Pier-Slab Questions 2

[alphanumericname]

Hello,

I am designing a pier-slab moment foundation to support an 85' pole that is loaded in two directions. The pier is 6'x6'x11', the slab is 20' x 20' x 3', and the slab will sit 10.5' below grade on excellent quality limestone. The excavated soil will be backfilled with gravel. It will have foundation design loads as follows:

Load Type	Maximum Load
Axial (incl. self weight, overburden, and loading)	742.15 kips (down)
Moment (X)	2232 kip*ft
Moment (Y)	2459 kip*ft
Shear (X)	40 kips
Shear (Y)	37 kips

I do not have access to any software to assist in this design, instead I am using an article from a 1927 engineering publication that was given to me by a coworker. I could not find the article with a cursory google search, but I've scanned a picture of the loading regions (see below) and it falls into the "Case II" loading category, which results in one corner being in uplift.


The geotech report for the site we are working in assumed we were using piles, and they gave us a tip strength of 8ksf, which I am taking to be the general bearing capacity of the rock. With the load being outside of the kern, and the loads/slab being so large, should I consider anything other than the bearing capacity of the rock? Thanks.

 

[HTURKAK]

..........but I've scanned a picture of the loading regions (see below) and it falls into the "Case II" loading category, which results in one corner being in uplift.

ex = My/P= 2459/742=3.3 ft
ey = Mx/ P=2232 /742 =3.0 ft

"Case II" is True..
You are expected to check the bearing stress , sliding check , and overturning.

The geotech report for the site we are working in assumed we were using piles, and they gave us a tip strength of 8ksf,

Ensure that the proposed strength of 8ksf is maintained at 10 ft below.

I found also an old graph to locate zero pressure line. Year 1927.. From the book FOUNDATIONS OF STRUCTURES ( By DUNHAM , 1960)

 

[alphanumericname]

ex = My/P= 2459/742=3.3 ft
ey = Mx/ P=2232 /742 =3.0 ft

"Case II" is True..
You are expected to check the bearing stress , sliding check , and overturning.

Ensure that the proposed strength of 8ksf is maintained at 10 ft below.

I found also an old graph to locate zero pressure line. Year 1927.. From the book FOUNDATIONS OF STRUCTURES ( By DUNHAM , 1960)

Thanks. I checked bearing stress and overturning (I forgot to include this in the original post), but I was not thinking about sliding. I'm assuming this is an instance where I need to utilize passive earth pressures. For overturning, I broke the area of the pullout cone that will be resisting uplift into manageable shapes and determined each resultant load and the subsequent induced moment. I'm going to use this, as well as the moment induced from the loads and self weight of the foundation, to calculate the resultant overturning loads. Is a factor of safety of 1.5 appropriate for overturning capacity?

 

[HTURKAK]

Is a factor of safety of 1.5 appropriate for overturning capacity?

If SF > 1.5, it is OK for service loads.

 

[alphanumericname]

If SF > 1.5, it is OK for service loads.

Great, thank you!