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# Etabs Seismic Mass

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 sagarmjadhav (Structural) 13 Feb 12 10:57
 Hi guys,Can any one help me with how to find Seismic weight of the structure.I have assigned Seismic load as a user defined coefficient(C) of 0.1 ,The seismic Mass source is Loads-DL+0.25LL.But when I run the file and check the results The base shear for earthquake load case is not equal to 0.1*(DL+0.25LL).Where the base shear comes out to be 2045.39 kips which is not the same as 0.1 times (Dl+0.25LL) i.e 2461.61 kips.Can any body guide what may be the reason??
 jdgengineer (Structural) 13 Feb 12 11:33
 I'm not coming up with the same loading as you.  I see that your base shear is in fact 2045.39 kips.  Your dead load reaction is 19,864 kips, and your live load is 4752 kips.  0.1 (DL + 0.25 LL ) = 2105 kips which is pretty close to 2045 kips.  While I could be wrong, I would bet that the discrepancy is the weight of the bottom half of the first floor columns.  I think that ETABS is most likely neglecting the bottom half of the first floor columns when calculating seismic masses.  Where's your diaphragms?
 sagarmjadhav (Structural) 13 Feb 12 13:06
 Hey thanks.But I think base shear is still the function of the whole seismic weight of the structure.Etabs is giving the base shear as 60 kips less i.e(2105-2045).Is there any other options I need to check or define before I define the seismic load in the define menu.Also I have defined and assigned the diaphragms in that model for each storey.Is it a bug in etabs or is there something I am missing I am not sure.
 ahmedhegazi (Structural) 18 Feb 12 1:58
 Go to file-Print Table-Input then select auto seismic loads and print them to file ,you will see detailed calculations
 saburo (Structural) 13 Jun 12 4:18
 I think if you have a look at the " Assembled mass", scroll down to the bottom, there will be a summary of assembled mass for each level and the total mass. The seismic mass should be total mass minus mass at the BASE, as bottom half of lowest level does not count. I think you should get pretty close numbers between your baseshear and 0.1xseismic mass. Let us know how did you get on.

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