Column Bracing to Increase Column Capacity

[NinerStruct]

This came up in the thread765-368271, 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

 

[TehMightyEngineer]

Hokie, hmmm, looks like you're right. I incorrectly thought they gave you an increase in compressive capacity for concrete filled tube columns but it appears not per the ASIC tables.

Maine EIT, Civil/Structural.

 

[hokie66]

Concrete filled steel tubes are used in some commercial building designs, medium to high rise buildings. Procedures have been developed for this, although I don't know the status of formalized code provisions in the various countries. But these are large columns, not skinny little tubes. Small prefabricated and filled "Lally" columns were used some years ago to give a boost in fire rating, and that may still be the case, although I don't know.

 

[NinerStruct]

StructSU10, I assume that after you adjusted your K, you still used the Ix of the original column then in your critical load formula? I’m sure that the paper will probably explain further, so I’ll take a look at it tonight. Thanks for sharing.

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?

 

[paddingtongreen]

Niner, don't confuse buckling load with yield stress, buckling takes place in the central section of the span, you can go up to your normal proportion of yield at locally. In the middle, you have the increased area. Buckling is listed as a stress limit in most books, which bugs me because it has little to do with stress.

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

 

[RFreund]

I'm not so sure about only using the Area of the Original column.

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

http://www.HowToEngineer.com

 

[SAIL3]

StructSU10...interesting paper, but, it would be beyond my practical capability to make use of it on a normal project.
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.

 

[paddingtongreen]

I'm not sure which, but one of our members has a paper on his site with an iterative procedure. You divide an axially loaded column into about six lengths, assume a curve over the full length, calculate the deflections and moments, calculate a new deflected shape due to the moments; repeat until they close. calculate a new Euler load and apply the current multiplier to get the capacity. The division points are placed at section changes so you might end up with eight. Without section changes, six gives a near perfect result.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[RFreund]

This thread may be of some use:

http://www.eng-tips.com/viewthread.cfm?qid=288365

EIT

http://www.HowToEngineer.com