Upon reusing an older calculation, 3

[FL_Sem]

Upon reusing an older calculation, we noticed that the rebar provided for concrete breakout in a Seismic Design Category D region, is insufficient in quantity. The foundation has already been poured. Are there any mitigation techniques that don't involve tearing up the concrete?

Also, when designing anchor bolts for overstrength load combinations, does the ductility requirement still apply?

Thanks!

 

[BridgeSmith]

Was the calculation incorrect, or is it insufficient per a newer code/specification?

 

[BridgeSmith]

If you do have to confine the concrete, you could look into fiberglass-reinforced polymer (FRP) external strengthening.

 

[FL_Sem]

Drawings were incorrect, didn't reflect what was calculated correctly, and wasn't checked for that.

 

[canwesteng]

Can you give us a sketch? Is the rebar being used to intercept the failure cone of anchors?

 

[FL_Sem]

@canwesteng, yes it is used to intercept the failure cone. It has been developed in only one direction, and As quantity has not been increased for lack of development in the other direction from the failure plane.

 

[canwesteng]

Can you give us a sketch then? Still quite vague, is it a pier, slab, pilaster, what direction is the load.. etc

 

[FL_Sem]

See attached

 

[dik]

pretty robust... how large is your anchor rod and is it high strength steel? Unless there is something special happening, I could have missed it.

Dik

 

[FL_Sem]

@dik, the bolts are 2" 105ksi steel. Almost 500kip uplift load. So, the (8) #7 bars don't cut it as far as also including development length requirement on either direction of the failure plane. Plus, this is in seismic design category D site, and ductility of bolt isn't met either.

 

[WARose]

So, the (8) #7 bars don't cut it as far as also including development length requirement on either direction of the failure plane.

Just looking at it.....I'd agree. But with all that tie/confining steel, perhaps there is a chance you could (instead) try to make it work like a embedded RC column perhaps? (Assuming it's tied properly.) I did something similar once myself.

 

[FL_Sem]

Thanks, all. The way I see this design is:
1. To maintain ductility requirement, divide load by utilization ratio of anchor bolt steel.
2. If concrete fails in breakout, design rebars for the above calculated load.
3. Develop (in this case, Ldh, hooked bar development length) the bars both top and bottom of where the ~33Deg failure plane from bottom of anchor bolt meets the rebar.
4. If depth of concrete restricts development, increase rebar quantity by ratio of reqd Ldh/available Ldh.
It is the last bit of point 4 that hasn't been done.

 

[dik]

the bolts are 2" 105ksi steel. Almost 500kip uplift load

Thanks... would have really checked that one...

Dik

 

[FL_Sem]

@Dik indeed!

Can anyone think of a solution for this that doesn't involve tearing out the concrete? One of my colleagues suggested welding a plate to extend the exist base plate and add post-installed bolts to relieve the stress on the existing bolts and its dependence on concrete. I am not sure how that will work.

 

[WARose]

One of my colleagues suggested welding a plate to extend the exist base plate and add post-installed bolts to relieve the stress on the existing bolts and its dependence on concrete. I am not sure how that will work.

Sounds like a plan to me. If what is in the concrete won't work.....a extension is likely in order. You'd ultimately be sharing the load between the old and the new.

 

[sandman21]

How close to 500k are you? The rebar is good for 432k as shown, two legs transfer load to the bottom of the footing. One select one option from ACI 318-14 17.2.3.4.3, if you select d you do not need to comply with a. Is the 500k conservative? Can you lower loads by using the other options in ch. 17? Adding post-installed anchor will not help your condition. If you have access from the side of the footing, you could potential drill new anchor holes and bolt to a plate under the footing.

 

[KootK]

4. If depth of concrete restricts development, increase rebar quantity by ratio of reqd Ldh/available Ldh.

Gawd no. Two reasons:

1) In doing this, you're basically making your intentionally ductile rebar path non-ductile. Bond failure in rebar anchors isn't meaningfully ductile so the A_req / A_prov trick is baloney.

2) Looking at the failure planes involved, the rebar hardly makes any difference to the situation at all.

When working on these things, it's crucial not to bury your head in the sand and blindly follow the APP D stuff. A quick graphical study of the geometry involved should quickly lead one to the conclusion that the rebar provided in this detail is pointless. You essentially need to head down one of two paths:

1) Is the concrete app anchorage okay without the rebar, following ACI 318-14 17.2.3.4.3 option D? If so, you're done and no remediation is required.

2) If #1 doesn't work out, then we need to start contemplating serious remediation efforts that would likely not rely on the U-bars at all, developed or otherwise.

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.

 

[Agent666]

You're only going to increase capacity if you increase the breakout area, so any post installed anchors will need to go beyond the current breakout surface which is very deep. So it doesn't seem that practical.

If you put new bolts in close proximity of the existing you don't increase the breakout surface very much at all, you have to got much further from the existing bolts to make a significant difference to the breakout surface and hence the capacity.

 

[FL_Sem]

@KootK the concrete is well over 100% utilization for said load. Anchor bolts is just fine. So this is going to not be an easy fix, sounds like!

 

[CANPRO]

Could you use post-installed rebar to reinforce the breakout cone drawn by kootk? You’d probably need a lot of anchors and large embedment to develop the bar on both sides of the failure plane...but if the other option is to remove and replace, it might be economical.