HSS Pier Base Plate Encased in Concrete

[WaterfrontPE]

I've got a 16x12x0.375 HSS, the bottom of which will be encased in a 20" CIP slab with the base plate anchored to the concrete below. The pier is experiencing a significant moment, and I'm looking for a resource to evaluate how the bearing resistance from the concrete encasement will reduce the moment from the top of the slab to the base plate. Any ideas on how to do this?

 

[KootK]

I don't know how to evaluate the stiffness of that load path but I have some ideas for what I'd be inclined to do to improve it:

1) Weld some deformed bar anchors to the sides of the HSS, at the top and bottom of the slab, and do your moment transfer that way. Make smart choices with respect to corrosion etc.

2) Run your pier rebar up into the slab to create some fixity between the pier and the slab. Done this way, the base plate will still see the moment but the pier itself should be shielded from much of it.

I'm sure that the embedment on its own will be somewhat effective but my concern there is the the slab shrinking away from the column over time or there needing to be compressible material between the slab and the HSS. Those things would tend to make it such that the HSS would need to see a fair bit of displacement before it would engage the slab in bearing.

If you share a sketch of your anticipated detailing, I might be able to provide a more targeted response.

 

[WaterfrontPE]

Thanks KootK, this is helpful. I don't share the same concern, but only on the basis that the structure will not bear sustained loading. It will be loaded in emergency situations only, so there shouldn't be separation over time. Any separation that occurs from loading can be inspected and injected after the fact.

The problem I'm having is detailing the base plate connection to the slab below. It's fairly close to the edge, and analysis shows breakout in tension induced by the moment (the slab below is by others, and they don't play nice). I've run that analysis under the conservative assumption that the column (HSS) base is not encased, knowing that if the anchorage works for this scenario, the encasement only adds more resistance.

I like your idea of welding deformed anchors, though it still gives me the similar problem of evaluating what the load transfer actually is through the encased section, and what the reactions at the base of the HSS are. That piece is critical to ensure adequate anchorage to the slab without a breakout failure.

 

[dhengr]

WaterfrontPE:
Consider welding some reinforcing into the HSS and to the top of the base pl. before welding it to the HSS. Fill the HSS with several ft. of concrete. Otherwise, you will have trouble with bearing stresses and bending in the weak direction of the face shells of the HSS. That is, you can’t count on 12” or 16” of bearing surface/face, on a hollow section, given the weak bending in the center portion of the face shell. The bearing and the bending will be concentrated near the corners, where the stronger/stiffer web shells exist. Maybe some shear studs on the outside of these web shells would help get the reactions out into the slab.

Edit: Weld some 6’ long rebars to the stronger/stiffer web shells, horizontally and extending back into the slab, away from the breakout face. On the face opposite the breakout face, weld rebars (6’ + 16” + 6’ long) horiz. out into the slab. Put some hairpins in the slab to contain the breakout surface/cone.

 

[WaterfrontPE]

dhengr:
Thanks for the thoughts. I've already checked the HSS for local buckling, but filling the section with concrete certainly won't hurt. I'm going to check shear studs on the sides. The 20" slab I'm referring to is actually a 13" slab and a 7" slab in two lifts, so I'll only be able to rely on the 13" slab for that resistance. Thanks!

 

[WaterfrontPE]

I should also clarify that the base plate analysis shows a failure when running it as a CBFEM, but within acceptable limits (only just) as a rigid plate. Given the concrete encasement, I think the rigid assumption is valid, but would like to find additional literature detailing load reduction considerations for concrete encasement. Thoughts?

 

[phamENG]

Waterfront - one question I'd have is how much control do you have over the construction of the slab? What are the chances that you go through all of this and then when they build it they box it out with an isolation joint and you lose all of your connection to the larger slab? That wouldn't be good.

You mention that CBFEM is saying NG, but rigid plate barely works. How think of a plate are you using? Can you alter your connection detailing to make rigid plate assumptions accurate?

 

[WaterfrontPE]

That would definitely suck, but it's highly unlikely. I'm using a 1" plate, no plastic deformations from the CBFEM analysis, so I feel good about it. The CBFEM ignores some of the anchors that it calculates not to be in tension, which shrinks the breakout cone. That's why I'm trying to find a good resource to understand the interaction with the concrete. If the concrete reduces the moment exerted on the plate, we're dealing with more of an axially loaded scenario. I came across this Link which seems to have some good emperical data.