Built-Up Column - Double Channel Axial Load Capacity Approach

[EastEng1012]

I am evaluating an existing surge tank at a hydro-plant for code compliance. The surge tank consists of steel riser and cylindrical tank with 8 built up columns supporting the tank at the springline. The columns are braced with rods and struts at 3 locations. The built-up column consists of double channels (W12x30) with a continuous back plate and on the opposite side V lacing. The lacing and plate are connected by rivets. The built-up member has been checked and does not contain slender elements.

My question is concerning the approach to evaluating the axial capacity in accordance with AISC Specification Section E. I understand that I have to modify the slenderness ratio according to E6 to account for shear deformation.
1. Should I evaluate the column considering the section properties of just the double channels and ignore the continuous back plate? If I do consider the backplate, would the shear center need to be calculated?
2. Does Section E4 Flexural torsional buckling apply to this section?
3. Should my computed critical stress be similar to the values in Table 4-22 of the AISC manual?

Thank you!

 

[JoshPlumSE]

1) That's what I'd do. It's just so much easier to deal with if you consider it a doubly symmetric sections.

2) I don't think FTB applies to this section. Especially if you analyze it as a doubly symmetric shape. Torsional buckling should be much of an issue either, you might check it for academic curiosity. But, I'd be very surprised if it controlled.

3) Yes, I would think you would match these values pretty closely.

 

[KootK]

1) That's definitely what I'd do. Were you to include the back plate, you would need the shear center.

2) I agree with Josh. While those modes technically always apply, they're highly unlikely to govern except for members severely lacking torsional stiffness (angles, cruciform,..). I believe that FTB is also irrelevant if your weak and strong axis unbraced lengths are identical as weak axis would still govern then. Torsionally, you've effectively got either a wide flange here or, depending on your gap dimension, effectively a tube. Both of those possess meaningful torsional stiffness which works in your favor.

3) I would expect so if you're entering the table with slenderness parameters consistent with both channels acting compositely.

I understand that I have to modify the slenderness ratio according to E6 to account for shear deformation.

With the continuous plate there, you shouldn't have to do this for straight strong/weak axis buckling. Torsional properties would be affected by the shear stiffness of the lacing but it's hard to imagine anybody going into that much detail in routing work.





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.

 

[EastEng1012]

Thank you both Josh and KootK!!!