Column Steel Base Plate Design 5

[krus1972]

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.

 

[dbuzz]

That doesn't sound like an appropriate solution. Two 0.75" plates won't be a 1.5" thick continuum, in the way that a 1.5" thick plate is.

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.

 

[JStephen]

There's not an easy analysis for the case.

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However, if a small plate is welded around the edges like that, the analogy is no longer valid, and the plates can't flex independently anymore.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.

 

[UcfSE]

Can you use some gusset plates to stiffen the base plate so that a 3/4" thickness works? I'd try that first if it were me. Can you adjust the dimensions down to the minimum they need to construction to reduce the thickness? I'm guessing you can't but I know that sometimes base plates are sized to be a little bigger than is really necessary for ease of construction. Maybe some plug welds would help you connect them so they act as one piece in addition to your groove welds, but that sounds like a lot of labor and time instead of just ordering bigger plate.

 

[dbuzz]

That's a good suggestion, re gusset plates to reduce bending moment and minimum plate thickness, if you are constained by having to use 0.75" plate.

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 (

http://www.steeltips.org).

 

[corus]

To weld two flat plates together then you can use slots or holes to weld them together. Make sure that the distance of the edge of the slot from the edge of the plate is twice the plate thickness. Refer to BS 5135, or equivalent.

corus

 

[DSB123]

corus,
BS5135 is a withdrawn/superseded standard and hence should not be used anymore. Always use the latest standards.

 

[Lutfi]

I would insist on the contractor to adhere to your design. You are trying to save him money by using what he has in stock. You and the owner have nothing to gain.

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

 

[haynewp]

I agree, I get these type of questions a lot from contractors. He should have put this in his project bid to begin with. If it's just one plate, it probably could have been bought for the amount of time you have already spent thinking about it.

 

[corus]

DSB123, Thanks for the comment. On checking, relevant standards would be BS PD 5500 and prEN 13445 with regards to fatigue, fracture toughness requirements, NDT flaw assessment and welding procedures. The guidance notes in BS5135 regarding welding of two flat plates would still be relevant, though the assessment of the welds should be to the latest standards.
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

 

[UcfSE]

I think dbuzz was correct in that his point is that without a proper connection to ensure composite action you need 4 plates stacked (not welded) to achieve the same capacity as 2 plates acting compositely. I would try to help the guy out if it works, but I wouldn't go too far with it. I wouldn't at all let them just use 0.75" in lieu of the 1.25". I'd make them do something like gussets or multiple plates or you will get a bunch of flak about overdoing it or doing it wrong it the first time and they may start picking your design apart from that point.

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.

 

[JStephen]

I disagree with the comments above that basically say "don't even consider it". By all means, consider it, and approve the change if it is reasonable. But you shouldn't have to do a lot of analysis on it, either, unless the fabricator is willing to pay the cost.

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.

 

[haynewp]

Yeah, approve the change on your time to save him money. Keep doing it and see where your budget is at the end of the month. I've been in this situation more than once.

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?

 

[HgTX]

Mightn't you also get prying action between the two plates? Slot welds can help to some extent but then there's the question of where to put them.

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

 

[dbuzz]

I'm from the "Lufti school" on this one. I'd hate to be in the dock justifying to a judge and jury, against contrary expert opinion, as to why I accepted a built-up plate when 1.25" plate was readily available at a modest, or even insignificant, cost implication to the overall project.

Surely all the preparation, welding and, presumably, NDT testing for the fabricated option is more expensive than new 1.25" plate anyway.

 

[fuchsjr]

If you weld the plates together, you should check the shear stress to insure that the weld is adequate to insure composite action for the two plates. Just a simple matter to check the shear forces along the length of the plate and then calcuate the VQ/It value.

 

[SlideRuleEra]

I recommend Lutfi's advice also.

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.