Pin base column using epoxy anchors 1

[hm_structural]

I am designing a moment frame to support a heavy ductile iron pipe. I have designed it as an intermediate moment frame with pin base columns. I am not able to deliver the uplift with over strength factor to the foundation with epoxy anchors set within the column flanges (typical pin base connection detail). Ideally, I would like to cast-in-place the bolts in a new footing, but due to some site constraints I have to go with with epoxy anchors in existing 17" thick slab. Is it an acceptable practice to set the anchor bolts outside the flange (detail similar to a moment base column) for this case. I need a bigger breakout cone for the anchors which can be done by setting the anchor bolts outside the flanges. Any other suggestions are welcome, thanks!

 

[Greenalleycat]

I am dubious with this whole philosophy to be honest.

Firstly, if you want a ductile moment frame then I think the only acceptable type of base detail is a genuine pin consisting of a single large greased rod that truly cannot take rotation
Otherwise, any form of base detailing involving epoxied or cast in bolts will lead to moment generated at the base which will affect your design
If this isn't explicitly considered then you are likely to generate lower moments at the knee than expected and will have a stiffer structure than anticipated, which may throw your whole design off
This is true regardless of where you epoxy the bolts - outside the flanges will be worse, sure, but even in the middle of the web a moment couple exists

Secondly, I don't like the idea of relying on epoxied bolts in such a critical overstrength application
I just don't trust epoxy that much and it's fundamentally a brittle failure which is much harder to depend on than casting in some bolts with a baseplate into a stirrup cage
This is especially true when you consider the moment at the base which will generate larger tensions in the bolts
That part is up to you though

Perhaps you could fabricate a bracket that you bolt down that supports a pin connection to take shear and uplift only that you then bolt on site to the portal leg
See the link below for some ideas of what I'm meaning

https://www.tboake.com/SSEF1/pin.shtml

 

[canwesteng]

The connection could attract moment either way. Your best bet is to try and size the anchors so that they will yield before breakout, to give a ductile failure mode.

 

[human909]

Engineers and codes are living in fantasy land if they think your typical column and baseplate connection will behave like a pin. Keeping the bolts inside the flanges will make it a little less stiff, but it is still going to be fairly rigid.

Nothing wrong with modelling the connection as a pin if the expected rotations are minimal or acceptable for your foundation. However if your end up with high rotations then don't be surprised if your connection remains somewhat stiff and something else rotates (eg your foundation).

If you really NEED a pin connection then it should be detailed as such:

 

[hm_structural]

Thanks everyone for the feedback. I ended up with cast-in-place anchors and baseplate assembly set on a new footing on top of the existing concrete slab, thereby eliminating the need for epoxy anchors.

 

[Greenalleycat]

Good option mate. I've significantly reduced the amount of epoxy I use in my designs now
I'll use it for secondary structure or lightly-loaded portals only
Any significant portal frames I cast in the bolts for surety