Column Embedment into Base Plate

[ClarksonEng]

A legacy practice of my engineering group has been to cut a hole in our base plate and embed an 8x8x1/4 HSS column 5/8" into the plate. The new manager wants detailed calculations on each structure, including base plate design. It is my understanding that the individual that detailed our base plates went with this design due to the moment on the structure. Unfortunately, this is a unique design and I cannot find any similar calculations to analyze this plate. The TIA, AISC, and ASCE 113 don't answer this question. To continue with my analysis of the base plate, how do I determine if this plate is adequate? The plate is A36 and column is A500 grade B, design loads are as follows:
Bearing Case: Shear = 4kips Axial = 9 kips Moment = 55kip*ft
Uplift Case: Shear = 5kips Uplift = 6kips Moment = 49kip*ft

Any direction would be appreciated.

 

[structSU10]

there must be some other reason for this detail. I do not see how a 5/8" embedment helps with the moment. It would seem to hurt it due to the step in the baseplate right at max moment location.

That said, the weld transfers your forces from the column to the baseplate, and should be analyzed for all combined actions. I think the baseplate should be assumed as 5/8" thinner than its specified thickness for a bending check per AISC.

 

[JAE]

Agree with structSU10.
This seems like someones "clever" idea that was adopted years ago without any real benefit and results only in added cost.

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[KootK]

- Unless you're full pen welding the donut hole to the rest of your base plate, I'd consider only the donut as being effective.

- Utilities running up the inside, right?

- I've evaluated similar poles assuming that prying action in the base plate ties the column walls down. Similar to monopole splice connections. Kinda depends on your base condition. Grout bed? Double nuts elevated?

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.

 

[ClarksonEng]

No grout bed, we use leveling nuts, the plate rests 2" above TOC.

In some cases we do have utilities running up the inside but not for this structure. This structure is for supporting rigid aluminum pipe (conductor,I'm in the substation group for the utility). It is a 2 column structure with a beam across the top supporting all 3 phases.

 

[KootK]

If you don't need the hole then lose it. That will make for a better performing, cheaper, and easier to design base plate.

With levelling nuts and the detailing as shown, I'd expect this to have a very limited, and difficult to determine, moment capacity.

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.

 

[BUGGAR]

In the utility industry we've used monopoles thru holes in baseplates so we can use a fillet weld on both sides of the baseplate for moment capacity without full pen welds. But not quite like your detail. Yours looks like they wanted a flat bottom, something we didn't have a need for.

 

[ClarksonEng]

KootK, we do need a hole for galvanizing draining at least. I'd like to ultimately get rid of this design to reduce labor costs, just wanted to know if there was anyone out there that utilized a similar design or has any ideas so I could determine its strength.

BUGGAR, I think ours is like how you describe yours, in my opinion the details aren't shown correctly. There is a hole completely through the plate with the column "suspended" 5/8" deep into the plate.

Thanks for the input

 

[chris3eb]

Clark, does your detail show the hole going all the way through the plate? It looks like there is still some plate below the HSS post (maybe a cap?), but I don't know how they would get that 3/16 fillet weld in if that's the case.

 

[ClarksonEng]

chris3eb, yeah it goes all the way through, the section view is confusing and I think should be revised.

 

[KootK]

With the hole, your evaluation procedure may need to be one of these:

1) moment conn btw plate and sidewall. Not much there with 1/4" walls.

2) model plate as discrete strip beams.

3) FEM

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.

 

[MotorCity]

We have welded an HSS to the bottom of a baseplate to serve as a shear lug, but it was simply welded to the bottom of the baseplate (i.e. no hole in the baseplate). Based on the section, it looks like the HSS is embedded into half of the thickness of the baseplate, which makes it even more difficult to fabricate. So, my best guess is that it is an attempt at a shear lug (if you turn the detail upside down).

 

[MotorCity]

Sorry, scratch my last idea.....thought it was a short stub of HSS....not a full column

 

[kingnero]

Thickness of the plate does not make sense if you're using thin walled HSS columns.
I'd say this detail needs being revised, based on my non-expert opinion.

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[wannabeSE]

I've never seen something like this. Maybe the intent is to have a big hole in the plate to promote better galvanizing on the inside and the 3/16" fillet weld on bottom seals the connection between plate and the HSS. These are just guesses. Maybe call the galvanizer.

 

[mronlinetutor]

have u check if the hss will punch through the base plate. if the base plate fails at the perimeter of the hss, then there will be no support for the hss column.

 

[ClarksonEng]

@BUGGAR, I also work in the utilities, we use this design a lot due to galvanizing fluids and water drainage. Do you have any examples of on to calculate the strength of this connection. We've never had one fail but going forward and making this design "standard" I want to be certain on how strong it is.

Once I do get a grasp on how strong this is, I'd like to get it re-detailed to show the hole going completely through the plate and the column suspended halfway through the plate (5/8"). This has led to confusion in the past but our internal fabrication shop has done so many of these that they know what we are looking for.

 

[Lomarandil]

I understand the drainage and galvanizing issues, but we typically dealt with that by a 2"-3" diameter hole in the base plate, which I imagine is substantially cheaper to fabricate (presuming the weld connection advantage of your detail isn't absolutely critical to the design).