Strengthening of Steel Column 1

[strucguy]

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.

 

[paddingtongreen]

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.

 

[csd72]

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.

 

[Lion06]

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.

 

[csd72]

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.

 

[ToadJones]

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.

 

[strucguy]

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.

 

[slickdeals]

Can partial length plates be used to strengthen columns?

 

[Lion06]

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.

 

[csd72]

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.

 

[JLNJ]

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.