, maybe something like this.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hssbrian]

KootK, I will try to address what youve suggested:
1) I agree with you when mentioning that constructability and cost play a major role here, but because of circumstance and this project in particular, it seems hard to get the owner to agree
2) Did try what you recommend as checking first, they do not want the wall in the slab thickening and it doesnt work either
3) Actually your drawing shows what I decided to recommend and this didnt go as well with others hence we were highly encouraged to look for other ways...
4) you bring up a good point and might incorporate something for undermining

thank you for the great suggestions... it is a "simple" problem but due to owners wishes we had to look for other ways..

 

[hssbrian]

KootK, is there a reason for your second option you are still using an epoxy dowel???

 

[KootK]

I agree with you when mentioning that constructability and cost play a major role here, but because of circumstance and this project in particular, it seems hard to get the owner to agree

Odd. If cost isn't a priority for the owner, what are the owner's requirements here? If cost is no matter, drill some piles down deep and throw a meaty grade beam over top of them. That'll work.

KootK, is there a reason for your second option you are still using an epoxy dowel???

Belt and suspenders approach that:

1) Also helps to suspend slab on grade from retaining wall if undermining occurs,
2) provided another path for the overturning loads to go and,
3) Keeps a gap from opening between the retaining wall and the slab thickening.

The dowels aren't strictly necessary. If it's a problem, lose 'em.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

hssbrian,
You indicated above that you would use bars in each face. That is not practical in 140 blocks. If they don't work centrally placed as KootK shows, the wall doesn't work, but it should with the light loading you specified. Make sure the mortar droppings are cleaned out at the bottom so the bars adequately lap with the starters. With the L shaped footing, I see no need to dowel into the existing slab, as the lean will be toward the slab.