load on strip footings 1

[tjhubo]

When determining the load on a strip footing, how do you consider the possibility of the load transfered by the basement floor? Some guys think we don't need to look at it. What's you opinion? Thanks.

 

[craigory28]

If you are speaking of a slab on grade, we typically do not show it bearing on the footings. We like to keep it seperated so it will move seperate from the footings.

If you are speaking of a structural slab (i.e. have some bad soils and are only providing fill at the strip footings.) as a basement floor I would definitely include it in the footing design.

 

[tjhubo]

craigory28,

Thanks a lot for your reply. I agree with you on the points you made. what I am concerning now is if we have a slab on grade with large span, maybe the settlement of soil between two footings would cause some extra load on the footings. I know it is less likely to happen because the settlement of the footing is usally large than that of the slab.

My point is if we need to check to settlement of the slab before we sperate it from the footing.

 

[npthao121]

tjhubo
I do not understand your idea well. Are you consdering to check the effect of basement slab load on the soil bearing capacity for your strip footings?
If the basement slab are not separated from the building structure (frame, tie-girder…) of course we have to count the basement slab load transferred to strip footings via grounding tie-girders. In this case we have load pressure under footing bottom, just check the soil bearing capacity to verify the whether it is satisfied or not.
If the basement slab are separated from the building structure, so the basement loads are not transferred directly to the strip footings. But we still have to consider this load as a surcharge. The actuall soil bearing capacity must be decreased by this load value. And then just check the strip footing with decreased value of soil bearing capacity.
Hope to help you.

 

[tjhubo]

npthao121

Your idea is very helpful to me. Would you like to tell me which reference I could follow to calculate the surcharge, or how to decrease the soil bearing capacity?

Thank you evry much for your help.

 

[npthao121]

Tjhubo,
For example your total load (deal + live + ....)on the basement is 1T/m2. so you just simply minus this value from your bearing capacity. let say your original capacity is 10T/m2 then just minus 1 and the remaining is 9T/m2. use this value to check your strip footing.
Hope to help.