ETABS: Seismic storey response plots do not match base reactions?

[ilyas415]

Hi all,

I'm working on a 9-storey building and I want to extract the seismic cumulative shear forces from the storey plots.

Upon closer examination, it seems like there is a mismatch between the base shear shown in the storey response plots and the base shear for the same load case in the base reactions table. See below:

There is a similar difference in the base overturning moments.

Granted, it's not a huge difference. But I've seen another ETABS model produced by a colleague showing much larger differences.

Does anyone know why this might be happening?

Thank you,
Ilyas

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Ilyas

 

[dadomago75]

Hi Ilyas,
I am interested to see if anyone has an answer because I just found out the same thing. The difference is 1% but still I would expect the same value.
I presume you are talking about the equivalent static right?
The funny thing is that if you select all the 6 cases il the EQS load pattern (EQX, EQX+e, EQX-e, EQY, EQY+e, EQY-e) and check the results (EQ1, EQ2, EQ3, EQ4, EQ5, EQ6), all the base shear along the same direction match, but if you compare the base shears for one direction with those for the perpendicular direction, there is a difference.
Given that the equivalent static is purely based on floor mass, there should be no difference at all.
I did the same with the response spectrum. I created 6 cases RSX+e, RSX-e, RSY+e, RSY-e, RSX, RSY to compare with RS which is the ETABS default load case that includes the accelerations in both directions (U1, U2) with the code default eccentricity.
I found that the single cases are exactly the same (as I would expect), whereas the ETABS default one is actually higher (in my case +35%)
The problem is that I don't know what is the math behind the ETABS default algorithm.
I checked in all the documents I could find but found nothing.

Cheers
Davide

 

[PicoStruc]

The base reaction moment are calculated about X=0,Y=0,Z=0 Zeros coordinate. This is not overturning moment of the building.