Piers on Footings

[dburch6]

I am performing the structural design for a middle school. Traditional steel framing with mostly HSS columns. The top of my interior footings is -1'-4" and the top of my perimeter footings is -2'-0". I will not be using concrete piers on top of the interior footings but another engineer is recommending that I use concrete piers on top of the perimeter footing and set the top of pier to -0'-8". I do not really see the point in adding the concrete piers. It seems to me that I can just make the steel columns a little longer and set the column bases on top of the footings at -2'-0". Can someone help me understand why I ought to use concrete piers in the perimeter condition? I do have several braced frames along the perimeter of the building as well.

 

[phamENG]

Do you have corrosive elements in the soil? Bringing the steel up closer the surface could help minimize exposure to those elements leaching in from the native soils.

Lot's of braced frames? Could be that the other engineer is concerned about sequencing and being able to adjust the braces. Assuming they'll end encased in concrete from a foundation wall/slab edge, once that happens you're stuck with what you've got.

Most of the steel buildings I've done have had the steel down at the -2' except for one where I had highly corrosive native soils.

 

[JLSE]

why is your TOF at -2ft? Is it a SOG with finished elevation at 0ft? I would typically set the TO pad footing at TO perimeter footing, which would be -8" (edit- assuming typical 4 in slab with thickened edge). If there is retaining, I would use a pier to bring the column above retaining forces.

 

[dburch6]

phamENG - The native soils are not corrosive as far as I know. And I require any steel below grade to be painted with bitumastic paint for corrosion protection. That's a good point about the braced frames. However, I specify fully welded braced frames so there really isn't any opportunity for adjustment anyway.

JLSE - The top of footing is down -2'-0" to get the bottom of the footing to frost depth which is -3'-0" in my area. This is also the standard depth of perimeter footing that the architect I'm working with uses.

 

[phamENG]

One potential advantage of the piers, if used at all of the columns would be that the column base elevations are all uniform. That could reduce the already pretty small chance of a fabrication error.

This other engineer - is he/she in your office? Ask them why - my money's on "well, I've always done it that way..." but who knows? Maybe it'll be an enlightening moment for all of us.

 

[1503-44]

We put well coated steel pipe in the ground all over the place. It is also cathodically protected and it still manages to corrode. Not much different than steel sheet piling. Why not.

What's your building inspector say?

 

[dik]

...am I missing something? Why would your steel be that far down for all steel? As 1503 notes... corrosion and longevity would be a real issue.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[1503-44]

I always thought that 2ft of steel column is more expensive than 2ft of concrete (cast at grade level).

But I really kind of think what you should do may depend more on the details of how the floor slab ties in with all this and joins up with the wall materials, water proofing, insulation, flashing etc.


If you wind up with something that looks more like columns encased in concrete floor slab anyway, what was the point of putting the steel baseplate down that low?

 

[dburch6]

Thanks to everyone for your responses. It seems like corrosion is the biggest concern. What I'm struggling to understand is why a column with a base at -1'-0" or -1'-4" is less at risk than a column at -2'-0". The difference in elevation seems negligible to me from a corrosion perspective.

 

[phamENG]

Where are your column lines relative to the exterior wall? Are they inline (hidden) or inset? If they're inset, then I'm guessing your foundation wall runs past the column and you have a slab block out that gets filled in after all the dead load is in place. That block out encases the steel column below FFE in concrete. If it's painted with bit and encased in concrete, then you should be fine. I've never loved this detail, but it's how it's done around here and I can't figure out a better way.

If they're hidden the wall you can still do this, though your base plate probably sticks out of the foundation wall and will be exposed. Properly coated and with free draining soils you don't have too much to worry about. And in this situation, bringing it up to -8" could put it outside of the wall and piss off the architect.

 

[1503-44]

"bringing it up to -8" could put it outside of the wall and piss off the architect."
Thats a good reason to do it in my book.

Concrete is not cheaper than 2' of steel?

 

[phamENG]

Once you factor in extra form work for the pier, reinforcing, and probably an extra pour if they pour the footing first and then come back and pour the pier, it's a wash at best or possibly more expensive than 50lbs or so of steel (if it's even that much).

Nucor-Yamato beam prices are around $60cwt for small wide flange as of two weeks ago, so bump it up because HSS is more expensive and a bit of a markup going through the fabricator (no increase in labor or anything else as it's just added material length)...call it $100cwt. So that's about a dollar/lb. So you're adding $50 per column by making it it 2' longer. I think the pier will cost more than $50.

*I'm pretty sure that price list is FOB Mill, so shipping will drive it up a little more, but not much...on a per pound basis shipping really isn't much.

 

[1503-44]

Maybe we all need to start doing that.

The only real potential clash I can think of is that you have steel works mixing it up with concrete works. I generally like to have concrete set for a week and those guys off the site before steel works move in, but then you got to get them back to do the slab with the columns stuck in it. Then you have steel workers on top of green slab. I dont like that part.