unbalanced vertical force in 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.

Engineer @ LA, CA, USA

 

[ishvaaag]

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.

 

[leenlee]

Thank you but too hard to understand.

So if SCBF used for top story level of building or one story building, I do not need to consider unbalanced vertical force on beam???

 

[ishvaaag]

that's obviously an interesting question, because you are referring now to the beam, not the existence or not of one member to meet any vertically unbalanced force there.

The structural logic demands that if a force exists and is not met some way must be in another way, so it is clear that for inverted V-braced of 1 story without central vertical strut the beam must meet that force. I transcribe from the quoted book...

"The intent of the ZIP configuration is to tie all brace-to-beam intersection points together, to force all compression braces in a braced bay to buckle simultaneously, and thereby to distribute the energy dissipation (damage) over the height of the building."

so as long as you have two levels and one column linking the central nodes it is clear that you abide under the intent of a zip frame.

Outside this, you are in a single story inverted V configuration, and the unbalanced load on the beam still to be met, so it seems to me that if you want to except the beam of the effects of the unbalanced load you should in this case add the central column for the unbalanced load. That is, there is also exception to just 1 story in the sense that to free the beam of the effects that the braces would exert on it on seismic action you need to add the central column-like member.