Expert: Arshad Khan - 12/2/2009

Question
Dear Arshad Khan,
I hope that you are fine and keeping well. I have a question regarding torsional irregularity ...
Etabs summary report gives 3 things
1)Max Deflection
2)Avg Deflection
3)Ratio of max to average.

UBC says that we should use story drifts to determine torsional irregularity and story displacements to determine.
To use that , what i do is i take Maximum deflection and average deflection and find the deflection at the other end and repeat it for all stories. then I find drifts and apply the story irregularity check and find the torsional amplification factor.
for a better understanding of above you can check my excel sheet at http://cid-60157d8f9414c461.skydrive.live.com/self.aspx/Public/Plan%20Irregulari...
Now i was reading FEMA-451 Design examples(2003 NEHRP) and it calculated the torsional irregulatiy just by using the ratio of Max Deflection to Avg Deflection.
I am pasting it here

For forces acting in the N-S direction:
Total displacement at center of mass = δavg = 3.640 in. (see Table 6-9a)
Rotation at center of mass = 0.000189 radians
Maximum displacement at corner of floor plate = dmax = 3.640 + 0.000189(102.5)(12)/2 = 3.756 in.
Ratio δmax/δavg = 3.756/3.640 = 1.03 < 1.20, so no torsional irregularity exists.

Now, i am confused , shall i use story drifts or story displacements to check torsional irregularity and is my method of checking torsional irregularity correct..

Best Regards ,
Muhammad Umar


Answer
Hi Muhammad,

From what I can make out by way of a quick review, your spreadsheet looks OK, and your interpretation of both UBC and FEMA-451 is correct. There are some differences in the two approaches (as you have noted) and some discussions/debates on this subject can be found in these sites:

e-imo.imo.org.tr/DosyaDizin/WPX/.../td/.../15-D-2GunayOzmen.pdf

http://books.google.com/books?id=osR-emZbyJYC&pg=PA23&lpg=PA23&dq=UBC+torsional+...

www.seaint.org/.../S.S.1997%20UBC%20S.D.%20Provision.pd

http://books.google.com/books?id=2vjB0mMDRjEC&pg=PA269&lpg=PA269&dq=UBC+torsiona...

I suggest you follow the approach that suits you and one that is consistent with the codes applied in your country, and make appropriate references in your documentation.