Civil Engineering Association

This is one of my example Analysis and Design of a two storey Building using ETABS.
The Loads are :
1. Live Load
2. Dead Load
3. Static Seismic Load (UBC 97 associated with our Local Code)
4. And Dynamic Seismic Load (Response Spectrum)

Design code : UBC 97
Hopely can be usefull.

Thanks and Regards,

erimajus

do you have any calc notes or detail drawings?

In ETABS v7 there are two options to define mass:

1. Define the mass element masses and additional masses. The building mass associated with the element mass is determined by multiplying the volume of each structural element times its specified mass per unit volume. Additional mass can also be specified on the Assign menu.
   
2. Define the mass from a specified load combination.
   

Whereas in V8 and v9 of ETABS there are three options:

1. From Self and Specified Mass
   
2. From Loads
   
3. From Self and Specified Mass and Loads
   

Typically we assign a self-weight multiplier of 1 to one dead load case only and a self-weight multiplier of 0 to all other load cases. You can assign self-weight to its very own load case, perhaps named SELFWT or you may want to include the self-weight in the same load case as your other dead load.

In the posted example of Analysis and Design of a two storey Building using ETABS, Option 3 was selected to define mass wherein the self-weight was included in the same load case as other dead load with a self-weight multiplier of 1. I think this will double count the structural elements self-weight in determining the mass. In my opinion, Option 2 should be selected. (Open the uploaded file 3DframeModel3.edb and see the Define Static Load Case Names and Define Mass Source dialog box)

If you want to use Option 3, then you should assign self-weight to its very own load case named SELFWT without including the load case SELFWT in the Define Mass Multiplier for Loads. (Open the uploaded file 3DframeModel2.edb and see the Define Static Load Case Names and Define Mass Source dialog box).

If you want to include the load case SELFWT in the Define Mass Multiplier for Loads, then select Option 2 (Open the uploaded file 3DframeModel1.edb and see the Define Static Load Case Names and Define Mass Source dialog box).

Running the files that I have posted, 3DframeModel1.edb and 3DframeModel3.edb the EQX Story1 Story Shears is 450789.86 Newtons and in 3DframeModel2.edb the EQX Story1 Story Shears is 450643.49 Newtons.

Running the original file 3Dframe.edb, the EQX Story1 Story Shears is 669405.06 Newtons. The difference in the results obtained from 3DframeModel1 or 3DframeModel3 is 218615.2 Newton.

To prove that the structural elements self-weight has been counted twice in the determining the mass, I selected Option 1 (only self-weight of structural elements) in the file 3DframeModel1 and run the analysis. The EQX Story1 Story Shears is 218615.2 Newtons, which is equal to the difference of EQX Story1 Story Shears between the original file 3Dframe.edb and 3DframeModel1.edb.file.

Files:

Hi. The mass must be assigned following the Benny explanation. Be carefull about the mass definition, because the DEAD LOAD (selfweight) mass assignation is taking duplicate the correct value. Use the option 2 to define the mass asignation, and use the DEAD LOAD value as 1 (selfweight load only), the other dead load cases (for example, OVERLOAD) with 1 also, the LIVE LOAD as %(0.25, 0.5 or the using code value) and the any additional vertical load cases participating in the dynamic analysis. Best regards.

Please kindly re-upload Benny's files in post #5 above
Thank You!