Foundation Design - Strip footing for a series of column in frame structure

[Tahir Khan]

Hello, 
         Good evening to everyone. I am designing a strip footing for two storey building. I prepared the model in etabs. I got the support reaction and from that i calculated the strip width = 6'
When i made a Shear diagram using the support reactions (acting downward as point loads) and soil pressure (acting upward as UDL) i got value of critical shear = 108 Kips and one way shear capacity = phi*2*Sqrt(fc')bwd = 68.5 kips for? foundation thickness = 15 inches ... is that right, i mean do we need to calculate one way shear in such cases ? and i got some abnormal kind shear force diagram why is that so? in some point it only goes upward (positive values)  - anyway where i am getting wrong? 
Bearing capacity = 0.6tsf = 1.32 ksf

[UmarMakhzumi]

Tahir,

Is your building a frame structure or a confined masonry structure?

Anyway, I don't see any mertis of providing strip foundation under a frame structure. You should go for deeper isolated foundations and if they are overlapping, then go for a thicker raft.

Thanks.

 

 

[Badar (BAZ)]

You are getting unexpected Shear force distribution because, I believe, you have assumed  the uniform distribution of bearing pressures under the strip.

Find out the resultant  of series of point loads, and then provide the dimensions of bearing area in such a way the geometric centroid of the bearing area coincides with the resultant of series of point loads.

If you can't do that, then do the finite-element-modeling.

[WR1]

Share the diagrams you calculated? What do you mean the shear force goes only upward? You mean like a cantilever in isolated footing? If yes, then strip footing is not a cantilever but like a continuous frame.

And one way shear, yes one-way shear could also be a problem (bigger problem than the 2-way punching shear) even in isolated footings. Dont under estimate it. For example if the bearing reaction too huge.

[UmarMakhzumi]

Tahir,

I missed out the attached image while replying earlier (on cellphone 3:00 am in the morning). I was assuming your layout to be a rectangular frame one where columns are way apart. Anyway,  with reference to the attachment provided, your certain columns are very close. If I were you, I would have the centre three columns on 1 combined foundation and right two columns in another combined foundation and keep the other three as isolated foundation.

5 hours ago, BAZ said:

Find out the resultant  of series of point loads, and then provide the dimensions of bearing area in such a way the geometric centroid of the bearing area coincides with the resultant of series of point loads.

This would be correct if your strip/ combined foundation can be considered rigid. You need to have enough foundation depth with respect to spacing between columns. If you do like I said, and use this approach for sizing combined foundations, things would work out as columns are closely spaced and the assumption underlying use of geometric centre is satisfied.

Thanks.