Evaluating K value for Columns in Platforms out of Braced Plane 1

[JeffLam]

Hello all,

I have two questions I'd like some advice on. I hope with your expertise I can work through these columns! I am just a junior structural working in mechanical industry with no senior mentors.

Scope of the story - platform design was outsourced by a FEA company and they made our columns rather oversized using formed channel back to back that is .5" thick sheet metal. Turns out they use ky and kx values equal to 2. Per AISC, since there is no actual bracing and straight up columns, I will agree, but there are semi possibilities that will reduce K under the right conditions. (which leads to my next 2 questions)

Question #1.
I'm trying to determine the K value for these columns on a structural platform. I remember reviewing sidesway inhibited and uninhibited in the AISC manual, but usually it's in one plane.
I'd like to know what you guys think about the k values for columns in other planes parallel to the picture I have provided.

Question #2.
Does this frame concept allow me to use column K values equal to 1? I feel like during earthquakes, it would allow the struts to direct transverse loads into the column creating a combined loading "beam-column" effect. Regardless, I'd like to know if this would be considered sidesway inhibited because technically I see that where the struts connect to the column is actually helping to restrain AND lower the length of the column. (k =1 and L is lower)

input would be greatly appreciated thanks!!!

Much Thanks!

Jeff, EIT

 

[JeffLam]

Picture attached to the post!

 

[JAE]

For question 1 - you have braces in one corner of the platform but not in any other areas. The columns aligned with the brace, in the direction of the brace, would be braced with K=1.

The other columns will be subject to sidesway and in those other areas and directions you would need to have either fixed bases or moment connections at the tops of columns to resist lateral loads. These columns would be either K=2.0 or K=1.2 (approx.) based on the conditions.

For question 2 - the knee braces are similar to effectively creating a moment connection at the top of the column. With a column pinned base, you would have sidesway so your K would be equal to 2.0 but the length of the column would be from the base to the bottom of the knee brace, not the full length of the column.

If the base was fixed, then k=1.2 +/- and the length again from base to knee.

 

[JoshPlumSE]

Jeff -

You can probably take this as a response to question 2, but it is really a general response to any K factor questions.

If you use the AISC Direct Analysis method (which essentially relies on a P-Delta analysis to account for column buckling) then you can always use a K value equal to 1.0. That's actually the major strength of the DA Method, it takes most of the guess work out of setting K values for these atypical cases.

On another note, the 2005 ACI code has some criteria (10.11.4.2) about what constitues a sway vs. non-sway frame. Essentially, you calculate a stability coefficient:
Q = P*Delta / (V * H)

If this value is less than 0.05 then you can be considered non-sway.