Shear verification of pedestal and mat foundation 1

[geo321]

Hi,
I am analysing a foundation mat with the columns, being on the same axis, are being supported by a thick pedestal (thickening is from above).
I am checking the one way shear and appears that at at d away i am out from the pedestal area and located in the thinner raft.
How do you check one way shear in such case ? What thickness do you assume ?
Thank you

 

[geo321]

Ant comments please ?

 

[WARose]

I'm a little confused as to what your situation actually is......but in a mat or a elevated two-way slab, I typically start considering the shear as one-way once you get outside of the two-way failure perimeter. It's a bit tricky as to what you will call your failure plane. (Obviously a short spike due to a stress concentration isn't going to blow you out.) But I typically average the stresses over a reasonable width. (Maybe a width of "d" if you are away from a support.)

 

[geo321]

Warose thanks for the reply. I attached a sketch explaining my situation. As mentioned I am averaging over the entire panel and not taking the peak stresses. I would like to know how do you deal with one way shear and what is the thickness u adopt

 

[WARose]

I don't know if you are doing this by FEA or with hand calcs but....for FEA, I just compare the shear stresses to the allowable (i.e. Φ2√f'c; with a slight increase in the shear stress output to account for the fact you are typically modeling the full thickness, but for shear consideration, "d" is the thickness to use). For hand calcs, I would pick out a section with elevated soil pressure (near the column) and check the one-way there. Sort of like you do it in a spread footing calculation......but you have to pick your "footing" out of the mat.

 

[JoshPlumSE]

This is just like an inverted drop panel, right? Where there are two thicknesses, ts (slab thickness) and (td) drop panel thickness.

I would suggest that two way shear (at d/2) would probably control. Then you'd check punching shear in the drop panel at d/2 (based on the thickened area) from the face of the column. Then you would check punching shear in the slab at d/2 from the face of the thickened area (with d based on the thickness of the slab).

If you want to check one way shear, then go ahead and check it as well. But, it shouldn't control. If d ends up being in the slab itself (rather than in the thickened region) I would suggest that the thickened region is really a short pedestal. And, I would check one way shear at d from the face of the pedestal.

 

[geo321]

OK JOSHlump. thank you. i was thinking of the same thing (a drop cap below a column) but needed some confirmation.
thank you once again.