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






RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
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.
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
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?
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
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.
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
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?
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
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.
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
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.
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
Mike McCann, PE, SE (WA)
RE: PEMB Foundation - Hairpin Used in Combination with Footing-Soil Friction to Obtain Required F.S.
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)