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Bracing Bottom Part of Knee Bracket To The Weak, or Minor, Axis of Columns

Bracing Bottom Part of Knee Bracket To The Weak, or Minor, Axis of Columns

Bracing Bottom Part of Knee Bracket To The Weak, or Minor, Axis of Columns

(OP)
Can we connect the bottom part of knee braces to the weak, or minor, axis of columns?

It is my natural engineering judgment to want to brace the bottom part of the knee bracket to the strong, or major, axis of the column. Is this a correct judgment?

If I have to bracethe bottom part of my knee bracket to the weak axis of a column, I would place it as close as possible to the part of that column at the joints, or where that column is properly braced in both horizontal directions. I do not want to place ("dump")unnecessary loads on the weak axis of the column, and not especially, at the mid-point of that column, that is not braced in any other way.

I have been told by some other engineer that we always brace against the weak axis of the columns. According to him, this is because the moment of inertia, I, about the weak axis is lower. Is this a correct association? I believe he might have meant that it is good to brace in the weak axis because this will lessen the unbraced length about the weak axis, but this is hardly the same as placing a knee bracket there, and "dumping" loads against the weak axis.

RE: Bracing Bottom Part of Knee Bracket To The Weak, or Minor, Axis of Columns

If you are in a higher seismic area you do not want to use knee braces at all.

Bracing a knee into the weak axis of a column (if it is allowed) can work - you just need to check the lateral thrust moment in the column along with its axial loads under the required load combinations. Also need to probably take into account second order effects per your steel code.

It can work - just depends on your weak axis & axial combined capacity.

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RE: Bracing Bottom Part of Knee Bracket To The Weak, or Minor, Axis of Columns

Seismic issues aside, the frame will be more stable with the braces coming in to the weak axis of the columns. Framed that way, there is no possibility of torsional or lateral torsional buckling in the columns. Of course, all other things being equal, the frame would be stiffer with the columns oriented string axis. And the connections would be more conventional.

I rarely do knee braces anymore. I almost always find that extended end plate bolted moment connections are a more attractive alternative.

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.

RE: Bracing Bottom Part of Knee Bracket To The Weak, or Minor, Axis of Columns

(OP)
Thank you so much for your responses, JAE and KootK.

A quick question for KootK; what did you mean by, " ... with the columns oriented string axis", when you said, "The column would be stiffer with the columns oriented string axis".

Also KootK, I see your point about the knee brace preventing torsional, or lateral buckling in the columns, but I've already design my columns to resist that, even without the bracing. Does your argument about bracing causing the frame to be more stable still hold true?

JAE, you said that bracing a knee into the weak axis of column can work, and if it is allowed. Besides higher seismic areas, are they any other conditions why this would not be allowed?

From both your responses, I gather that while it is okay to brace a knee into the weak axis of a column, this might not be the best solution? Is that a fair assumption. Is it better to brace the knee into the strong axis of a column, as one might deduce instinctively?

RE: Bracing Bottom Part of Knee Bracket To The Weak, or Minor, Axis of Columns

No - only seismic restrictions on the use of knee braces to my knowledge.

Bracing a knee into the weak axis is fine but may not be efficient.

Check out Eng-Tips Forum's Policies here:
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RE: Bracing Bottom Part of Knee Bracket To The Weak, or Minor, Axis of Columns

Quote (OP)

A quick question for KootK; what did you mean by, " ... with the columns oriented string axis", when you said, "The column would be stiffer with the columns oriented string axis".

String axis = strong axis.

Quote (OP)

Also KootK, I see your point about the knee brace preventing torsional, or lateral buckling in the columns, but I've already design my columns to resist that, even without the bracing. Does your argument about bracing causing the frame to be more stable still hold true?

Yes, it does still hold true. Lateral torsional buckling is about a member trying to resist bending via strong axis resistance and then flipping over to the weak axis position. If you start at the week axis position, there's no potential for this. That being said, if you've designed for it, you've designed for it. The point that I was trying to make is that, from a member stability perspective, bracing to the column weak axis isn't any worse than bracing to the column strong axis. It may, in fact, be better.

Quote (OP)

From both your responses, I gather that while it is okay to brace a knee into the weak axis of a column, this might not be the best solution? Is that a fair assumption. Is it better to brace the knee into the strong axis of a column, as one might deduce instinctively?

I only think that bracing to the strong axis is preferable for the two reasons that I mentioned above:

1) That column orientation is more efficient when it comes to providing frame stiffness.
2) That column orientation may lead to more conventional connection detailing.

All other things being equal, it would behoove one to orient the columns such that the knee braces tie into the strong axis. However, if there are reasons to orient the columns the other way in your particular case, I see no cause for concern.

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.

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