Metal Building Horizontal Reactions

[JimWebb1224]

How big of Horizontal Reaction can be withstood by a #6 Hairpin around the Pier Cage x 20' Long? I'm seeing numbers like 33K Horizontal Reaction. The metal Building Supplier will use a 4 Bolt Pattern for Anchor Bolts. I believe they are calling for 1" Dia. Bolts. I had planned on using a 24 to 30 times the dia. calculation to determine the final a.b. specification. That will embed them in the pier cage (I was planning on 4 each #6 Verts w/#3 ties at 12" o/c). The Grade of the Reinforcing Steel would be Grade 60.

 

[dm3415]

The way I approach those calcs is to figure the friction of the slab on the base below. Don't use visqueen if you use this hairpin design or you lose your friction. The hairpin engages a certain amount of the slab around the hairpin and the weight of the slab reduced for the appropriate load combination times friction should equal your horz. force. Be mindful of control/construction joints too. You must also check the tension capacity of the rebar. I've never seen it happen but you can check concrete crushing at the area the bar makes its hairpin around the anchor bolts.

Another method if you have columns on opposite sides of a building lining up is to build a grade beam with the hairpins in a 90 around the pier. You figure the development length of your reinforcement and run the hairpins past each other in opposite directions. Your outward thrust is then counteracted by the opposing footing.

 

[JimWebb1224]

I guess I'm unsure of how you compute "friction".

If I use a #6 Bar with 20' feet of embedment bent @ 45 degree angle to the pier I would get a Floor Area of about 100 SF. That would mean the 6" Slab plus the extra concrete poured with the Slab could probably withstand about 9 or 10 Kips. Is "friction of 2 too much to use?

The Grade 60 reinforcing Steel 20' long should be able to withstand 100,000 lb. load? (42,000*1.75/12*20)

 

[dm3415]

Remember from basic physics class: Friction force = coefficient of friction of concrete on gravel (or whatever your base is) x weight of concrete slab or any other dead load. You find the required area of dead load and lengthen your rebar to get that much area. Width of slab to engage is at your discretion, I would keep it under 5 feet.

You can find the coeff. of concrete on base in any soils literature or from your geotech, (.35 to .75) is in the ballpark. You would never have 2.0 as a friction coefficent, that would mean the friction of a 1 lb block of concrete on anything would have a friction resistance of 2 lbs.

Again, you will have to reduce your dead load per the ASCE load combos for seismic or wind (.6DL or .9DL).

For the rebar; if your phi from LRFD design is .9, you factor your loads and find if phi x 60ksi x area of #6 (.75in^2) is greater than the required lateral load
Check development length per ACI 318-05 or 08 sec 12.2.3 to make sure that doesn't control the length of your hairpin.

On the alternate option I mentioned, I meant the rebar does a 180, not 90 degree.

I just read the last statement you made, you should really talk to someone in your office or a more experienced professional, that would be a dangerous situation you're walking into.

 

[msquared48]

Twice the tension capacity of the bar, factoring in the reduction for trhe geometry of the hairpin.

Mike McCann
MMC Engineering

 

[dm3415]

That too, thanks Mike!

 

[jrh009]

Generally when I get to the 35k range or higher I start to look at using continuous tie rods, pier to pier. Rods threaded and connected with threaded couplers etc, or continuous rebar sized for the tension load pier to pier.

You seam to be at your limits for 20m hairpins especially if there isn't any primary slab reinforcing, which in effect can tie both columns to each other.

You can also use passive earth pressure from the pier and slab haunch if there is one. As a rule I don't use passive earth pressure in my designs as you never know what will occur in the future as to site works.

I agree with dm3415 that you may want to ask some one in your office to review your solution. 35k in a rigid frame is not a huge load, but certainly it needs to be properly accounted for.

 

[jheidt2543]

Do a search on "hairpin design" and you will find that this topic has been discussed a number of times, there are some very good discussions in the archives.

Two references that you can look for that have good discussions on this topic are:

"Metal Building Systems", by MBDA and Computerized Structural Design, 1980. This is out of print, but you might find it in a used book website.

"Metal Building Systems - Design and Specifications", 2nd Edition, by Alexander Newman, McGraw-Hill, 2004. Look at Chapter 12.

One concern I've always had it that if you use hairpins to resist the horizontal column reactions, then you need the floor slab in place! Many times the floor is not poured until the building is enclosed. This can be more of a problem in the northern snow country if you get a snow on the roof before the floor is poured (the snow load generally contributes more of the horizontal live load than the wind does - but not everywhere in the country).

Also, when using hairpins or tie-rods, you make the assumption that no one in the future cuts the floor slab out for a machine base and takes the hairpins or tie-rods with it!

I have also seen where the metal building is designed to sit atop of a wainscot wall (actually columns within the wall). Obviously, the column has to be checked for the horizontal loads above the floor too.

Just a few more things to consider and keep you awake at night!

 

[Stumpy]

I would never rely on hairpins. My preference is to tie opposite piers together with mild epoxy steel.

In my opinion the hairpins work "on paper" only if you have enough reinforcing steel in the slab to tie the piers together, and you need to use bars hooked into the piers to achieve that. No trench drains, machine foundation pits, etcetera.

Friction between the slab and subgrade is highly speculative; with a vapor barrier it is negligible.

In Iowa and Illinois, where I practice, frost depth is 42". Since it is good engineering to ignore passive pressure of any soils within the frost zone, ties are about the only real option left.

Engineers that allow hairpins are probably saved by the fact that the passive resistance of soils in the frost zone can be (temporarily) significant. Again, not good design and hard to defend in court.