Single diagonal braces orientation in 3D braced frame system 3

[TTTKAO]

Hello All,

I am designing a simple braced frame system with single diagonal alternative braces. my colleague recommended to reverse the single brace orientation at the other braced bay. the reason is to meet the condition- one braced bay at tension condition and the other braced bay at compression condition under the lateral load. I attached pic for the detail information. by the way, i am following AISC 360 due to low R factor(R=1.5)so i am not replying on the steel ductility.

1. is this common practice? I can't find or remember any example like this
2. is there code to address this requirement ?

Thank you for your help and advice!

 

[Lomarandil]

Short answer -- no, it's not a common practice or really beneficial. You'll be designing the system to work with lateral load in each direction (I presume), so while it may be theoretically "better" to have one brace in tension and one in compression, that's not necessary. And it can potentially introduce some torsional effects which can be detrimental (on top of additional construction complexity).

For a related discussion, see this recent thread

 

[JLNJ]

If this were a 4-sided frame I might run the braces in opposite directions so that the different exterior elevation views are all similar. It might make it easier on the detailer.

Other than that, no.

 

[human909]

To be blunt colleague is wrong. Steel is just as stiff in compression as it is in compression. So unless you are designing compression struts to buckle then your colleague's logic is wrong. If the struts do buckle under lateral load then you'll have a torsional issue.

For lateral loads there is a very good reason why you find tension members in pairs. You likely already know why. If they are compression & tension members then you would expect equal stiffness in both directions and the brace direction doesn't really matter. Though if they alternate directions within a bay then you'll get less axial load on cross members.

I design these sort of steel towers all the time. Due to the presence of penetrating equipment I normally opt for compression struts throughout for consistency as cross bracing gets in the way. Mechanical or architectural reasons mostly dominate. In the absence of these I often start with brace pairs meeting at nodes on opposite corners of the square.

Note your tower seems to show significant eccentricities in your bracing system which would put excessive loads on your columns.

 

[TTTKAO]

Thank you all of the replies and explanation. I am not considering the compression member buckling. my colleague's suggestion may be true theoretically after compression buckling, but with some uncertainty as well.

My load is very small, due to clashing with other members, the tower have to be have eccentricities.

 

[TTTKAO]

Could you explain more "In the absence of these I often start with brace pairs meeting at nodes on opposite corners of the square." i didn't get this point. wondering if you could show a pic what you normally do for four columns tower.

Thank you so much in advance.