Adding eccentricity using "coefficient lateral force"

[saburo]

HI there
I am currentlly using version 8.3.5 and wanted to achieve the accidental eccentricity (10% of the building width perpendicular to the eq direction) that NZS4203 requires.
I made some loadcase ecc+ and ecc- for x and y direction trying to get that, but it seemed that ETABS is not putting any torsional effect on the model. I checked the storey shear and deformed shape as well, but it does not make any difference. I dont want to assign diaphragms because the slab is quite flexible. Is this a bug? (ps i could override the eccentricity by difining the diaphragms @ each storey.)
The other thing is that if i specify the eccentricity ratio as 0.1 using "coefficient function", will this mean 10% of what? Am i putting 10% x story shear of torsion in each story?
Thanks in advance

 

[Lixx]

Accidental torsion can only be used together with rigid diaphragm. If you don't specify rigid diaphragm in you model, ETABS will not introduce accidental torsion. Besides a structural model without rigid diaphragms needs to be analyzed as a full 3D structure in lateral analysis, which makes adding accidental torsion to lateral earthquake effect very difficult, if not impossible.

 

[saburo]

Thanks for your reply Lixx. I have contacted the guy in CSI regarding this and got the same answer as you have got.
I finally had to put some diaphragms and chosen one of the corner points within the diaphragm and assigned the global torsional force to the point.

 

[bugsfinder]

I want to kindly ask you something about eccentricities in Response Spectrum Case (v8.3.5 and higher).
1. What does it means "Eccentricities Ratio":
0.05 = 5% ?
2. The program multiply this eccentricity ratio with the building width perpendicular to the Earthquake direction?
3. If I have a Response Spectum Case containing input on 2 directions (X and Y), what means in this case "Eccentricities Ratio"? The program will took the value and multiply it with the building widths per X and Y for each input component?
Thank you

 

[saburo]

Hi Bugsfinder
Speaking of eccentricity ratio, i have made 1 storey square building to check what this means, the ratio turned out to be the percentage of the building width perpendicular to the quake direction. If your buildding is 10m wide, eccentricity 0.05 gives 0.5m x external force that has been calclated within ETABS.

I have not checked the eccentiricity effect with the spectrum case, but i guess the eccecenturity will be generated in each direction first like static case and they will be combined using modal combination method.

I know that we can specify the eccentricity ratio in the modal analysis too, but wonder how to differenciate the sign of the eccentricity. In the static case, there are tickboxs to specify either negative or positive. Can we put -0.05 as a negative eccentricity ratio?