Railing Posts Anchored with Non-Shrink Grout

[n8structural]

We provide engineering services for a stair and railing manufacturer in the Midwest. Typically railings are welded to the top of stringers or in some cases welded to a baseplate that is anchored down. However one common detail in the railing industry is to core a hole into concrete and grout the railing post in place with a high-strength non-shrink grout. This is a standard practice and I see this all the time. For instance a 2" post (square or round) is grouted in a 3" diameter core some dimension (3"-4").
My question to you is how would one go about proving this base connection meets the ASCE load requirements of 50 plf or 200# in any direction? Typically a 43" high railing with a 4'-0" post spacing would have a base moment of 8.6 kip-in. How do I provide evidence that this detail is sufficient. NAAMMM shows this type of condition but does not address how it is designed (from what I can find).
My concern is not with the design of the post itself, but rather with the connection of the post to the concrete. Would this be a modified example of an anchor and therefor could be designed for concrete breakout strength per ACI 318 Appendix D? A common product I see specified is Por-Rok (a high strength non-shrink anchoring grout). Obviously this does not work with an aluminum post, as embedding aluminum in concrete is not a good idea.
Any insight or ideas would be appreciated.
N8structural

 

[KootK]

Great question. So far, this is what I've been doing:

1) Convert your moment and shear into a force couple / stress occurring within the concrete.

2) Check bearing stresses on the concrete.

3) If you're near an edge, as you often are, jury rig appendix D provisions to try and deal with concrete breakout in a rational manner.

Obviously, it's three that's the hardest, especially if you're coring and supplemental, feel good reinforcement isn't an option. Sadly, I know of no design guide or established method for this. Hopefully someone else does.

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.

 

[TehMightyEngineer]

I've seen some construction grouts that are non-shrinking and claim extremely high bond strength in cored holes with embeded steel items. I'd see about contacting some of them for recommendations and any testing data. One we've used often in the past is A.H. Harris Construction Grout. Otherwise I'd do this as KootK suggested.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[n8structural]

Thank you for your input. This was the approach I was taking.
Thanks again.
N8structural

 

[wannabeSE]

Periodically, this subject is discussed. A couple years ago, slickdeals attached calculations from shop drawings that appears to be fairly good. But, I haven't spent the time to verify the methods, the pdf is tucked-away with my references for the next time I need to check a grouted post close to the edge slab. See

http://www.eng-tips.com/viewthread.cfm?qid=357753.

 

[RHTPE]

Something to consider about non-shrink (NS) grout - while the overall net effect is little, if any, change in dimension, the NS grout actually expands during its initial curing and shrinks back to its starting dimension when curing is complete. I have seen way too many edge break-outs resulting from this phenomena.

I have also seen a number of cases where the rail post AND the grout have been pushed up and out of the core-drilled hole from a few years of freeze-thaw cycles. Makes for an ugly railing system.

I would specify corrugated sleeves rather than core-drilled holes AND a small rebar tie wrapped around the sleeve to combat the expansion-shrinkage behavior of the grout.


Ralph
Structures Consulting
Northeast USA

 

[TehMightyEngineer]

The construction grout I posted provides data on it's expansion rate and showed very little expansion from what I recall when batched properly. But, yes, if this data isn't provided or if it does expand significantly then definitely something to consider and your recommendations certainly wouldn't hurt.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com