The effective length of strut in a X- Bracing

[Angre]

Hi,
In a X-brace , is it possible to take advantage of tension member countering the buckling of compression member ?
In that case is thee any literature or paper wherein this is quantified in numerical terms.
Thanks

 

[structSU10]

yes, see attached paper.

 

[JAE]

And sometimes not depending on how much tension is in the other brace...the paper may discuss this but I've read many articles that suggest it isn't the case where it ALWAYS works or always doesn't.



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[KootK]

I believe that the design standard for latticed transmission towers gives some middle of the road, easy to apply guidance on this.

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.

 

[Lomarandil]

For laced members, I've seen the compression leg (lace?) allowed K=0.7 to account for a tension member connected at midspan.

 

[Gumpmaster]

Recent research also shows that you probably can't take a reduced K factor when members are in the post-elastic range (such as special concentric braced frames).

 

[Angre]

Hi Lomarandil,
Am I correct in assuming that K is applicable to the full diagonal rather than half the diagonal.
For e.g if the brace frame is 10 x 10 square then effective length will be 0.7 x diagonal = 0.7 x sqrt(2)*10 = .7 x 1.42 x 10

 

[Hokie93]

This topic was addressed in a 2013 (3rd quarter) Engineering Journal paper ("Stability Design of Cross-Bracing Systems for Frames") by Eric Lui and Xiaoran Zhang. The paper is available (free of charge to AISC members) on the AISC website at

http://www.aisc.org.

The paper concludes with an easy-to-follow, 6-step design procedure.

 

[Lomarandil]

Angre -- correct, applied to the full length of the diagonal.

 

[JAE]

One other thing to consider (and Hokie93 - I'm not sure if the paper covers this or not) - is that a typical X-brace bay may produce compression in both braces from dead loads and other vertical loads, that when combined with lateral wind or seismic result in your tension brace having less tension in it and your compression brace having more compression in it than a more simply lateral hand analysis might show.

This has the effect of reducing the lateral bracing ability of the tension brace on the compression brace.


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[RPMG]

I would ignore it in strength because the tension capacity is much higher, and that's the failure limit. If this is a serviceability issue like lateral drift, I see no problem accounting for the compression strength.

 

[Leftwow]

JAE I see what you are saying, two columns could deflect toward each other and cause compression in both bracing. WHICH I believe as why to cables aren't used very often for bracing anymore. I honestly don't like setting them as tension only but I conclude I will set my K values 0.7, as one engineer discussed earlier.