Direct Embedded//Buried Steel Columns for Buildings

[Jc67roch]

I used to work in the electric utility industry, and we used direct embedded steel poles on transmission lines all the time. I now have a situation with a proposed interior building mezzanine where the allowable soil bearing is poor, and we need to go to bedrock for bearing. The bedrock is about 15 feet down. I am thinking that direct embedded steel columns, bearing on a concrete leveling pad in the base of 2 foot diameter hole might get me the axial bearing capacity I need along with the lateral stiffness (from the embedment and floor slab at grade) for the lateral loads on the mezzanine. I don't know that any has done this before, but seems the same premise as a pole barn structure.

I don't think corrosion will be a significant concern given the interior location, surrounding concrete slab, and that in my experience most corrosion takes place in the first 12" of soil or so as it needs oxygen. However, I could add a sacrificial steel collar at ground line to provide extra steel cross section if corrosion is a concern (this is done on transmission poles).

Thoughts?

 

[TXStructural]

You could do this, with a steel baseplate on the bottom of the column for bearing, but you will likely get corrosion if the fill is soil. You do not say that you will filling the hole with concrete, but that is the way to go if you are set on dropping the steel column into the hole. This also provides better fixity and avoids buckling of the buried backspan. The alkalinity of the concrete will passivate and protect the surface of the steel (this is how reinforcing steel stays intact over time.)

You say that you are embedding the columns for moment resistance, so be sure you have flange and web stiffeners at the point where the column exits the slab. I would not embed a hollow section like this, since it cannot be effectively stiffened as a one-piece column. Generally, a leave out would be around the column at the slab would prevent slab movement from compromising the column, and having the stiffener at slab level could present a problem. I would probably have a round form (same as the bore hole) extend to top of slab and fill to that level as the column is embedded, to allow vertical slab movement.

The other issue will be compatibility with the other structure. If the mezzanine is founded on rock and the surrounding building is not, there could be issues.

 

[paddingtongreen]

I also worked in the transmission line business for a while and I never even heard of, or would consider, directly embedding steel poles. We used drilled in piers and bolted the poles to them. Where were you, in an arid desert some where?

I really can't imagine any director of code enforcement allowing it for a dwelling.

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

 

[JStephen]

Around here, the have those big steel monopoles. Half of them are mounted on a circle of anchor bolts, half of them just stick out of the concrete with details unknown. They're all galvanized. (And this is Dallas, not especially arid).

Anyway- some other issues. If you'd normally have a steel erector doing their work after the concrete is finished, then you'd have to have both onsite to work this. If the steel erector doesn't have a good way to hold (or maybe to adjust) the exact location when the beam is set, you could have issues. And you may run into the "We've never done it like that before" syndrome.

Utility poles should mainly have a moment load, not much vertical load, and that detail could be poor for vertical loading- may still require a baseplate, for example.

 

[paddingtongreen]

If embedded in concrete, okay. But OP clearly indicates it is in the soil.

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

 

[TXStructural]

In Dallas soils, I would not embed a steel column into soil, encased in concrete or not. If you do pursue this method, galvanizing may be the way to go. We tend to get water migration into any void and through any crack, and then we get severe drying. This frequently drives corrosion. Do you know the sub-slab conditions, i.e., what the excavation and backfill were? These could contribute to success of your design.

I did forensic structural engineering in the area, and our stock in trade could have been sub-slab and sub-structure moisture-related problems. I also spent 5 years working in a 890,000 building where the slab had continually heaved, year by year, for more than 20 years, due to moisture-driven expansion of the underlying clay, at a rate of about 1/2" per year. The vertical rise was nearly uniform throughout the slab (a bit higher on the irrigated, uphill side of a nearly flat lot), and was 8-10 inches on the high side when I left there.

 

[Ron]

Why complicate the construction with techniques that are seldom if ever used? Construction quality is poor enough without introducing new variables. No, I'm not against innovation and trying new things...I do it and so do many other engineers; however, your needed construction is not difficult using currently accepted and tried methods.

 

[IsaacStructural]

I agree with Ron, why not go with a short Rick caisson?

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[Jc67roch]

Thanks All.

I would be backfilling these embedded columms with concrete.

Our original design had them on augered concrete piers (caissons). However, we are looking at the embedded option to help reduce hole size, and save money on rebar and anchor bolts.

Money is the big driver on this project, so we are looking at all means possible to shave some costs.

 

[TXStructural]

Rebar will cost far less than the similar length of column, and sticking with common grade anchor bolts is economical. A slightly larger pier is not really much more expensive, since it is the cost of mobilizing a drill rig that probably costs the money. Don't mistake economy of material with economy of construction. I'd stick with common means and methods unless a real game-changer is available. I like innovation, but it seldom saves money the first few times. The lest expensive option would probably be a steel braced frame or a shear wall (ICF, CMU, CIP concrete) between piers, rather than moment connections at the column baseplate to pier connection.

I agree that dropping a steel column into a boring, truing it up, maybe hanging some beams on it, and then concreting it in seems like an easy solution, but I doubt the contractor has ever done it, so it would be priced at a premium. Also, how much overhead space do you have to hoist the columns and feed them in to the boring. (How about space for the drill rig?)