Column Bracing to Increase Column Capacity
Column Bracing to Increase Column Capacity
(OP)
This came up in the thread765-368271: Old Design flaw, what to do., where existing columns were noticed to be under-designed. In discussing how to remedy the issue, I was looking to simply decrease the braced length of the columns by adding a bolted T-C brace in each axis, angled at 45 from the column up to the roof structure. In most cases, I only have access to two sides of each column due to existing ductwork, which is why I only had one in each axis. My concern with this approach was that the brace would be adding an additional horizontal load on the column from the roof structure as the existing roof beam deflected under snow load, and actually cause more harm than good when I add the moment and have to start using the interaction equations.
I wanted to post the question in this forum to get others' opinions on this as well.
Thanks
I wanted to post the question in this forum to get others' opinions on this as well.
Thanks






RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
What about welding on reinforcement plates to the column?
Maine EIT, Civil/Structural.
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
I've begun to do a quick Risa 3D model, in which I add the physical brace and record it's axial load from only snow. I then convert that to it's horizontal and vertical components and add them as joint loads for snow only at the brace elevation, turn off the physical brace and set the braced length of the column in the properties. Does this make sense? Otherwise, the model imposes dead load horizontally on the column as well.
TehMightyEngineer, I had thought about increasing the radius of gyration by adding tubes to each face, but wanted to explore the braces first, as it would be less material and less demolition. That's my next exploration though, and how to figure exactly how much of each column needs reinforced.
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
Maine EIT, Civil/Structural.
RE: Column Bracing to Increase Column Capacity
The columns are loaded now and are assumed to already be in single curvature since they are not blemish free. Adding a brace now might force them into "S" curves under additional loads (roof or snow), or reinforce the existing single curvature, but either way, snow load will also put a diagonal force into the column from the roof level, and push the column back into single curvature. The vertical component of the brace will be some of the load that would otherwise reach the column from the roof system.
You cannot add the brace and say it is effective in bracing the column and then say it is not effective in bending the column.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
I think bracing in the direction of the joists is more complicated than I initially thought. I hadn't thought as much about bracing effect on the joists, only that they wouldn't put as much load in the brace as the beams would, and that was my main initial concern.
I know that there are cases where adding the braces would be beneficial, but the more I look at it, I just don't think I have the right scenario to adequately implement this type of fix. I think that the columns are just such that reducing the braced length barely works out for axial; adding any moment puts me over on interaction equations. As I change the the brace point to reduce Lb, it only makes the moment worse in the column. If I had a scenario where I didn't need to increase the capacity quite so much, and had beams in both directions, I could easily justify bracing in two directions.
If the footings are oversized enough, could I assume that the base is fixed and use a K=0.8? Or is my only other option to add steel to the section in order to increase the radius of gyration? If so, can I reinforce over the middle 1/3 of the columns? I thought I saw another thread about that earlier. I will try and find that.
Any other suggestions are welcome.
RE: Column Bracing to Increase Column Capacity
-Honestly, I don't know enough about how a non-prismatic column section would affect the capacity, but at the end of the reinforcement you still have the same axial load, cross-sectional area, K, Lb, and r, so I fail to see how that solves the problem. I would just reinforce the column top-to-bottom and be done with it.
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
Did you consider my note above about filling the columns with concrete? Should be doable and result in a minimum of demolition assuming the foundation can take the extra load.
Maine EIT, Civil/Structural.
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
Hopefully I explained that well. Attached is a paper on using the finite difference approach to find the buckling load of a non-uniform column.
RE: Column Bracing to Increase Column Capacity
I doubt that concrete filling a 4" x 4" column, 19' high, is indeed doable, and doubt that it would help much anyway.
RE: Column Bracing to Increase Column Capacity
Maine EIT, Civil/Structural.
RE: Column Bracing to Increase Column Capacity
RE: Column Bracing to Increase Column Capacity
Either way, I think I need to increase my capacity by so much that simply reinforcing say the middle 50% of the columns won't accomplish much. But that does get me thinking, if you’re reinforcing the column, but only from just above the slab to just below the beam connection, you're really only increasing stiffness and can only count on the increased r, right? So in my critical buckling load calc, I'd still use the gross area of the original column. If this is the case, I need to nearly double my r, which is more than I can reasonably accomplish by simply adding a small tube to two sides of my column. What would I need to do to be able to count on the additional area of the reinforcing in my axial capacity?
RE: Column Bracing to Increase Column Capacity
If you cannot increase "r" then you have to check the modified column buckling load and if you need it, backfit an equivalent "r". There is an iterative procedure or you can use the method(s) stated in the posts above.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Column Bracing to Increase Column Capacity
Say you welded plates on each face of the EX. HSS column. When load is applied to the HSS column it will deform. Well that deformation is going to deform the plates also. Therefore there is shear transferred from the column to the plates and back again I suppose. At this point you have a built up column but it is non prismatic as discussed earlier. What if you placed a brace at the transition point near the top of the column. Then you treated the column as two separate columns with "moment" connection at the brace point. Apply the horizontal loads, P-Delta, etc. It's getting late for me so hopefully this makes some sense...
EIT
www.HowToEngineer.com
RE: Column Bracing to Increase Column Capacity
Paddington...yes, I also agree that it boils down to an eguivalent r....but, unfortunately, not easy to claculate with any degree of confidence.
I had a somewhat similar situation a few yrs back. The following is an example as I cannot remember the exact details. It was an unbraced beam, say 24ft long with a W8x24 for the first 5ft at both ends and a W16x45 in the middle section. The problem was how to calculate the allowable bending stress or the equivalent rt. of the compression fla of the W16x45. I posed the question to AISC and they could not come up with solution. What I ended up doing was to bound the problem by taking the rt of the W8x24 for the full length but using the stress calculated on the W16x45. So one solution for the OP would be to take the r of the original section for the full length to calculate the allowable stress and calculating the actual stress from the reinforced section. I realize this is a conservative approach but have not found any practical alternative.
RE: Column Bracing to Increase Column Capacity
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Column Bracing to Increase Column Capacity
http://www.eng-tips.com/viewthread.cfm?qid=288365
EIT
www.HowToEngineer.com