Base Plate On Existing Concrete - Anchorage

[ATSE]

I have an existing 12" thick equipment slab for mechanical equipment. Slab has single layer of minimum reinf.
We are installing a large structural steel canopy (sunshade) over equipment. Very high seismic zone (SDS=1.2g).

The canopy columns are square tube steel HSS5x5x1/4. I believe it is good practice and probably a code requirement to develop the moment strength of the column thru the anchor bolts.

Based on Hilti and Simpson ESR reports and ACI 318 Appendix D, the max embedment permitted for a 3/4" dia rod is about 8" (min slab thickness = hmin = heff+5*do).

I know that 8" of the best post-installed epoxy anchor in the world will not "calculate out" to develop the yield strength of a 3/4" dia F1554-36 ksi. However, I believe that this is because of the very high factors of safety on epoxy bond stress values.

As an alternate, what about tension pull load testing each and every installed epoxy anchor to, say 80% of anchor bolt yield? This will demonstrate adequate "in-situ" bond. Is this reasonable and within the realm of Code acceptance?

I believe the only other choice is taking the equipment out of service, breaking up the existing concrete, and using cast-in-place anchors in a new thickened concrete equipment slab.

Any comments or other ideas are appreciated.

 

[JedClampett]

I disagree that you need or want to develop the moment strength of the tube. Analyze it as a pinned base and design it as such. If you try designing a base plate anchorage for moment, you're going to be sorry.

 

[ToadJones]

for the sketch he shows, a pinned base won't be stable, will it?

 

[Lion06]

I agree with Jed-

There is no reason to develop the moment capacity of the post into the slab. Design and detail it as pinned.

 

[Lion06]

Toad-

You're right, it won't. Fix the base, but only design for the moment needed, not to develop the moment capacity of the HSS post.

 

[ToadJones]

can you use a different column arrangement so as to eliminate the cantilever columns?

 

[DCBII]

You shouldn't have to develop the moment capacity of the HSS. Analyize it as a pinned base. If this column is part of a lateral force resiting frame (like a moment frame or braced frame) you may need to factor the loads up by the overstrength factor though.

 

[ToadJones]

Am I missing something here?
If you analyze this thing as a true pinned base, it will flop right over, no?

 

[Lion06]

Toad, I think everyone did the same as I did. I didn't even look at the sketch and didn't realize it was a cant in both directions.

 

[DCBII]

It will only flop over if you are using a cantilevered column system (like a flagpole), however if you have fixity through beam-column connections you can pin the base.

 

[ToadJones]

SEIT- I was thinking maybe since it is Monday that my brain was still in weekend mode.
I do know that the subject of moment bases has been argue on this site many times.

 

[ToadJones]

OK-
All, please see the OP attached sketch

 

[Lion06]

DCB-
It is a flagpole, there is only one column with a cant'd arm in both directions. Pinned base won't work.

 

[DCBII]

I just looked at your drawing. I see the problem now. To the best of my knowledge you still do not have to design the bolts for the column capacity though. Even if you detail it as a "Cantilevered Column detailed as a Special Moment Frame" per ASCE-7, all of the special moment frame requirements pertain to the beam column connection and you don't have one of those here. Just make sure you're column section is seismically compact and design the anchors for the amplified seismic load (overstrength).

 

[JedClampett]

Sorry, I stated some facts without looking a the sketch. But I'd stay away from retrofit anchors to develop moment. Can you drill a concrete pier and embed your post into that?

 

[ToadJones]

I still say the easiest way to deal with this would be a canopy on, say, 4 columns rather than, say 2.
Then you can rely on a little frame action.
This, of course, assumes there is nothing in the way ...which I'm sure there is.

 

[Ron]

Core through the existing slab (12" dia.,min). Embed the columns in the pier or cast anchor bolts into the pier. Slab will then help resist the moment.

It will not work as a pinned connection.

 

[paddingtongreen]

Could you make a secondary connection to the wall, no moment involved, just + & - vertical load, that would relieve the base moment.
I agree with the others, design for the design forces, not the column capacity.

Michael.
Timing has a lot to do with the outcome of a rain dance.