SCBF: X-brace

[BAGW]

Hi all,

Is X-cross bracing allowed in SCBF SFRS? Is there any special requirement at the X-brace intersection? Could not find anything on this in the code.

Thanks

 

[dold]

Yes. If youre going with this approach i'd probably stick with the tension-only provisions in AISC 341. IIRC the braces and columns must be designed for overstrength. Connections (clevis...clevii(?), turnbuckles, whatever youre using are designed for overstrength*1.5). Essentially they recognize that there's a lack of ductility in this system so you're bringing the forces back up to a "steel not detailed" level of demand. I don't remember the chapter but just look in the index for "tension-only" and you'll find it.

That said, i've only ever used these exceptions for tension-only rod bracing. Not-so-much for HSS-type spliced middle gusset ordeals that i think you're talking about.

 

[dik]

I was unfamiliar with SCBF and from the net...
"The Mission of the Scared Cat of Burma Fanciers is to promote the quality, health and temperament of our breed, the Birman. As a non-profit organization, we intend to generate income in order to promote our club activities, donate money to the research of cat health issues and to support other clubs' efforts to do the same."

These acronyms just send me for a spin...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAGW]

@ dold

Tension only is not permitted in SCBF. Why not tension/compression? The splice at the intersection will be designed for the capacity of the member.

 

[dold]

Ah yes you're right. I was thinking about the OCBF tension-only exception which i've used for tension-only in SDC D projects. I.e., rod bracing for 'small' structures where R=3 isnt permitted.

But back to your question: I've never actually designed a single-tier x-brace SCBF. Only two-tier module types, where the 'x' occurs at an intermediate floor beam. The floor beam provides lateral stability at the connection. That said, if you look at 341-16 fig C-F2.15 it appears to show that this configuration is permitted, or at least addressed in regards to protected zones. Other than that, I can't find anything in the code that specifically addresses what you're asking. So we're back to square one.

I've got some details in my 'library' for SCBF connections at the X-intersection but I can't back them with any references.

So, two thoughts - spitball style:
1) Design the x-intersection gusset to develop the full compressive strength of the brace plus whatever material overstrength. I'd imagine it'd be a pretty thick plate and brace reinforcing.
2) Dig up some reference that lets you say the tension brace 'braces' the compression brace at the intersection and detail the gusset like you would the other gussets...

Interested to see if anyone else has input.

 

[straub46]

I think as long as you follow the 70%/30% rule for the braces, there isn't anything special you would need to do. I would agree with designing the gusset to develop the full compressive strength of the brace like dold said.

Here's a snippet from the AISC 341-10 Commentary showing X-bracing without an intersecting floor beam as being an allowable configuration:

Go Bucks!

 

[Deker]

thread507-430632

 

[hoshang]

I think as long as you follow the 70%/30% rule for the braces, there isn't anything special you would need to do.

I'm not aware of this. Where's this statement coming from?

 

[JoshPlumSE]

I think as long as you follow the 70%/30% rule for the braces, there isn't anything special you would need to do.

I think he might be referring to the idea that the "post buckling" strength of the compression brace should be considered to be 0.3 times its expected brace strength. For purposes of evaluating strength of members and connections for the case where the forces in compression / tension members don't balance (see section F2.3 - Analysis).

 

[straub46]

I was thinking more of the allowable tension resistance being between 30% and 70% of the total horizontal force from F2.4a, but if I remember correctly I'm pretty sure they're related.

Go Bucks!

 

[HTURKAK]

The following document ( Seismic Design of Steel Special Concentrically Braced Frame Systems nistgcr13-917-24 ) is useful for practicing engineers..