Embedded Column Base Plate 4

[mountaineers19]

When designing a WF column where the bottom 1.5 feet of it is going to be encased in concrete do you take the unbraced length from the baseplate to be conservative or is it okay to take it from the top of the concrete to try and trim down the column size?

 

[cliff234]

I usually consider the column length to be measured from the bottom of the baseplate. This may be a bit conservative, but I've been doing it this way for about 45 years and have never been "value-engineered" by anyone about this. I pay attention to other things that are of greater consequence - such as attention to details, constructability, showing reactions, producing high quality and complete contract documents, listening to fabricators when they have suggestions, following the requirements of AISC 303 (the Code of Standard Practice), etc. Depending on the footing size and baseplate detail, I have occasionally used k=0.8 (for non-sway columns).

 

[JAE]

If the concrete is just unreinforced decorative cover, I'd ignore it.

If the concrete is properly reinforced, I might be tempted to ignore the top 12" of the concrete due to possible long term concrete deterioration and go with the remaining concrete below (in your case only 6").

I always worry that someday, somehow, some engineer will say: Yes - go ahead and remove that concrete - it ain't doing nothin'

 

[bones206]

So you would select a column that would fail buckling design checks if the unbraced length was 1.5 ft longer? That seems inadvisable.

 

[lexeng18]

So you would select a column that would fail buckling design checks if the unbraced length was 1.5 ft longer? That seems inadvisable.

i dont disagree entirely with this mindset, but also for structures that are well-defined and that I perceive as having a low risk of future changes/modifications I have no issue designing repetitive elements with a utilization ratio approaching 1.0. that said, its not crazy to me to imagine a column section being controlled by weak axis buckling, where for example if the unbraced length went from 20' to 21.5' thats almost a 10% increase in compression utilization and could result in overstress.

 

[bones206]

It’s a judgement call for sure, but I don’t think the risk/reward would be worth it in this case.

If it’s a column that is going to be repeated many times over, maybe the economical gains justify being less conservative in your assumptions. But that assumption should have some rigor behind it, like testing or FEA buckling analysis. Just my opinion.

 

[EngDM]

Curious to what the general consensus is for calculating storey drift for embedded elements. Similar to OP, would you take length from top of concrete since the embedded column won't be doing much of any moving and go from there?

 

[bones206]

I’m more liberal with story drift. I’ll assume a some partial fixity at the base for better economy. I’ve never assumed a shorter column length for an embedded column, but given the right circumstances I would consider it. I believe AISC has been doing work on embedded column base behavior recently.

 

[lexeng18]

It might seem obvious but hasn't yet been stated--I think this this entire discussion hinges on if the column/anchorage is being designed as a truly embedded column (as per the latest edition of AISC Design Guide 1 for example) versus a normal column that just happens to be bearing below slab elevation by means of an isolation diamond (or similar).

For a true embedded column design there is absolutely a point of fixity somewhere between the top of concrete and bottom of baseplate depending on the model used. For anything else, the column length used should be to bottom of baseplate.

 

[WesternJeb]

I reinforced a PEMB column one time like this. We removed an interior column and distributed the load to the two adjacent interior columns. To gain the additional capacity (reduce the buckling length), we encased the bottom 6' of the steel post columns in concrete to restrain buckling and sold it to the owner as a bollard since there was a lot less redundancy. It has been this way for a few years now with no issue so it seems to have worked.