Columns with no base plates
Columns with no base plates
(OP)
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.
Thanks.
Alan L., P.E.






RE: Columns with no base plates
How did you check punching shear for an open section? Did you assume it was a solid block?
RE: Columns with no base plates
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.
RE: Columns with no base plates
Vertical forces are a different story.
RE: Columns with no base plates
RE: Columns with no base plates
RE: Columns with no base plates
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
www.cdeco.com
RE: Columns with no base plates
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.
RE: Columns with no base plates
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
RE: Columns with no base plates
RE: Columns with no base plates
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.
RE: Columns with no base plates
RE: Columns with no base plates
RE: Columns with no base plates
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).
www.SlideRuleEra.net
RE: Columns with no base plates
RE: Columns with no base plates
RE: Columns with no base plates
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
RE: Columns with no base plates