Eccentric looad on basement continuous footing

[nickky]

I have eccentric load on a continuous basement footing along the property line. The basement wall is all the way towards the property line and sits on the edge of the footing. I know of a method where some engineers try to introduce a counteracting moment by engaging the slab on grade with the wall, but I just didn't agree with the assumptions there. Anybody has any better suggestions as to how to counteract the eccentricity?

Thanks

 

[UcfSE]

Increase footing weight.

 

[youngstructural]

Just to think outside the square of your question, what assumptions don't you like? What you describe (taking some moment into the slab to offset the effect of eccentricity onto a foundation) is a well established design procedure. Perhaps we can offer some clarification or explanation to help you feel more comfortable in applying this technique...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[nickky]

Youngstructural,

Perhaps it will be a bit difficult to explain everything in a few line. but here are my problems with that method:

1- If I understood correctly, that method assumes a "rotational" mechanism while because of having the structural slab restraintment at the top the wall, no rotaion can happen to activate the "slab-on-grade reaction" to counteract.

2- Even if we accept the rotaional mechanism then It is not clear to me where will be the real hing axis for the footing to rotate. Is it the bottom corner or the geometrical centerline of the footing. (let's remember that we will have a little settlement on one edge while we have uplift on the other edge, means rotaion along the centerline). Now when we force the slab-on-grade to counteract what is the assumed point (or axis) of footing rotaion. (in other words along which axis we should calculate the moments?)

 

[msquared48]

I have to agree with Youngstructural here. I have used that approach for years with no problems.

I designed a large tilt-up building with a similar situation located on the property line next to a major railroad line with a downhill slope away from the footing to the rail line. Even with the vibration from the trains, the footing has held very well for the last 20 years. I've been watching it's performance over the years. No worries here for me with the design approach.

If you stll have concerns, just install concrete grade beams at 10 foot spacing or so back to another transverse grade beam line that intersect with your interior spread footings. Then you could take the torsional forces from the wall footing into the grade beam system. Perhaps that is a more direct approach for you.

Mike McCann
MMC Engineering

 

[asixth]

If the slab is cast in-situ with the basement footing, I don't see why the slab wouldn't stiffen the footing against twisting about its longitudinal axis.

If there was an isolation joint between the basement footing and the slab on grade then I can understand why moment wouldn't be put into the slab.

 

[slickdeals]

Using the slab is not uncommon. Facade attachments on perimeter beams on buildings often rely on the slab to transfer the moments. It is the same concept.

 

[youngstructural]

I can understand your hesitation regarding the rotation required to activate the soil force, and respect your caution when applying soil resistance. Many engineers forget to allow themselves .6% rotation for activating Ka active pressure; They should properly be applying Ko standing earth force pressure.... BUT, this is not an appropriate situation, at least as I see it, to be worrying about soil theory...

In this case I don't actually care how or where the rotation proper takes place, because I don't need it to activate the dead weight of the slab. What I do is consider the dead load of the slab as a cantilever away from the wall. Because your eccentricity is away from your buiding, this causes as LIFT on your slab, not a downwards rotation and thereafter soil pressure... Unless you can clarify further, I think you might have missed that one.

Also, we don't need to worry too much about the soil pressure due to the eccentricity other than under the footing proper, since without rotation (taken away by the slab's dead load) the remaining vertical load can be considered to act through the footing only.

Let me know if that's right for your situation, please. Hope the job's going well!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...