ETABS story shear

[Akeee]

Hello guys,
I have a question about the Tortional moment "T" from Etabs - Story shear. The problem is that i dunno how to manual calculate that, let me explain from the start: i calculate and verify in etabs story response the seismic force for every level ( i distributed the base shear on every level). The values are good. Next i calculate the overturning moment "My": Sum of Fi * zi and it checks too ( the values from column My are approximate the same). Next i wanted to calculate the torsion moment "T" are there i got stucked. I took the exccentricity - distance between center of mass and center of rigidity - from etabs - tables and "center..." (e = cm - cr), i even plotted on autocad the points to see if it's ok and it so. So the next thing i did is: Fi * e and it should give the T value but it's not even close, the values from etabs are 10x bigger on every floor level. To give you a numeric example Fi = 70 KN e= 3 m and the T = 10200 about...
What do i miss?
Thank you for your help

 

[DaveAtkins]

I was going to suggest you forgot accidental eccentricity in your hand calc, but that would not make it off by a factor of 10.

Could it be a math error?

DaveAtkins

 

[Akeee]

What do you mean by math error ? ETABS did math error ?
Accidental eccenttricity is 5% from the lenth so is 0.05 * 26.2 = 1.31m; add them to 3 => e = 4.31 m; 4.31 * 70 = 305.9 kNm and ETABS shows me at T that is 1126.5 kNm ...

 

[Akeee]

Print screen:

 

[asixth]

Those storey shears are about the global axis. How close have you put your modelled structure to the origin?

 

[Akeee]

Asixth, sorry i dont understand the question, what do you mean with origin?

 

[Akeee]

I looked at the model in 3D and i think you mean by origin the point where green global X, Y, Z axes are. This point is at the bottom left where the building starts, first column bottom left. You want the distance between CR and the origin ?

 

[Akeee]

Hmm i think i figure it out, the T = distance between the origin and the center of mass (also noted in etabs as XCM or YCM) * Fi (seismic force at the level i) i run some numbers and gives approximate values. Thank you very much Asixth, but i have to ask you, why is this ? WHy is not calculated - the torsional moment T - in the center of rigidity and it's calculated in the origin ? It doesnt make any sens, it would be a lot more helpful in CR?

 

[jayrod12]

Can't you then move your origin to the C.O.R?

 

[Akeee]

Don't know how..