straight headed anchor bolts vs bent AB's 1

[jimstructures]

1. Where in the codes does it say you can specify staight headed anchor bolts over bent anchor bolts? I know the research is in favor of straight headed bolts but I am dealing with some troglodites who say if it was good enough for my (you choose the person) its good enough for me or we have been doing this for 60 years and I am not going to change. I am looking for the research that supports this.

Jim

 

[paddingtongreen]

Are these the same troglodytes that taught you how to apply your, college gained, booklearning to designing useful things?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JAE]

I don't know of any restriction on bent AB's but in ACI Appendix D they do include the straight along with the bent.

I believe there is a bit of a reduction to the bent tensile capacities however. I'd start with Appendix D.

 

[paddingtongreen]

I don't know the present state of things, but until fairly recently (I realize that "recently" is in the eye of the beholder), we got a value for the hook and then added the value of the bond on the straight bar.

Walking away from that, intuitive, anchorage after years of using it successfully is difficult. The hook helped in the setting of the dowels on the main reinforcing, too.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[steve1]

Here's a link to some AISC information on the subject

http://www.aisc.org/DynamicQuestion.aspx?Grpid=6&QueId=1834&ste=_Preview&id=2100&type=0

 

[steellion]

FWIW, our company standard is straight anchor rods only, no hooked rods. This is the way the industry is moving and in line with AISC recommendations. That said, I don't believe there is anything code-wise preventing you from using hooked bars.

 

[Lion06]

I don't know of any code provision that prevents it, but AISC recommends against it for an anchor rod subject to tension. The rod has a tendency to crush the concrete at the bend and then straighten and pull out.

We never allow a bent anchor rod. If they come in bent on shops, we send it back R&R asking for straight anchors with nuts per our typical details.

 

[paddingtongreen]

This started me thinking, we used to rely on the hook and not consider bond because of the smooth surface of the rod. For bigger bolts we threaded both ends and double nutted an anchor plate on the rod.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[msquared48]

Me too Michael. I think I will change my callouts in the future. Thanks.

Mike McCann
MMC Engineering

 

[jimstructures]

Thanks to everybody who answered my post and special thanks to those who gave me leads to check for sources to review. I am looking for citations for original research as well as any code instructions. I need to reread ACI 318 App D as well as th AISC citations mentioned.

I understand the predictions for the load straightening the J bolt and pulling it out. My dissenters are saying that in 60 years of company history and hundreds of buildings it hasn't happened.

I do know that in 17 years of practice I have only seen 2 buildings that pulled columns out of or off foundations. One from an explosion of magnesium dust in the plant and one from a direct hit by a tornado. I believe the bent anchor bolts can pull out easier than the straight bolts.

I did see pictures of another case where the contractor who placed the foundation used 5 gallon buckets with the anchor bolts pushed into the concrete to provide the piers. In that case the building only lasted until the first decent wind (no where near a design event) and promptly fell over.

Again thanks for the responses.

Jim

 

[aayjaber]

Does the shape of the bend have any effect on reducing/increasing pull out?

It is reasonable to assume that a short semi circle or a short section at the end of J-Bolt will facilitate crushing the concrete and ease the pulling out.

How about bigger bends? let us say converting the J-bolt to a large L bolt where the lower part of L is long? How about bent threaded bolts?

Without real data and proper experiments to support the claims it is hard to believe that straight is better than bent in absolute terms. May be the kind of bends we took for granted is where the problem and not the concept of bending the bolt or the rebar.

 

[IJR]

Pals

Lets talk UNHEADED straight bars or LONG HOOKED bars.

Intuition says you will have a hard time pulling out an UNHEADED straight bar out of concrete if the bar is well embedded into the concrete. Current research IMPLIES that there is no calcs to support that intuition. But AISC says well, if your headed anchor is too short, go ahead transfer the load to the straight reinforcement ensuring development length.

There I get confused, why reinforce the insufficient HEADED with an unheaded straight or hooked.

I guess the whole thing against straight unheaded bars or bent bars has to do with incomplete research

respects
IJR

 

[Lion06]

IJR-

Developing the straight, headed anchors into the pier reinforcing is invaluable and I don't see anything counterintuitive about it. This will only get you out of one failure mode check in App. D. (which is the concrete breakout), you still have to check all other failure modes.

For the headed anchor, we assume all load is transferred to the concrete at the head. If you develop the rebar above the plane of the anticipated failure surface then you've taken that failure mechanism out of the picture. What about this doesn't sit well with you?

The difference between a straight, headed anchor and a rebar is that a straight headed anchor is assumed to have all the load concentrated at the head, while the rebar transfers the load along its length.

 

[dik]

I can see if a bar is tensioned that there can be a progressive loosening of the bond at the load interface. The tensioning can be a result of uplift or shear development.