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Strengthening of Steel Column

Strengthening of Steel Column

Strengthening of Steel Column

(OP)
I am looking for some guidance on strengthening existing steel columns for a major renovation project. All columns are A992 Gr. 50 steel and will be reinforced with A36 grade plates welded to the flanges. But, I am having hard time estimating the compressive strength of the stiffened member. Is there any guidance available on estimating Fcr for a member that uses two different grades of steel? Your thoughts are greatly appreciated. Thanks in advance.

RE: Strengthening of Steel Column

There is a paper out there from the 4th Quarter, 1988 Engineering Journal called:
"Reinforcing Loaded Steel Compression Members" by J.H. Brown.

I can't find my copy of it.
I know it is available through AISC for free for members and for $10 otherwise.

Not sure if it covers different materials.  

RE: Strengthening of Steel Column

(OP)
Thank you for your responses.
I did go through the papers...but, I couldn't find a case where two different material strengths are addressed.

RE: Strengthening of Steel Column

Do you have to use A36 plate?  When I have this issue, I normally call for A572 Grade 50, and then I don't have to worry about it.  I don't think there's much of a price premium.

RE: Strengthening of Steel Column

(OP)
Gumpmaster...we did specify Grade50 plates. But the fabricator is asking if they could use A36 plate steel. I can't just say no without giving him a valid reason.

RE: Strengthening of Steel Column

If the column is in the elastic buckling range, could you not consider both to be A36.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Strengthening of Steel Column

(OP)
paddingtongreen -

The KL/r for the column isn't big enough for stresses to be in the elastic range.  

RE: Strengthening of Steel Column

Wouldn't a column with a large value for KL/r be more likely to in the elastic range?  

RE: Strengthening of Steel Column

(OP)
Toadjones -

You are right. But, the KL/r isn't large enough for the column to fail because of inelastic buckling.

RE: Strengthening of Steel Column

(OP)
type in my earlier post

"....fail because of elastic buckling"

RE: Strengthening of Steel Column

Can you assume 36ksi for the whole section and get it to work?  You're not only increasing the area, but you're also increasing the radius of gyration, so the allowable stress goes up, too.   

RE: Strengthening of Steel Column

EIT-
I have read the second paper your posted in the past and I'm not sure I'd ever go that route. The whole process seems way too academic to me.
Thoughts?

RE: Strengthening of Steel Column

I have had the same problem with 50 ksi wide flanges with 36ksi cap channels....I am trying to remember how I handled it.  

RE: Strengthening of Steel Column

(OP)
StructuralEIT-

I could definitely do what you said. But, wouldn't that be conservative and force me into using thicker plates? I just wanted to know if there is any other rational approach to the problem, and get away with a little additional steel as possible.

RE: Strengthening of Steel Column

On consideration, I think you have to go with SEIT's suggestion, the A36 will be connected to the A992 but if you must not strain the material at the interface beyond the A36 limit, it becomes a limit for the A992 as well.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Strengthening of Steel Column

What about this?  

Use the new kl/r to get the allowable stresses in the plates only (call fa1).  Use the new kl/r to get the allowable for the WF only (call fa2), then subtract the existing stress from the WF (call fe).  Use the difference (call fa3 = fa2-fe).

Hopefully fa3 is close to fa1 so that you don't need to have different strains in different parts of the column to reach full capacity.  I would probably use the lower of fa3 and fa1 on the entire section.  I don't think I would want to get any more refined than that.

My biggest concern is that if the plates yield while the WF is still elastic, they are going to soften and effectively reduce the E of those plates.  That would have detrimental effects on the overall column.  I would limit fa3 above to no more than fa1.

RE: Strengthening of Steel Column

Sounds like a good article for next quarters Engineering Journal.  

RE: Strengthening of Steel Column

I agree with Structural EIT, it is important to remember that higher grade steel still has basically the same E which means it has to yield further to achieve the higher yield strength.

RE: Strengthening of Steel Column

ToadJones (Structural)      
24 Aug 10 15:31
"Sounds like a good article for next quarters Engineering Journal."

Start writing!  

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Strengthening of Steel Column

When I have done this I have treated everything as the lowest yield strength because of the strain compatability issue mentioned above.

I cannot see any way around that.

RE: Strengthening of Steel Column

csd-

If the column is already loaded, you can have a strain correlated to 14 ksi already in the column (just for talking purposes).  If you now add 36 ksi plates, you have 36 ksi left to yield in the 50ksi column which provides for strain compatibility with the 36 ksi plates.  That is a rare condition, which is why I noted above to use the lesser of fa1 and fa3 - I agree with you 100% on the strain compatibility issue.

RE: Strengthening of Steel Column

Yes I thought about that, but how do you weld to it while it is under stress? Also need to make arrangements at the top and bottom for transferring load onto the new plates.

Not saying its not possible, just not sure how it could be achieved outside a laboratory.

RE: Strengthening of Steel Column

J.H. Brown's paper covers load transfer at the ends.

Also, I think as long as you are primarily using longitudinal welds and not welding across the column web or flanges welding under load can be done.  

RE: Strengthening of Steel Column

(OP)
Thanks for the responses, fellow engineers. I checked the AISC Design Guide 15, and it clearly mentions that there are two different schools of thought on the effect of initial stress on the ultimate strength of the stiffened column. I am going ahead with the assumption that the initial column stress doesn't impact the ultimate strength of the stiffened column, as the retrofit is being done for progressive collapse enhancement anyways.

RE: Strengthening of Steel Column

Can partial length plates be used to strengthen columns?

RE: Strengthening of Steel Column

Sure. I know we only do a single allowable stress check for the column, but realistically, a section with a smaller I, r, and A at the pinned location won't negatively impact the buckling capacity of a column - assuming the kl/r is very low.   

RE: Strengthening of Steel Column

Yes but what you would need to do is use the original area for stress and the increased area for Kl/r calculation.

Technically it is a stepped column but if the ends of the plates are close enough to the top and bottom then it shouldnt make a difference.

RE: Strengthening of Steel Column

More food for thought (especially for "ultimate strength" design)

Tensile strength of A36 is 58 ksi
Tensile strength of A992 is 65 ksi.

Not really all that much different, is it? Don't over-complicate the analysis.
 

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