Steel Built-Up Column

[nlh146psu]

I am analyzing a building that was constructed with the columns placed with high eccentricities. They are gravity columns and are now failing in combined bending and axial due to the eccentricities in both directions. My solution was to weld (2) WT4x14 to a W8x31. I used Enercalc to get the section properties. However, it does not give the Zx or Zy for the new shape. Can anyone help me with an equation to calculate these values so that I can finish my combined bending and axial calc for the members to propose this fix to the contractor?

Thanks!

 

[Lion06]

I've typed this out too many times to do it again. I just copied this from a previous post that I made (it was my post, so I didn't plagiarize it).

I would also be VERY careful using Enercalc to come up with composite section properties as I've had some serious problems with it. I've had numbers pop out of it that weren't even really close. I would definitely double check it by hand.

 

[nlh146psu]

I'm sorry, I am new to this site. I cannot see the solution to my problem. I see that you said you copied it, but I'm not sure where.

Thanks again.

 

[dougantholz]

SEIT was just talking about his warning, not the solution. The warning is valid, and really valid for all computer programs - be able to do it by hand. Only use the computer to speed up an understood process.
Now for the solution, use S, not Z. Conservative but it will solve your problem faster. Otherwise look for shape factor for the shape you are building, that will get you from S to Z.

 

[Lion06]

Sorry, I don't know why that didn't show up. I copied and pasted.
Let's try this again.

Find the PNA by locating the axis about which there is an equal area of steel above and below. Basically, divide the total As by 2 and determine what axis will satisfy that condition - That is your PNA.
Next find the centroid of the area above the PNA and the centroid of the area below the PNA.
Now multiply the area of steel above (or below - the will be equal) the PNA by teh distance between the centroids of teh areas above and below.
This will give you the plastic sections modulus in in^3.
It should be noted this is only valid for a homeogenous material such as all steel with the same yield stress. If you have a different yield stress (which it sounds like you probably do with the plate below - you can spec a 50 ksi plate ) or follow the same procedure as above with a slight modfication.
Instead of locating the PNA where there is an equal area of steel above and below, locate it such that you satisfy statics above and below (i.e. find the axis about which when the entire section yields you have an equal C and T). This shouldn't be that hard to do and I don't feel like writing out an equation. I would find Mp first in this case and back calc into Z.
Sum moments of each portion of the cross-section about the PNA (making sure to use the correct Fy for the plate if it happens to be 36 ksi) and that will give you Mp. I'm not sure if you can get a true Z since Fy is not constant.

 

[Lion06]

The last paragraph or so probably doesn't apply to your situation as this was specific to the last post this was in.