one way shear in thick foundation raft 2

[chekre]

Hi guys,

I am designing a building foundation which consists in a thick raft with pedestals around the central core and columns. Do u check one way shear over the entire width of the raft or for every pedestal?

I generally check it over the entire width since that, if it will fail in shear, it will fail over the entire width, I don't see a mode of failure where one part will break and the adjacent one will remain ok !

 

[BAretired]

One way shear over the entire width would need to be checked, but the critical section could be oriented in any direction. For example, a pedestal near a corner of the raft might need to be checked on a section oriented at 45 degrees to the edge of raft.


BA

 

[JoshPlumSE]

I tend to look at the foundation similar to elevated slabs and define "column strips" and "middle strips". I check the 1-way shear for each of these strips. I believe the PCA notes gives a similar procedure for elevated slabs with some explanation about why you should check 1-way shear in the strip rather than just checking for punching shear.

Why extend this to foundations? Well, it's the same basic structure, just upside down. Right?

Granted, for "raft" foundations, you are not as likely to be controlled by shear as you would be for the much thinner elevated slabs. But, I would still caution against assuming that you can engage the entire width of the slab in shear.

 

[WARose]

For slabs I’ve tended to do the two-way check by hand and checked the stresses in my model for the one-way (which seldom controls). But doing it that way you are running into potential concentration factors and are also assuming a very conservative failure plane. If I get overstresses, a lot of times I’ll take that into consideration before upping the thickness. But it makes a good first pass.

 

[chekre]

I think checking one way shear for columns and middle strips is too conservative.
section 11.11.1 of ACI 318-08 states that beam action (one way shear) where each critical section to be investigated extends in a plane across the entire width.

That's why one way shear will rarely be critical in flat slabs.

 

[JoshPlumSE]

Chekre -

That's a new section of the code. It wasn't there in the 2005 code that I was looking at. Thanks for pointing that out... star for you.

I should get a new / updated copy of the PCA notes to see if that example I referred to has been updated at all. Because it was pretty clear in the older version of the PCA notes that they believed one-way action should be considered.

 

[slickdeals]

Please read one way shear discussion in Mat Foundations in this NEHRP publication (

http://www.nehrp.gov/pdf/nistgcr12-917-22.pdf)

Refer to Page 15 and Page 18.

 

[slickdeals]

I don't agree that you can check for shear based on full width of raft. "Entire width", I assume refers to bay width and not width of the entire slab. It is easier defined for a footing. FWIW, PCA Notes example and McGregor check it based on a bay width.

 

[slickdeals]

Josh,
The code requirement in 11.11.1.1 (318-08) was 11.12.1.1 (318-05)

 

[WARose]

Please read one way shear discussion in Mat Foundations in this NEHRP publication (

http://www.nehrp.gov/pdf/nistgcr12-917-22.pdf)

Refer to Page 15 and Page 18.

That’s how I’ve done it (i.e. as the article suggests, on p.15: comparing peak stresses to allowables; which is what I was talking about above as far as stresses go).

Interesting though: the allowable(s) they suggest fall into the range of 2*f’c^0.5 and 1*f’c^0.5. I’ve typically used 0.85*2*f’c^0.5. Perhaps I should re-think that.

 

[JoshPlumSE]

Re-reading that section in the 2005 code, I realize it doesn't define "entire width". Perhaps it means the entire width of the design strip, bay width.... It's not as crystal clear as it seemed the first time I read it. Thanks for setting us straight slickdeals.