Lap Splicing #11 to #14 Bar 1

[structeng1985]

Hello, I know that most codes limit lap splices to #11 bars and smaller. I'm wondering whether splicing #11 bar to #14 bar such as that between a column and oversized shaft, with a non-contact lap, is considered a #11 bar lap splice and thus allowed, or a #14 bar lap splice?

 

[KootK]

I assume that lap splicing is prohibited for the larger bars because the force developed in the bars grows with d^2 whereas the bond surface only grows with d^1. At some point, bond stress just can't keep up. Based upon that, I feel that it is acceptable to do the lap splice that you are proposing based on the logic that the force you will develop in the #14 is no more than that which you can develop in the #11 (for you to verify based on your situation).

In all likelihood, the more important thing is probably the presence of lateral ties that would complete the strut and tie design of the offset lap if you chose to look at the problem that way. I consider this to be one of the better, contemporary articles on that subject: Link

 

[BridgeSmith]

I agree with KootK's logic of the force being transferred not exceeding the tension capacity force of the smaller bar. I believe there's a statement to that effect in the commentary of the AASHTO spec. For smaller bars, per the AASHTO spec provisions, the lap length when lapping different sized bars is the shorter lap length of the bars being lapped. So I think you can lap them and use the lap length for the #11 bar.

Edit: In looking for the comment I referred to, not only didn't I find the comment I believed was there, I made a rather surprising discovery, or rather a non-discovery. In the current version (9th Ed.) of the AASHTO spec, I can only find the statement about using the smaller lap length when lapping different sized bars in the compression lap section, but not in the tension lap section.

 

[structeng1985]

Perfect, that all makes sense to me. I'll likely use the STM for the offset lap like you suggest and look for commentary in the AASHTO spec. also. Thank you.

 

[bugbus]

I assume that lap splicing is prohibited for the larger bars because the force developed in the bars grows with d^2 whereas the bond surface only grows with d^1.

But this is why larger diameter bars have proportionally longer lap lengths. So really the bond surface is proportional to d^2 also.

I believe the reason is simply that there is a lack of experimental basis for lap lengths of large diameter bars.

The Australian code allows bars up to 40 mm to be lapped, which is pretty close to your #14 bar.

 

[KootK]

But this is why larger diameter bars have proportionally longer lap lengths. So really the bond surface is proportional to d^2 also.

I see your point, thanks for making it. I think that the situation is made more complex by the fact that:

a) bond stress is not expected to be uniform along the length of the lap.

b) development / lap performance is often limited by slip behavior.