Steel Column Reinforcing

[KOTOR1]

I have an existing column with additional load coming down due to a new mechanical unit (about 2500#) on the roof. The existing column was already over stressed due to the existing loads (as per my calculations by about 10%). The column is an HSS4x4x1/4 (14'-8" tall) w/ 28k DL and 13K LL (existing loads). I am assuming the column to be fully unbraced.

Would reducing the unbraced length by providing braces be the way to go or can I reinforce the column in some other way. The owner prefers not to replace the column due to various medical equipment around the column.

 

[hokie66]

You could either brace it or reinforce it by welding on some plates. For the reinforcing solution, keep the section symmetrical. The best approach will depend on factors like access space, whether you have other elements which can resist the bracing force, etc.

 

[csd72]

It depends on how the numbers work out, if t is the slenderness that dictates the design then any tpe of stiffening or bracing would do the trick.

You could potentially weld an angle to each corner to increase the buckling resistance.

 

[DaveAtkins]

Does the base plate have four bolts? If so, did you assume K < 1 when determining the column axial capacity (fixed base, pinned top)? You will be surprised how much this will improve the column's capacity.

DaveAtkins

 

[Lion06]

The first question I would ask is did you do your design check using LRFD? Your LL/DL ratio is very low, so you will get a healthy benefit by using LRFD.

If you use 14.67' unbraced length, you get an allowable capacity, using 13th ed. ASD, of 37.75k (you have 41k +2.5k add'l) - so you are overstressed by 9% w/o the new load and 15% w/the new load. If you use LRFD, you have a capacity of 56.7k. Your Pu is 54.4k + 3k add'l for a total Pu of 57.4k. Now you are ok w/o the new load and over capacity by 1% with it. That looks fine to me.

 

[haynewp]

You have to be careful when recalculating column capacities when adding on new plates to an overstressed column, if you go that route. Example; if the stiffness modulus has gone to zero at the existing column, you can't apply r=sqrt(Itotal/Atotal) for the new built-up section to calculate the new total column capacity.

 

[swivel63]

StrEIT's solution sounds good to me.

 

[kslee1000]

StrEIT's approach is technically sound. However, one caution, if it turns out no strengthening required, one should ask is there potential problem on other components, and adjacent structures. Especially the justified new load (useing LRFD) exceeds the original design capacity (from ASD) by more than 15%.

 

[KOTOR1]

We are thinking of reinforcing the column. What kind of sections are a general for a stiffening the column ? I was thinking of using a C section (C4x7.25) on two sides of the column to stiffen it up.
Are there any code regulations for strength of built-up columns?

 

[Lion06]

The two channels would work, you could also try an angle at each corner.