Use of U-Shaped anchor bolts at base plates 1

[sponcyv]

I am designing sound wall foundations. The sound wall manufacturer is showing anchor bolt sizes and configurations, but I am checking for anchorage to the foundations (drilled caissons). One wall particular has very high overturning forces and thus very high tensile forces. They are showing U-Shaped anchor bolts here. My question is does anyone know how to design for U-Shaped anchor bolts? Appendix D does not cover this. Side face blowout is for headed anchors and I'm not sure if this applies to the U-Shaped bolts. Picture attached.

 

[KootK]

I'd be tempted to treat the u-bolt as a pair of hooked anchor bolts. App D has provisions for that.

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.

 

[sponcyv]

Thanks KootK,

My thoughts were similar in that the crushing at the radius may control. A colleague of mine gave me the idea of placing 180 degree bent rebar looped over the straight portion of the U-Shaped anchor bolt. The idea is to develop it below the anchor bolts to resist the tensile forces.

 

[KootK]

I'm sure that the extra rebar would help matters. However:

1) It's tough to quantify that help and;
2) I don't know that you've eliminated the issue of crushing in the radius.

Do you have to use these U-bolts? I'd much prefer long, conventional anchor bolts with double nuts effectively lapped with rebar.

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.

 

[KootK]

I'd also prefer to use down turned hairpin rebar effectively lapped over the U-bolts. The tops of the hairpins would be at the top of your foundation pier.

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.

 

[sponcyv]

The amount of load on these bolts fails side face blowout using hex heads. Lapping with the rebar as far as I'm aware does not alleviate this. My understanding is that the anchor bolts don't really bond with the concrete and the anchorage is developed at the bearing of the hex head. The way the forces transfer to the caisson reinforcement is that the cracks that develop through the caisson reinforcement and the caisson reinforcement is lapped properly above and below the crack.

 

[KootK]

My understanding is the same.

If an alternate anchor bolt is permitted, I'd recommend going with threaded rebar. That increases the effective lap length required but eliminates the blowout issue.

Another option could be to use a stiff, rectangular bearing plate spanning between the anchor bolts. This is the same concept as the U-bolt but more convincing. The bearing plate could be a small channel if you need the extra stiffness. I guess this suggests another way to check the U-bolt. If the U-bolt could work as a bearing plate spanning between the vertical bolt legs, and concrete bearing stresses are okay, maybe you're good to go as is. You'd also need to do this check if you were using the hairpins that your colleague suggested.

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.

 

[KootK]

Is this connection purely tensile? Or is there a moment in the plane of the U-bolt such that you'd have tension on one anchor and compression on another?

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.

 

[sponcyv]

KootK,

Thank you for your input. The other option was a plate as you mentioned, but it is more costly. The actual forces from the wall panels to the post base plates are moment and shear. The moment resolves into the T-C couple and the tension is placed on two of the anchor bolts or one U-Shaped anchor bolt.

 

[oldrunner]

Have you run some numbers of what the forces are in the bolts? We've done many poles with tension and compression in the anchor bolts and I would basically stay with a traditional A.B. arrangement with long enough anchor bolts with a nut and template to transverse the pull out forces with shear-friction. I particapate on a A.B. committee and which will be tele-meeting on Thursday morning and we might discuss your detail. It would also be interesting to see the rest of the reinforcement at the top of the footing - like any extra spirals or hoops.

 

[KootK]

with a nut and template to transverse the pull out forces with shear-friction.

Do you mean that the blowout concrete cone is held in place by shear friction through the vertical pier reinforcement?

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.

 

[oldrunner]

Probably. But the A.B.s are pretty far into the interior of the pier.

 

[sponcyv]

oldrunner,

I don't see how this force is transferred via shear friction. The force will be transferred to the vertical caisson reinforcement once the concrete has cracked and the crack develops through the vertical steel. This places the vertical reinforcement into tension, not shear. Are you talking about a typical footing where the top horizontal bars would experience shear friction?

 

[BUGGAR]

I like the threaded rebar concept. Make sure you have the development length you need in the footing pier structure and then just make sure you stab them into the soil below your footing. Then call them through-bolts and APP D doesn't apply.

 

[dcarr82775]

Put rebar in to keep the breakout from occurring and extend the bolts far enough into the pier to get a not contact lap splice. Then Appendix D sulks in the corner unused and unloved.

 

[Leftwow]

We never use J bolts in our work. ACI code 318 doesn't like J bolts either. You get more strength from a headed anchor bolt tied into a rebar loop that goes down into the footing.

 

[hokie66]

U-bolts are a lot better than J-bolts. They don't eliminate the concrete crushing issue, but do eliminate the issue of bolt straightening and pullout.