Column Steel Base Plate Design
Column Steel Base Plate Design
(OP)
Hello,
I have a steel column that requires a 1 1/4" Base Plate by design.
The contractor would like to use his 3/4" plate he has in stock to make the base plate.
I would like to double up the plates to make a 1 1/2" base plate. Other then using a groove weld along the plate perimiter to connect the two plates, what other criteria should I consider in welding the two plates together so both act as ONE 1 1/2" plate? Could you please point me to code references in regard to this issue.
Thank you all,
Jeffrey Krus P.E.
I have a steel column that requires a 1 1/4" Base Plate by design.
The contractor would like to use his 3/4" plate he has in stock to make the base plate.
I would like to double up the plates to make a 1 1/2" base plate. Other then using a groove weld along the plate perimiter to connect the two plates, what other criteria should I consider in welding the two plates together so both act as ONE 1 1/2" plate? Could you please point me to code references in regard to this issue.
Thank you all,
Jeffrey Krus P.E.






RE: Column Steel Base Plate Design
Consider the elastic modulus of the different sections (where t = 0.75”):
1 x 0.75” plate
Z = (b*t^2)/4
= 0.25 b*t^2
2 x 0.75” plates
Z = 2 * (b*t^2)/4
= 0.5 b*t^2
1 x 1.5” plate:
Z = (b*(2*t)^2)/4
= b*t^2
You can see that you need four 0.75" thick plates for the same stength in bending.
It seems to me that buying some 1.5" plate is alot cheaper option than expensive fabrication effort to build up a section from 0.75" plates.
RE: Column Steel Base Plate Design
I've seen this done before, by the way, this fabricator is not the first one to think of it.
If the two plates are not attached to each other, and in fact have no friction between them, you get the case described above, where they are much weaker than a single thicker plate. If the fabricator has trouble understanding why this is so, you might make a comparison of a cable and a solid bar- the cable is flexible because it is lots of little bars. A similar situation exists with leaf springs, where the multiple leaves are used to increase flexibility of the structure.
However, if a small plate is welded around the edges like that, the analogy is no longer valid, and the plates can't flex independently anymore. But I don't know of any other analysis for this situation, either.
RE: Column Steel Base Plate Design
RE: Column Steel Base Plate Design
However, a thicker base plate will almost always be more cost effective because of the high labour cost associated with the stiffeners.
Check out "Practical Design and Detailing of Steel Column Base Plates" by Honeck and Westphal at the Structural Steel Educational Council (www.steeltips.org).
RE: Column Steel Base Plate Design
corus
RE: Column Steel Base Plate Design
BS5135 is a withdrawn/superseded standard and hence should not be used anymore. Always use the latest standards.
RE: Column Steel Base Plate Design
Let me give you one way to look at it: Imagine yourself in court being asked to justify you allowing the use of sub-design thickness or a combination that has no sound engineering back up!
The contractor/fabricator/erector will abandon you like a red-hot piece of coal. You will be all by yourself. Just do what is right and insist on it. Do not compromise.
Good luck
RE: Column Steel Base Plate Design
RE: Column Steel Base Plate Design
On a further note, dbuzz's assessment of two flat plates welded together is not correct as you do not simply add the two separate inertias together to obtain the combined inertia, just as in any fabricated section (ie. a box section) you do not add the separate inertias of each plate together to derive the overall inertia. The procedure should be to look at the bending stresses in the welded plates combined (1.5" thick), and also to look at the shear stresses in the welds that join the plates.
corus
RE: Column Steel Base Plate Design
Do you work with this contractor often? If he has tried to pick at you throughout the design or given you problems before I would do what Lutfi says and let him just use the correct plate size. By the time you put in your time for redesign and they weld up or use whatever your solution is the savings will be minimal if there even are savings. Still, if he is new or you work with him a great deal without a bunch of hassle, I'd help him out if its reasonable. Don't forget that its also extra coordination and such so that the 0.75" doesn't just get put on as a direct replacement. Don't ever let him just substitute something that doesn't work, safety is paramount and so is covering your butt down the road. You don't want the cheapest solution, you want the cheapest solution that works.
RE: Column Steel Base Plate Design
Be very wary of the "my way or the highway" approaches. As a fabricator, we see that with a few customers. If they are very knowledgable in every aspect of the work, it's not so much a problem. But we more often get these kinds of responses from engineers that just don't have a clue, and the result is that the owner spends a ton of extra money for nothing. It seems like the larger the organization involved, the more you run into this.
RE: Column Steel Base Plate Design
He should have included this in his estimate in the first place. Why don't you just stack a whole bunch of 3/16" plates on top of each other?
RE: Column Steel Base Plate Design
Have the contractor propose a design for you to evaluate. Let them hire a PE to do this if they need to. Don't waste your time thinking up solutions for them.
Hg
RE: Column Steel Base Plate Design
Surely all the preparation, welding and, presumably, NDT testing for the fabricated option is more expensive than new 1.25" plate anyway.
RE: Column Steel Base Plate Design
RE: Column Steel Base Plate Design
As for the other part of your question about code references - the best codes, with few exceptions, only cover issues that ARE acceptable. They are usually mute on approaches that ARE NOT acceptable. There is a good reason for this - on any given subject there is going to be an almost limitless variety of "solutions" that are not acceptable. If a (partial) list of unacceptable "schemes" is presented, then the reader could (mistakenly) assume that any approach not listed as "unacceptable" could be used.
In your case, if the design (by code) indicates that you need a 1.25" thick (minimum) baseplate, I doubt if you will find suitable references on how to make "thinner" sizes work.