How irregular is too irregular? 1

[TycoSpot]

I have a building where the footprint of the second story is 6 feet narrower than the first story. This offset occurs only on one side of the building- i.e. only along one perimeter wall line. It's a large building so the 6 foot offset is relatively minor. The overall square footage of the smaller second story is more than 95% of the overall square footage of the first story.

Does this qualify the building as irregular per ASCE 7-05 table 12.3-1? I designed the building as "regular" but now I have a plan review comment telling me to justify that it's not irregular. Any help or advice would be greatly appreciated!

Thanks!

 

[Lion06]

Does this offset affect the lateral system in any way? Is there an out-of-plane offset of the lateral elements?

I can't think of any other irregularity the situation could qualify for.

If you have a comment to "justify" that it's not irregular, I would just go down the list from 1-5, and say that it doesn't fall into any one of these categories that ASCE 7 defines as an irregular building.

Offsets in floors are pretty common, a 6' offset is nothing.

 

[TycoSpot]

Yes, it does affect the lateral system as both perimeter walls above and below the offset are shear walls. I'm trying to justify the offset by saying that the 6' is small. Also the floor diaphragm between these walls is a pretty substantial concrete on metal deck diaphragm that I'm thinking can transfer any offset load.

I'm just not very good at "argueing" with plan reviewers...

 

[SAIL3]

Since irregularity concerns mostly affect how the structure behaves under lateral load, I would check for any unintended or unusual load redistribution both locally and globally.
One concern that comes to mind right away..how does the offset affect the assumed stiffeness of that line of shear walls?
Since the shear walls are offset 6', the lateral shear has to be transferred from the top shear wall thru the 6' deck to the bottom shear wall.....shear flow can accomodate this as long as all the resulting loads are accounted for and the stiffeness of that 6' deck is equal or greater than the stiffeness of the shear walls.
The question for me would be the overturning loads from the top shear wall...they don't line up with the lower and if resisted by a beam and not a local col. could increase the flexibility of the top shear wall and thus reducing the stiffeness along that perimiter. Kinda long-winded, but if there is some doubt that it does not qualify as irregular, then one has to analyze it as is and try to account for all the loads in the given situation.

 

[JRGse]

You do have a horizontal irregularity per T12.3-1 Type 4, however you do not have a vertical irregularity (T13.3-2). Hence, you would be required to follow the restrictions listed in section 12.3.3

 

[steve1]

http://www.structuremag.org/article.aspx?articleID=1227

 

[cvg]

anybody else have difficulty reading the above posted article? It looks interesting, perhaps a version could be posted that is not garbled...

 

[csd72]

no problems here cvg, maybe you need to update your web browser.

 

[cvg]

got it, I had changed the zoom level in internet explorer and that apparently garbled the content of the web page...

 

[TycoSpot]

Thank you so much! Everyone has been very helpful! If you think of anything else that might apply or help me I will continue to appreciate your support.