Columns with no base plates

[alumpkin]

Nedd backup data to confirm columns sitting on concrete footing with no base plate. The situation is a Hss 6x3X1/4 column with a 27 kip load. It has no base plate but sites on a thickened slab that sites on a two foot wide footing. Punching shear is not a problem. Only localized crushing may be an issue. The load is way below the allowable of approx. 70 kips for a 10 foot tall column. Is a baseplate really required? The stress is approx. 6600 psi (27,000/4.09 sq in.) on 3000 psi concrete. This is not an elevated slab. Thoughts, references, other?

Thanks.

Alan L., P.E.

 

[TTK]

Why not just provide a base plate in the shape of the column to reduce bearing stress?

How did you check punching shear for an open section? Did you assume it was a solid block?

 

[alumpkin]

This HSS section is part of a metal stud framed building. It forms one side of a transfer lintel for the slab above. It is already in place and will be difficult to fix. Nevertheless, if needed, a base plate will be installed by cutting out a slice of the column and inserting a baseplate.

For punching, I would assume a solid block but there is no way this load will punch through 20 inches or so of reinforced concrete.

So, I am trying to find some experience or reference that I can cite that supports sitting a column on thick concrete.

 

[whyun]

Does this column carry any lateral forces? If so, what would resist the horizontal force at the base? Consider adding a welded angle with an expansion anchor each side.

Vertical forces are a different story.

 

[steve1]

Allowable bearing pressure is limited to .35(f'c) according to AISC 9th ed., Specification J9. Your bearing stress of 6600 psi is way over the allowable. Suggest you add base plate. Depending upon the situation it may also be possible to add some angle iron around the perimeter of the TS to act as baseplate.

 

[UcfSE]

I agree with steve1. We provide a base plate to prevent a failure in crushing the concrete. It also makes it easier to connect the column to the footing.

 

[Lutfi]

Colleagues,

I think the base plate provide secondary function besides distributing the pressure over a larger area. I also provide "pinned end condition" for the HSS. I have not heard how it is pinned! I think there might be a stability issue here!

Another concern, how do ensure 100% even force distribution all points of contact between the column and the footing? You can mill the ends of the HSS but I do not see how you can have a nice level concrete surface.

I also agree that how lateral loads are transferred if no anchor bolts are provided?

My two cents worth.

Regards,


Lutfi

http://www.cdeco.com

 

[alumpkin]

This HSS 6x3x1/4 is an internal column with no lateral load associated with it. It is also part of a load bearing metal stud wall that has a 16 gage wall panel welde to the HSS section, too. It cannot move laterally.

I do know that H-Piles regularly sit on partially weathered rock or solid rock with no base plate and carry load. So, I was trying to salvage a difficult situation with some backup data.

If nothing else comes up, then I will have the contractor drill a hole 4 feet up the column and fill with high strength grout. This will provide the necessary bearing area for the column. Load is transfered through friction inside the HSS section.

If anyone else has seen or read any literature that talks about sitting a column on a concrete footing without a baseplate, pleas let me know.

Thanks.

 

[rday]

alupmkin,

H-piles also have side shear acting to support imposed loading.

I think you need a baseplate. I would not be comfortable depending on internal grouting for bearing area or axial load transfer.

Rik

 

[CTW]

I'm curious, was this a construction mistake or was the column designed this way?

 

[miecz]

Don't count on friction between the grout and tube to carry the force. All grout shrinks (a little), even "non-shrink" grout. When the grout shrinks, you will lose your friction.

AISC J9 allows for .70*f`c bearing presure for your situation. I would shore the load and weld a couple of angles to the base of the tube.

 

[TFL]

IF IT IS PART OF A METAL STUDD WALL- IS THERE A TRACK BELOW IT FROM THE METAL STUD? THAT MAY HELP

 

[onteng]

I would weld a clip angle on each side of the column in the plane of the stud wall. Anchorage to the slab could then be achieved and bearing taken through the angles.

 

[SlideRuleEra]

alumpkin - I can appreciate your situation. Frankly I believe that the concrete (without the base plate) will carry the load, but it will probably be virtually impossible to prove it. Even though the 6000 psi load exceeds the theoretical (3000 psi) concrete compressive strength, it is like comparing apples and oranges.

What you describe sounds like a point load, applied to a small, thick area, with that area is confined by a relative large mass of unloaded concrete. The column might pulverize a token amount of concrete, but it is certainly not going to "drill" thru under the stated conditions.

By comparison the 3000 psi concrete value was arrive at by the method that we all are familiar with - "crushing" an unconfined cylinder that fails by more or less "spliting open sideways".

With that said, I agree with the others that you should "do something". Your idea of grout inside the member sounds like a very reasonable, cost-effective appoach. jmiec brings up an excellent point about not depending on friction. Perhaps you could drill the tube wall, install thru-bolts in the area that will be grout filled. Then these grout encased bolts can transfer load to the column independent of friction (might take a good many bolts to handle 27 kips, but you should have space within the 4 foot grout length).

P.S. Your H-pile analogy sounds good to me. From first hand experince - during the driving of "true" point bearing H-pile on "hard" rock, the steel ALWAYS "loses" if overdriven (steel deforms, bends, etc. while the rock remains intact).

http://www.SlideRuleEra.net

 

[UcfSE]

With all the trouble of drilling and grouting it seems like you may as well just weld a small base plate on it, even if one dimension matches the tube width so it doesn't stick out of the wall. You could probably get away with something like a small cap plate as long as you have some way of connecting the column to the footing. It does need something, column stability requires a pin at the bottom. The H-pile analogy sounds reasonable to me except that rock is a lot harder than concrete and is much better confined under the earth. That's enough to make a huge difference besides the fact that the piles are field tested unlike typical tube columns.

 

[DRC1]

The one concern I would have is water and dampness. The steel concrete interface of a foundation slaballways seems to attract dampness. A little rust at the edge of a base plate is no big deal, but a little rust in a 1/4 in tube could be.

 

[alumpkin]

Guys,

Thanks for your repsonses and information. I have forged ahead and prescribed a base plate to be installed beneath the columns. That problem is solved.

Concerning the lateral stability, I am quite convinced that having the prefabricated metal stud bearing wall that is shot to the conrete foundation using Hilti anchors and welded to the deck above is a stable system. Then my lintel column is welded to the end stud in this wall vertically up its length on both sides. The metal stud wall has crc bracing at 4'-0" c/c as well. Therefore no anchorage is required. That problem is solved.

Thanks again for the conversation.

Alan

 

[Ron]

alumpkin...you will likely have a lateral load as many codes require a 5 to 10 psf loading on internal walls.