Question - Steel Frame Single Diagonal Bracing

[jtjc23eng]

Hello all,

I was taught in school that single diagonal bracing was not allowed per AISC Seismic Provisions 4a. System Requirements "Along any line of braces, braces shall be deployed in alternate directions such that..." Recently, I heard a different opinion from another engineer. So my question is:

1. Are single diagonal bracing systems not allowed?
2. Is this rule only for buildings? Is it bad practice to do this for non-building structures/frames?


Thanks!

 

[human909]

I don't know the code or that rule but a quick search reveals it an that you are not quoting or representing it accurately.

"
4. System Requirements
4a. Lateral Force Distribution

Along any line of braces, braces shall be deployed in alternate directions such that, for either direction of force parallel to the braces, at least 30% but no more than 70%
of the total horizontal force along that line is resisted by braces in tension, unless the available strength of each brace in compression is larger than the required strength
resulting from the overstrength seismic load. For the purposes of this provision, a line of braces is defined as a single line or parallel lines with a plan offset of 10% or less of the building dimension perpendicular to the line of braces. " {continues}

To answer your question:
1. Yes single diagonal bracing is allowable.

2. This should be made clearing by reading the scope of the document you are referencing.

"The Seismic Provisions for Structural Steel Buildings, hereafter referred to as these Provisions, shall govern the design, fabrication and erection of structural steel members and connections in the seismic force-resisting systems (SFRS), and splices and bases of columns in gravity framing systems of buildings and other structures with moment frames, braced frames and shear walls. Other structures are defined as those structures designed, fabricated and erected in a manner similar to buildings, with building-like vertical and lateral force-resisting elements. These Provisions shall apply to the design of seismic force-resisting systems of structural steel or of structural steel acting compositely with reinforced concrete, unless specifically exempted by the applicable building code."

 

[Agent666]

If you're talking about a structure with all the braces operating in tension for one direction of load and all in compression in the other direction (i.e. every brace facing the same way), then no you are not allowed to do that. It leads to a very pinched hysteresis curve if you're reliant on any sort of ductility.

Can you have a single brace in a bay, yes, maybe that is what Human909 has interpreted it as.

But at a minimum you need another brace going the opposite way to balance the system so you meet the requirements around the strength in each direction being within any ratio in your code(s). Otherwise you just get a ratcheting effect, drift just keeps increasing on one direction under cyclic seismic actions, eventually risking collapse as the drift builds up.

https://engineervsheep.com

 

[jtjc23eng]

Thanks for the responses. Follow up question.

I am wondering if a steel braced frame for a pipe support would fall under the scope of AISC Seismic Provisions. Its not a building but might fall under "Other structures are defined as those structures designed, fabricated and erected in a manner similar to buildings, with building-like vertical and lateral force-resisting elements." What are you guys' thoughts?

Customer wants a single braced system for the pipe support frame, but I want to have braces going both directions for the hysteresis curve reason. If this were a building, it would be an easy no. Since its not a building, I am struggling to find a code reference against this.