effective length of knee braced columns 2

[bjb]

Is there a reference someone can point me at that has a method for determining the in-plane effective length of knee braced columns? It seems that it would be conservative to use K=2 times the full length of the column, but that's how I would do it without having a better procedure. The column in question is steel, one-story, with an actual length of approximately 21'.

 

[SlideRuleEra]

Here are a couple of links that may help:

http://www.aisc.org/MSCTemplate.cfm...tomSource/Faq/SteelInterchange.cfm&FaqID=1887

and

http://www.hnd.usace.army.mil/techinfo/ti/809-04/ch7b.pdf

 

[dbuzz]

What proportion of the column length of 21' is above the brace point? If it's only a few feet, then it's not going to be a very effective brace.

 

[JAE]

I typically use the length of the column from the ground, to the knee brace intersection as one segment of column to consider...and the length of column within the knee brace triangle as a second segment to consider.

Each are two separate design checks (in the plane of course) with a single check out-of-plane.

So the k for the lower in-plane check would be 2.0 with the L being the length from the ground to the brace intersection.

The k for the upper segment would be 1.0 (assuming pinned end at beam) and the length L is the length from the brace to the beam.

 

[dbuzz]

JAE, spot on!

 

[vmirat]

The Structural Engineering Handbook (Gaylord and Gaylord), Table A4, has K-factors for several column conditions. Since bjb didn't mention the column fixity at the bottom, but did mention the use of K=2.0, I would assume that the column in question is fixed at the bottom and pinned at the knee brace and top beam connections. Table A4 has a K value of 0.7 + 2k for fixed-pinned-pinned. "k" in this formula is the ratio of short column length to long column length (i.e. k < 1). If this is a pinned-pinned-pinned condition, then K = 0.7 + 0.3k. In each case, L is the distance between the end and knee brace.