Anchor Design: Concrete Breakout Code Comparison 1

[SixkHz]

I'm checking anchors subjected to tension for concrete breakout, according to the 2004 Canadian concrete standard CSA A23.3-04. I believe the anchor design guidelines are almost identical to those outlined in ACI 318.

The question I have arose when my boss gave me his old 1984 code (CAN3-A23.3-M84) as a reference. The concrete breakout resistance that I calculated for the anchor group based on the 1984 code was almost 2x greater than the resistance calculated according to the 2004 code.

I've attached calculations from both codes for comparative purposes. I'm hoping someone who is familiar with the codes can shed some light on why there appears to be such a big descrepancy in the breakout resistance.

Thanks in advance.

 

[sandman21]

2x seems alittle low, it appears to be based on ACI 318-05 App. D and UBC 1994. So yes ACI App D will give far lower loads than any of the old code equations.

 

[SixkHz]

So is Appendix D just really conservative?

I'm sure there is a better understanding of anchor failure mechanisms now than there was in the old codes. If these equations are more accurate, wouldn't any anchors designed according to the old codes be under-designed (theoretically)?

 

[sandman21]

Yes it is.

Depends on your definition of better understanding. lol

But theoretically yes you are correct.

 

[anchorengineer]

The main difference is you are designing for cracked concrete. Cracked concrete gives very large reductions. Re-run your calcs for uncracked concrete and they will be much higher.
You can also go to Hilti's website and download their Profis Anchor software. You can now design embed plates with studs! Very easy to use and fast.

As far as the old code being unconservative... the concrete has to crack first before it is uncracked / If the concrete is not in tension then you're getting the uncracked capacity... Embed plates on the bottom of slabs is where I would be concerned. Basically tension zones.

http://www.anchorengineer.com

 

[SixkHz]

Thanks sandman21 and anchorengineer for your help.

 

[rscassar]

Pardon my ignorance of the design code you are referring to, but where would you ever assume uncracked concrete (except for post-tensioned design)? The flexural calculations are based on a cracked concrete cross-sections and their is quite often restraint against temperature and shrinkage.

 

[anchorengineer]

kikflip - Compression zones. Top of slab, footings... If you have SDC's c-f you have to assume all concrete is cracked. A&B...judgement call.

http://www.anchorengineer.com

 

[sandman21]

You don't have to assume anything for SDC C-F. Uncracked concrete is when Mcr is greater than Mu, cracks are defined as anything over .012. Compression zones are a safe to assume uncracked, top of slabs, footings, walls, grade beams, these can develop cracks under service loading and then would need to be analyzed as cracked. Generally going from crack to uncracked is not going to make a huge difference, 40%, the ductile requirement is the real killer along with edge distances.