Metal building foundation anchor bolts 3

[DoubleStud]

It is my first time designing foundation for a prefab metal building. I have a sample project from an old project at my firm, but I feel like the old project was not designed properly. The metal building has 15k uplift force. It seems the foundation did not have enough DL to resist this. I should probably make sure the concrete, soil above footer are 1.5X heavier total than the uplift? Also the bolts seem too weak to me. They were only (4)-3/4" all thread that are embedded 10" deep and spaced 4" apart. It seems to me these bolts need to rely on the corner rebars and need to be deeper than the development length of the corner vertical rebars. Am I right? Also the horizontal load (outward) is quite big as well. I think I need hairpin to resist this horizontal load. The old project did not have any hairpin into the slab. I just want to make sure I am right before I go question the old design. What do you guys think? I want to embed my bolts about 27" to go past the development length of the corner rebar. Is this an overkill?

 

[KootK]

1) It sounds as though you need a benchmark for what represents practical, current design within industry. To that end, I recommend the book below.

2) I would say that most of your concerns are indeed valid, at least to some degree.

I want to embed my bolts about 27" to go past the development length of the corner rebar. Is this an overkill?

3) Probably not. Some things to explore that might improve the situation:

a) This is not so much of a concern if your anchors could work in pullout breakout without the aid of reinforcing.

b) You might be able to offset lap the anchors and corner bars for only the actual demand rather than Fy. This is ambiguous since, in ACI, there is a prohibition on prorating development length for lap splices. I don't personally feel that it applies to this situation however. Others may well disagree.

 

[phamENG]

It is my first time designing foundation for a prefab metal building.

I'm sorry.

You're pretty spot on with all of your concerns.

Foundation dead load should be found using 0.6D+0.6W if using ASD or 0.9D+1.0W if using LRFD.

By lateral load, are you referring to tension in the slab (caused by dead, roof live, and snow loads on the rigid frames) or wind load that's positive on one wall and negative on the other? Or both? Hairpins are good for the wind loads and preventing side face blowout, but less so for the tension in the slab. That's because you're turning your entire slab into a tension element, which complicates joints and adds a lot of rebar (not to mention the scenario where the owner tells the contractor after the design is done not to put in the slab because they want to wait for tenant build out - I've seen it!). For that, it's usually best to have a discreet tension element below or integral with the slab.

Bolt embedment depths is nothing special - just do it the way you normally would. But be careful - the spacing is usually too close for epoxy/adhesive anchors, and the MBS manufacturers often provide standard holes in the base plates which makes for a nearly impossible tolerance requirement. Make sure that's clearly spelled out on your foundation drawings.

 

[JStephen]

Some of the design standards on anchors have been updated considerably in the last 20 years or, so the "old" design may not be incorrect as such, just not current.
On the 1.5 factor- that could come from load combinations or there may be requirements to anchor for the yield (or tensile) strength of the bolt, which may control.

 

[phamENG]

Good call on Newman's book, KootK. Forgot about that...

 

[DoubleStud]

a) This is not so much of a concern if your anchors could work in pullout breakout without the aid of reinforcing.

Can't do analysis based on Chapter 17 because the metal building column baseplate put the holes only 4" apart. It does not meet the 6 diameter spacing requirement. I feel like you have to use the reinforcing.

 

[KootK]

Can't do analysis based on Chapter 17 because the metal building column baseplate put the holes only 4" apart. It does not meet the 6 diameter spacing requirement. I feel like you have to use the reinforcing.

Surely, at worst, there must be some vehicle for treating the anchor group as a single, effective anchor for tension breakout. Slap an anchor plate on the bottom if you must although I try to avoid that when possible to promote easy concreting. That said, it is entirely plausible that you will need to use the reinforcing anyhow.

 

[phamENG]

DoubleStud - check the spacing requirements again:

...minimum center-to-center spacing of anchors shall be 4d_a for cast-in anchors that will not be torqued, and 6d_a for torqued cast-in anchors and post-installed anchors.

So as long as you're not torquing down the anchors, you can go closer.

 

[DoubleStud]

phamENG, thanks. I am showing my inexperience here, but what does torqued really mean? Tighten the bolts is not torqued?

 

[phamENG]

I am showing my inexperience here

Not really. It's not well defined. Some people take a hard line and say a nelson stud is untorqued, anything with threads is torqued. I take it as if it's not pre-tensioned (which is dubious in concrete anchors anyway), it's not torqued. If that were not the case, there would have been an out crying against these base designs a long time ago (or at least one would hope there would have been...).

 

[JedClampett]

I've lived through this many times, including very recently. The metal building designers are not going to do you any favors, particularly with regards to bolt spacing and edge distance. What I like to do (this is probably too late for you) is put the girts on the outside of the columns. This at least adds 8 inches to their standard inadequate edge distances. The concrete breakout, which still might not work, needs to be taken by hairpins. I also needed to specify Gr. 55 bolts as the mild steel 3/4-inch bolts didn't even work for shear steel strength (note that you have to check the bolts in pairs, per code).
As far as having enough concrete for uplift, once you get the bolts to work, figure out the moment capacity of your thickened edge and slab. See how much of the concrete that includes. Hopefully that's more than your uplift. You might have to apply a .9 factor, but 15,000 lb. of concrete is not really that much (about 4 cu. yd.).
I've been complaining about anchorage to concrete for PEMB for more years than I care to remember. It seems that their designers are intentionally oblivious to anchor bolt design. And like entropy, the problem just seems to get worse.

 

[DoubleStud]

Thanks KootK, I bought that Newman book. My design without reading the book was pretty identical. But now I feel much better after reading the book. As expected, you can basically ignore Chapter 17 because with small spacing and small edge distance, your anchor design for tension and shear will be very small and will not meet the design load. My columns are very close to the edge. The client does not want the pilaster to stick out around the perimeter of the building. So my outer bolts are only 3" away from the edge. So basically you need reinforcement to help with tension and shear. Spent too much time on this project but next time I have a similar project, I should be able to finish it really quickly! Thanks!

 

[JLNJ]

Maybe you can use the two inside bolts for the entire load, gaining yourself a little more edge distance.

Often the 4 bolts are there as a matter of course and not for strength, especially if you go with grade 55 bolts as suggested above.

 

[phamENG]

Maybe you can use the two inside bolts for the entire load, gaining yourself a little more edge distance.

I wouldn't do this. The bolts will still be there (they have to be for erection stability and manufacturer warranty purposes just to name two reasons) and so the anchorage group will still behave as though there are 4 bolts (because there are). Just because two by themselves would "work" for a particular load case(s) doesn't mean it'll automatically behave that way. And when the outer bolts fail, you're right down to that 3-4" of edge distance again...maybe less depending on how the concrete fails.

 

[jayrod12]

Outer bolts at only 3" away form the exterior of the concrete are ripe for clashing with the reinforcing. Where I practice, 1.5" outside clear cover, plus 3/8" tie, plus 3/4" horizontal bar, plus allowance for ribs, plus bend radius on the ties, plus half anchor diameter. I never put centre of bolt closer than 3.5" to outside face of concrete, and I only do that when I'm absolutely bound to it. I argue for 4" minimum so I'm not having to deal with "The anchor bolts are clashing with the reinforcing everywhere".

 

[Lomarandil]

For traditional anchor bolts, Pham is correct. You need to both check the near edge bolts for 1/2 the shear load, and the far bolts for the full load.

For welded studs embedded as anchors (uncommon for PEMBs) Newman suggests that JNLJs logic is sound. By the time a concrete failure develops, load could shift through the base plate and rigidly welded anchors to the back pair.