Short Anchor rods (again)

[mjs611]

I know there are a few threads on short anchor rods but I have a slightly different situation. The anchor bolts are 'short' but there is still ~2.5 threads available above the top of the nut. The EOR is stating that we need to core drill out the existing anchor rods and install post installed anchors. There is information on using couplers, anchor welding, ext but the EOR is stating that the 'rule of thumb' is a minimum of 3 exposed threads above the nut minimum. AISC DG1 states you can get away with a minimum of 50% of thread engagement as long as you can prove through analysis that the engage threads resists the applied load. In my case, I have 100% thread engagement but admittedly the projection is less than I would typically specify. The column weight is less than 10 kips with a little bit of shear and moment.

The EOR's fix seems like A LOT of work over 1/2 a thread less than his rule of thumb and extending the anchor by welding or coupling doesn't make sense given there is full nut engagement. Thoughts?

 

[KootK]

I agree. Also, if I were the EOR, I'd feel a lot better about the connection as is with 1/2 a thread missing than I would about the post installed anchor repaired connection. It's a shame when compliance paranoia overrides good judgment regarding performance. Still, if you're out of spec then I believe that the EOR is within her rights to insist.

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.

 

[canwesteng]

0 threads are required to project past the nut. If the EOR doesn't have 3" of exposed thread written in his specs somewhere (doubt it), ask for a change order for the work.

 

[sandman21]

According to AISC, 15th ed. page14-10, anchors are required to extend a distance above the foundation that is sufficient to permit adequate thread engagement of the nut. This is requirement is the same as RCSC glossary, which state "Sufficient Thread Engagement. Having the end of the bolt extending beyond or at least flush with the outer face of the nut; a condition that develops the strength of the bolt." However, you could have a unique situation that would require additional extension.

 

[mjs611]

Thanks for the reassurance all. I agree that drilling out cast in anchors and applying epoxy installed threaded rods could actually reduce the capacity if something else goes wrong. And thanks for the 'compliance paranoia overrides good judgement' comment, made me laugh.

Sandman: Good find in the AISC manual. Secondly, I havent even started using the 14th edition let alone the 15th!

 

[dik]

2-1/2 threads are lots... criteria I use is if top of thread is at least flush with the nut.

Dik

 

[Rabbit12]

I agree with dik. What good does thread above the bolt do? About as good as runway behind you.

There is a nut, which I can't seem to find right now, that extends below the baseplate. They are specifically made for the situation of insufficient anchor rod projection you are describing. If I remember right they are tested to prove they can develop the strength of the rod. It'd be hard for the EOR to argue against their use. You'd have to enlarge baseplate hole slightly.

EDIT: These are what I'm talking about:

https://www.canam-construction.com/en/construction-products/elocone-nuts/

 

[Once20036]

Agree with all of the above - my requirement is always full engagement between the nut and bolt.
The product that Rabbit12 is referring to is Elocone Nuts by Canam. They work great, but they're not free.

Is there a requirement in the contract documents for this extension? An engineers rule of thumb isn't a project requirement unless its in there.

 

[dik]

FYI

From Bolt Science:

In summary the full height of the nut is to be used if you are to avoid thread stripping - that is - ensure that the thread protrudes through the nut. Also use a standard height nut as this is designed so that the bolt will break before the threads start to strip.

http://www.boltscience.com/pages/shortbolting.htm

and from Portland Bolt:

RCSC (Research Council for Structural Connections) Specification for Structural Joints Using ASTM A325 or A490 Bolts 2.3.2. Geometry
“Heavy-hex structural bolt dimensions shall meet the requirements of ANSI/ASME B18.2.6. The bolt length used shall be such that the end of the bolt extends beyond or is at least flush with the outer face of the nut when properly installed.”

http://www.portlandbolt.com/technical/faqs/recommendation-on-thread-engagement/

Both sites have added info...

Dik

 

[sandman21]

The same language can be found in the AISC 13th Ed. same page.