PEMB Foundation design with hairpins 4

[Ben29]

Could you theoretically have slab leave-outs (for future plumbing) in the yellow highlighted areas of the foundation plan?

Assume that you do not have columns creating thrust in the north-south direction.

 

[WinelandV]

bones,

In ACI 318-14, 17.5.2.9 allows the design strength of the reinforcement to be used instead of the concrete breakout strength. Figure R17.2.9a is titled "Hairpin anchor reinforcement for shear" and shows hairpins with a requirement of ld beyond the breakout cone. Usually by the time enough floor steel is engaged, the length of the hairpin is far beyond ld.

All that to say, the hairpin is both resisting the global thrust AND preventing anchor breakout.

Please note that is a "v" (as in Violin) not a "y".

 

[bones206]

Understood. I was just referring to use of hairpins in PEMB foundations back the olden days before Appendix D.

 

[StrEng007]

The most guidance I've got for this subject is from Alexander Newman's book, " Foundation and Anchor Design Guide for Metal Building Systems". The author disclaims that hairpin systems should be limited to small utility buildings and prefers deformed bar reinforcement over welded wire fabric.

https://www.accessengineeringlibrary.com/content/book/9780071766357

As I mentioned above, they certainly are an option.
It just has to be the appropriate one given the loads you're dealing with. I agree with others, what works on paper doesn't always feel right though.

 

[jerseyshore]

My graphic above is from the old Butler manual which is great for looking at these elements of cheap PEMB design.

I do agree that it's probably not a good thing to use for anything bigger than the small buildings, but Butler and similar companies designed a million of these back in the day. Sending a few kips of thrust into the slab isn't a big deal IMO, but when you get those PEMB shops with tons of force at the base it is sketchy for sure.

 

[DoubleStud]

How else would you do it then? Grade beam going all the way across? Use much larger pilasters that will stick out outside the building?

 

[hokie66]

DoubleStud,

For large industrial buildings, a time proven practice is to use piled foundations where the piles take the thrust. Pairs of bored piles are my first choice.

 

[phamENG]

Without piles, though - yes to the grade beam. Though it's less of a beam and more of a tension tie. That's my standard detailing now. To actually comply with the building code for design and detailing of structural concrete, it's waaayyyy easier than trying to use the slab.

 

[jerseyshore]

Yes grade beam which is a defacto tension tie. And pham is right that it is the better option.

 

[hokie66]

I agree that tension ties are a good option. But often in heavy industry, they are not possible due to subfloor utilities, trenches, etc.

 

[WinelandV]

My personal preference is the moment resisting footing, despite it requiring (usually) so much concrete.

I find that when tension ties and grade beams are proposed, the preferred placement is so far below the slab that a fair amount of moment is still generated that you end up balancing out at the footing anyway.

Please note that is a "v" (as in Violin) not a "y".

 

[campbelltc]

For smaller buildings on shallow spread footings with decent bearing capacity, I just design the piers and footings for overturning due to the kickout forces. The only time I used hairpins was a very large building with very think and highly reinforced concrete slabs with a long distance between joints. And even then, the legs of the hairpins were very long. The owner of the building was warned of the hairpins and the consequences of cutting the slabs.

My first boss hated the use of hairpins. His thought was "If someone cuts that slab, what becomes of the tie forces?" And I fully understand his thoughts. Building owners won't consult a structural engineer just to "cut a slab." But they'll call once the slab cracks or the building moves when the hairpins get cut.