Hairpin Design 2

[hdp321]

Has anyone else utilized more than one hairpin to resist large thrust loading? Most examples use one hairpin and this is fine for smaller buildings, but if the thrust is very high can two or three hairpins per column be utilized?

Tie rods cannot be used for this scenario.

 

[WARose]

I have.....mainly because the contractor (for some reason) didn't want a single #9.

 

[jerseyshore]

Of course you can. Not uncommon to see people using multiple #5 or #6 bars to minimize size and length. Here's an image I just found on google:

 

[dik]

Using hairpins, do other engineers treat the SOG as a structural slab, for ACI reinforcing limitations?

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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[hdp321]

I will need 3 rows and would like to also utilize the hairpins for shear resistance and pryout in the pier.

 

[Aesur]

While I'm sure you can use multiple hairpins, I don't see the point as 100% of the time the hairpin isn't the issue, the slab itself is. You need to develop the load into the slab and the amount of force you can develop into the slab is dependent on the geometry of the columns, how much slab you can grab before hairpins overlap (think concrete breakout and side face blowout with multiple anchors) and lastly can the slab distribute the loading across the building or grab enough weight to resist due to friction. I rarely have gotten more than about 10 to 15 kips capacity in a hairpin. I think many engineers seem to miss this concept as I have seen many designs with a single hairpin where the hairpin appeared to have been calculated based solely on the area of steel into an un-reinforced slab.

 

[hdp321]

There’s about 95 kips of thrust per column. The hairpins would need to be stacked to fit within the slab.

 

[jerseyshore]

95 kips holy hell; Aesur is right, the slab is the weakest link in that system and I don't see that working.

Even if you somehow got the math to work on paper I don't know if I would trust the slab and hairpins to even resist that type of thrust in real life.

 

[hdp321]

Any other suggestions? Design the pier and footing to resist?

 

[KootK]

Design the pier and footing to resist?

Most engineers seem to consider that the best, and most reliable approach. As usual, the issue du jour is the cost of that solution relative to the other alternatives. I think that we all like the tie member run across to the other side mechanically but, as you've mentioned, that usually runs into practical difficulties with respect to elongation, trench drains, future usage etc.

Any other suggestions?

I did some schematic work on one of these buildings earlier this year. It got me wondering if anyone has ever tried the scheme shown below as, kind of, an intermediate position. This would greatly improve the local connection near the thrust and provide a fairly reliable way to spread the tension tie load out across the entire width of the SOG such that, hopefully, there would be improvements with respect to elongation, reliability, and local interruptions in the slab on grade. For all the practical issues associated with a tension tie across the building, it's the most elegant mechanical solution by far.

Whenever I think that I've devised a better mousetrap, it almost always winds up being the case that I don't for some reason. I'm fully expecting someone to point out some deal breaking flaws with this conept.

 

[KootK]

Using hairpins, do other engineers treat the SOG as a structural slab, for ACI reinforcing limitations?

Nope, for better or worse. I believe that a true, concrete tension tie cannot be lap spliced according to code. Obviously, one does not see a lot of mechanical / welded lap splice in these setups.

 

[Eng_Struct]

I have not dealt with hair pins much but my initial thought would have been to try to lap the hairpin bar with the reinforcement in slab-on-grade (considering the slab is nominally reinforced). Laping the two reinforcement together, you would elimanate the concrete breakout/pry-out/pull-out issues and the mass of the slab would get engage via interface shear. The only limitation would be the spacing of the expansion joints as it will limit the amount of slab that will be engaged.

 

[jerseyshore]

StruEng1992, that's exactly how most hairpins are designed unless you have small loads. If you look at any design examples they all include the WWF or bars in the calculations to grab as much concrete as possible. I just think with 95 kips relying on hairpins only is a tough task.

 

[hdp321]

I agree with you jerseyshore, I'm not very comfortable using hairpins only for that much thrust. My design is for pier and footings to take everything, but of course the contractor had a heart attack because top bars may be required in the footing and the footing sizes are large.

 

[jerseyshore]

PEMB's and similar structures with thrust are a challenge for us engineers because unless a contractor does them a lot, most people will have no idea what you're doing and question every step of your design.

Hairpins? Heavy mesh or #4 bars EW in a SOG! Large footings and piers and crazy reinforcement!! I just built a 3 story building and it was less than this. This thing is just a light tin can storage shed. You could land a plane on this slab and foundation...

I try to be very upfront when starting or discussing these types of projects. I try to describe approx what the final product will look like so it's not a total shock when they see an "unconventional" foundation plan.

I haven't worked on a project with that type of thrust before, but for people who do deal with high loads, do you/ can you combine the hairpin capacity with the footing capacity to use them to work together?

 

[dik]

I'm not sure... given enough lap length, including staggered splice locations, I'm pretty confident they work.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

I've done the lapping of hairpins with the slab reinforcement. I've also done tension ties (usually poured before the slab, and with a bond breaker over the top, all bars mechanically spliced). I've only dumped it into the footings on tiny little buildings - it's crazy expensive and, since our frost depth is only 8", a "typical depth" footing gives very little passive resistance and friction is near zero.

Usually, the hairpins are just anchor reinforcement. I design them as Aesur mentioned - not to resist the thrust as part of the global load path, but just to make sure the thrust doesn't blow out the edge of the slab before it can get to the place I want it to go.

Options are essentially limited to a moment resisting footing/pier or a tension tie in or below the slab.

 

[hdp321]

Yes, the math and the design are the easy part. Convincing the client that you aren’t over designing the system is the harder part. The “I’ve never seen footings this large” comments are annoying when they don’t have the slightest clue what thrust is or how it’s resisted.

 

[Aesur]

I'm not sure... given enough lap length, including staggered splice locations, I'm pretty confident they work.

I believe what he is referring to is how the code says "Splices in tension tie members shall be made with a mechanical or welded splice.."

I like to remind clients that a PEMB isn't any cheaper than a conventional building, the difference is the structure itself is cheaper, but pushes most of the cost to the footings. I luckily work with pretty good and understanding contractors when doing these and rarely run into any issues with the contractor questioning the footings.

@hdp321 - out of curiosity why can't you utilize tie beams?

 

[dik]

...apologies, Koot... thanks, Aesur

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik