Unbalaced vertical force on SCBF

[leenlee]

Hello, all

I am designing chevron braced frame in one story tilt-up building.
And zipper column should be needed by unbalanced vertical force.
But spreadsheet attached shows Qb = 0 because this is one story building(= top story).

I just wonder if I do not need zipper column by spreadsheet or if I am missing something.

On AISC 341-05, there is no 13.4a exception but I am not sure about this.

My company purchaced this sheets. And Danial T. Li who made those says there is 13.4a exception and he believe that program does not have any problems.

Please have a time to reivew this and teach me.

Thank you all in advance.

 

[BAretired]

I haven't the faintest idea of what you are talking about. What is a zipper column? Maybe other forum members can clarify what you are concerned about, but I have no idea.

BA

 

[Gumpmaster]

I don't believe there's any explicit exception to the unbalanced force provision. I think you still need to either provide a zipper column or design the beam for the unbalanced force.

BAretired, a zipper column is an extra column at the center of a chevron brace. It helps with the collapse mechanism that occurs when the compression leg of the chevron brace buckles and all of the force in the tension leg is dumped into the beam. You either have to design the beam for this large force or add the zipper column.

 

[JoshPlumSE]

Options:
1) Don't design it as a SCBF. The requirements for the beam force goes down with an OBF.

2) Design the beam for that extra load. This is a real load that this beam can see.

What happens is that the compression brace buckles and then the tension brace is the only thing left to resist the lateral load. Any tension in the brace that is cannot be balanced out by the buckled compression brace ends up pulling down on that beam!

That is the reasoning behind the code provision. I've seen some cool pictures of chevron failures and it happens exactly as described.

 

[paddingtongreen]

Had to websearch "SCBF", I hate these acronyms that didn't exist before I retired.

"a zipper column is an extra column at the center of a chevron brace. It helps with the collapse mechanism that occurs when the compression leg of the chevron brace buckles and all of the force in the tension leg is dumped into the beam. You either have to design the beam for this large force or add the zipper column." Why is the compression leg buckling? If this is a sudden failure condition, don't make it the weakest link.



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Stazz]

Wouldn't it be better to add bracing along the compression brace and/or make the original members bigger. Adding a column adds a footing and a lot of extra juice.

 

[SteelPE]

Could you possibly design this structure using R=3? The design would then become much easier to perform.

The only benefit I could see by trying to design this using a higher R is to reduce the base shear of the building (which must be pretty high if you are using tilt up panel).

 

[BAretired]

paddingtongreen,

SCBF stands for "Sacred Cat of Burma Fanciers". Here is a link to their web page:

http://www.scbf.com/

BA

 

[FFENGINEER]

whats the dimensions of your building?

 

[leenlee]

It is 1164' x 400' with 40' high. And there are (2)-two bays frames in middle at transverse direction.

 

[ishvaaag]

I transcribe my entry in the AISC forum question:

In p. 234 of Ductile design of Steel Structures
Bruneau, Uang, Whittaker
McGraw Hill 1998

you see that a chevron shape with a bottom central equilibrating zipper is not labeled ZIP but STG (Strut to ground). There is exposed that as per one study by Khatib, ZIP bracings, that have not the zipper strut at the bottom level perform better against earthquake (p.236), so that must be the reason.

Also I examine a copy of AISC 341 recently downloaded and I don't see anything about in the code first section itself, yet

Fig. C-I-13.3. (a) Two-story X-braced frame; (b) "zipper-column" with inverted-V bracing

at the commentary to 13.1 has a ZIP frame without the bottom zipper.

In commentary to 13.4a says

"The adverse effect of this unbalanced load can be mitigated by using bracing configurations, such as V- and inverted-V-braces in alternate stories creating an X-configuration over two story modules, or by using a "zipper column" with Vor inverted-V bracing (Khatib, Mahin and Pister, 1988). See Figure C-I-13.3."

So that's it. AISC reckons the superior behaviour of the ZIP configuration without the zipper strut at bottom level.

That the Li sheet contemplates everything well I have not formed opinion about, since I have not entered the issue, nor have fresh in my mind now these matters; but it is clear that the preferred configuration at bottom level is without strut zipper out of better performance.