Loads to mass conversion_3 cases - diferent results

[zstone]

Hi Everybody
I have four Story building,
Flat slab with perimeter beams,
loads>
Dead - self weight of structure
Live - 3 kN/m2
When i convert loads to masses, i use 2 and 3 options
2. From Loads
Dead - 1, and Live - 0.5
3. From self and specified mass and loads
Live - 0.5

and booth cases give me diferent results,

I cant UNDERSTAND THAT

zstone

 

[slickdeals]

I am surprised that this is happening. I have tried that before never had a problem. Did you report it to tech support?

 

[zstone]

In fact the diference is small.

From case 2. (From Loads
Dead - 1, and Live - 0.5) i occur period T1=0.738 sec
From case 3. (From self and specified mass and loads
Live - 0.5) - i occur period T1=0.742 sec
I want just to be sure, about cases 2 and 3
which is prefered way to convert loads to mass 2 or 3,
because im new user in ETABS, i have make it before with other softwares, and i never havent problems. But in Etabs i like to find most prefered way

zstone

 

[slickdeals]

I don't think there is a preferred way. I would use option 2 if you don't have any additional masses (point or area) specified. And 99.6% (0.738/0.742) is close enough for any engineering we do.

HTH