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K Frame Tower Diagonal K value calculation

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Antnyt23

Structural
Joined
Jul 11, 2012
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US
Can anyone help me better understand the K value calculation for diagonal members of K frame tower sections.

This is my understanding of a K2 down frame. From table 4-7 The unbraced length of the diagonal is L1 = 1/3 L (1/3 total length of the diagonal). Your slenderness ratio is then determined by the larger of (l1/rmin, L2/rout, h1/rmin). Table 4-4 then has the effective slenderness ratios for bracing members. In my case my L/r > 120 allowing restraint to govern. I use case 6 in my instance due to partial restraints at both ends giving me KL/r = 46.2 + 0.615 L/r (I'm using a pipe member and the equation comes out as follows). K= 46.2 r/L + 0.615 = 0.75.

Now here is my question looking at a previous analysis conducted they use the K values of the diagonal members of a K2 down frame as K=0.27 and of a K1 down as K=0.4 (I did some testing and appears to be K=0.8/2 and 0.8/3 for the K1 and K2 respectively). I was just curious on the correct way to calculate the K value for a K frame.
 
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