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# Center of Mass

## Center of Mass

(OP)
I know it seems very elementary , but i have problem and doubts ,
well i want to know , when im going to calculate center of mass in each floor , without any software , should i calculate the weight of walls ? walls at perimeter and partitions ?
and what about that half of the height of walls above and below each floor system , , i mean what about it , ? is it necessary to be regarded in CM calculations , cause if it is , the first floor will be different ,,,
Tnx
!!

### RE: Center of Mass

Yes, there should definitely be some account given to the mass of wall. Most often I see partitions accounted for as uniform load and perimeter walls and cladding as line loads. Generally, 1/2 of the upper and lower wall mass is how it's done.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

### RE: Center of Mass

(OP)
you know , when i talk with friends , they just tell me that , forget about walls , and this half height difference in first floor , they can be neglected , no big difference in CM happens ,
... but i dont like to neglect ,
and about these partitions , why is that they say use uniform load , what about line load , i mean , is it wrong ?

### RE: Center of Mass

I'll have to refine my answer. I didn't read your post closely enough. The wall weight certainly affects the seismic mass significantly but often does not alter the CM all that much. Usually you can gague the impact by inspection as many floor plans will be such that the CM of the walls and slab considered separately are pretty close together. Obviously some judgement is required for cases where this would not be true.

Regarding uniform load treatment of partitions: it's just an approximation, like everything else that we do. Internal partition for many kinds of buildings often are quite uniformly distributed about the floor plan.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

### RE: Center of Mass

(OP)
Thank you , for your guide and help .
i really needed to know if surplus calculations are wrong are just to be more accurate , i was in doubt , ,
and by surplus i meant walls ))
okey-doke !

### RE: Center of Mass

You're most welcome Alireza. All conscientious structural engineers struggle with theses kind of issues.

When I first start a project, I'll use AutoCAD to quickly trace out the floor plate as a mass and the perimeter of the building as a polyline. You can use AutoCAD's MASSPRPOP command to quickly locate the CM's for the floor plate and the walls seperately. If they're close, I don't worry about it. If they're not, I'll give it a little more attention.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

### RE: Center of Mass

(OP)
Yeah , that is a real good command , useful , i will check it with masspprop too , again tnx )

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