PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S. 2

[waytsh]

I know the topic of PEMB foundations has been covered pretty heavily in this forum but I couldn't find this topic specifically addressed. I apologize if I missed it and this post is redundant.

Is there any reason that hairpins in the slab could not be utilized in combination with sliding friction under the footing to achieve the required factor of safety against sliding? Disregard passive pressure at this point since I understand there is some controversy about utilizing sliding friction and passive pressure together. For instance. assuming sliding friction under the footing achieves a factor of safety against sliding of 1.0 could hairpins into the slab be utilized to achieve the required 1.5 assuming they meet all the strength requirements, slab configuration, etc.

 

[Ron]

Yes...hairpins can be used to engage the slab; however, you will have to decide just how much area of the slab the hairpins engage for your friction calcs.

 

[waytsh]

Just to clarify, I will be using the friction under the footing not under the slab. I will be tying across the building with the slab and will not need to utilize friction under the slab to achieve the extra factor of safety. I don't see any reason I can't use these two resisting forces together but I wanted to make sure I am not missing something. Thanks for responding.

 

[Ron]

Whether or not you consider the slab friction, the slab mass will add to the footing mass depending on the influence area of the hairpins.

 

[KootK]

I'd say that this would depend on your details:

1) What is the elevation difference between top of slab and underside of footing?

2) Beyond the hairpins, what is the mechanism for resistance? Friction between SOG and soil? Tension tie to the other side of the building?



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[waytsh]

The elevation difference between the top of the slab and the bottom of the footing is 64". 40" tall pier with 24" thick footing.

The footings were designed to be independent. The intended mechanism for restraining sliding was supposed to be shear friction at the base of the footing and passive pressure strength of the soil using pretty conservative values and neglecting the frost depth. The only problem is a factor of safety was missed on the controlling load combination for sliding. At this point hairpins can still be added to tie into the slab and increase the restraint. The required factor of safety can be achieved with the friction at the base of the footing and hairpins; neglecting passive pressure altogether.

Thoughts?

 

[KootK]

Given the elevation difference, I feel that the hairpins and would be the stiffer load path and resist most of the shear initially. If the hairpin load path can yield in a ductile fashion and redistribute the subsequent shear to the footing, then I think that you're okay to sum the capacities. If the hairpin load path will fail in a brittle fashion, I'd be skeptical of you being able to develop both capacities simultaneously.

Can you not use friction and passive resistance together if you're willing to tolerate some significant movement at the ultimate limit state? I struggle with this aspect of foundation design. The Canadian Foundation Engineering Manual specifically cautions designers regarding using multiple mechanisms at once. But then a lot of commonly employed designs don't check out if you limit yourself in that way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[waytsh]

I see your point regarding the stiffer load path. I suppose the elongation of the reinforcing steel in the slab and the hairpin itself could yield. Of course there would be some cracking in the slab but the likelihood of this maximum load is low.

Passive strength and shear friction are not enough by themselves to obtain the 1.5 S.F. Unless you are talking about combining the passive, friction, and hairpins all together?

 

[KootK]

I was only thinking of combining passive and friction. I mistakenly thought that would have been sufficient.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CarlB]

What's the rationale for neglecting frost depth for passive pressure?
When frozen, the passive pressure (extent of soil engaged) would increase.
The only condition I can think of that would decrease PP is when soils are thawing, the thawing interval can have excess moisture and be "softer" for a period. I can see reducing the soils friction angle thus PP value in that interval, but include its mass so not reduce PP in the underlying soil.

 

[waytsh]

I'm not a geotech but every time I have asked about this they have told me I need to neglect it. I can't tell you why. I have just been blindly trusting their recommendation. Since there was no getoech report on this project I was trying to play it very conservative.

 

[msquared48]

I think the idea is to bear laterally on undisturbed soil, and frost penetration "disturbs" the soil, increasing the volume.

Mike McCann, PE, SE (WA)

 

[msquared48]

Personally, if the hairpins ever get beyond (2)#5's, I go to a direct tension tie across the bottom of the mainframe below the slab.

The first line of defense is the hairpin/tension tie. The dead load of the slab developed in friction will try to resist the spread if the slab develops a vertical crack from the tension somewhere. The friction can only be engaged if the slab cracks and lateral movement is precipitated. Then the passive pressure comes into play as the spread increases to eventually (hopefully) reach equilibrium.

That's the way I see it.

Mike McCann, PE, SE (WA)