Reinforcing existing steel column
Reinforcing existing steel column
(OP)
I have an existing steel column with 8ft deep trusses that frame into it on each of its 4 sides. These trusses will transfer new load to the column and so the column will need to be reinforced. I am looking at welding plates to the column to form a boxed section. The reinforcing plates will extend to just 4in below the truss bottom chord.
How would I go about analyzing the capacity of this new reinforced column? As the reinforcing does not extend the full height of the column (to the truss top chord where the load is imparted) is the reinforcing even effective given that there is a weak link, the top 8ft?
So if the reinforced column is 30ft tall and the unreinforced section is 8ft tall above it, should each segment be designed for the full unbraced length of 38ft or their respective lengths only (30ft and 8ft)?
My understanding is that it is the former, and so the column reinforcing as suggested is not effective at all. Intuitively, I feel that is not the case, but I can not rationalize it.
Any suggestions?
How would I go about analyzing the capacity of this new reinforced column? As the reinforcing does not extend the full height of the column (to the truss top chord where the load is imparted) is the reinforcing even effective given that there is a weak link, the top 8ft?
So if the reinforced column is 30ft tall and the unreinforced section is 8ft tall above it, should each segment be designed for the full unbraced length of 38ft or their respective lengths only (30ft and 8ft)?
My understanding is that it is the former, and so the column reinforcing as suggested is not effective at all. Intuitively, I feel that is not the case, but I can not rationalize it.
Any suggestions?






RE: Reinforcing existing steel column
Can you temporarily support the trusses and reinforce to the top? This will remove the "locked in" stresses and make the reinforcing more effective. Plus now you've reinforced the whole column.
If you can only reinforce part of the column and it increases your "r", there's a way to combine the "r"s over the whole length. This should increase the capacity of the column, maybe enough to just use the smaller section above the reinforcing.
RE: Reinforcing existing steel column
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Reinforcing existing steel column
If your column is not braced by the truss bottom chords, you'll probably want to treat it as a stepped column for buckling. Let us know if that's the case and we'll help you prosecute that.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Reinforcing existing steel column
For the new section, use the composite shape and analyze over the full height for buckling. You could do a two step stress analysis but I think the residual stress on the original column will not harm the performance of the boxed out section (probably the opposite).
If the trusses do not laterally brace the column at the bottom chord due to sliding or slots, then you should analyze the column as unbraced for the full 38 ft. I would recommend not changing the sliding condition.
For this case, you could model the lateral buckling deflection based on the column with 2 different curvatures corresponding to the two different column stiffnesses.
RE: Reinforcing existing steel column
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Reinforcing existing steel column
RE: Reinforcing existing steel column
Reinforcing the bottom 30' of a 38' column will do a lot for the capacity of a buckling-driven column. I know I was surprised the first time I tried to make it work.
Most practical approaches develop a modified effective length factor based on the ratio of lengths and Is of your two cross sections. If the column passes a buckling check with the modified K and total length, the unreinforced sections only need to be adequate for yield (or local buckling issues, I suppose).
Typically, if the radius of gyration of the reinforced section is at least 85% of the radius of the original section, the reinforcement can be considered "stabilizing" and the pre-load can be neglected. If not, you'll need to consider additive pre- and post-reinforcement stresses on your original cross section.
(all of this assumes that the column is in fact not braced by the truss bottom flange)
RE: Reinforcing existing steel column