gravity center of structure

[jo2ny]

Hi all,

I'm testing the otput of the center of gravity and total mass of a structure.

The model is simple, i attached it.

My question is: how are the COG and mass calculated because running a "hand made" verification the results are not the same:

Robot reactions: 1 Sw = 0.79 kN = 81 kg

                            2 Ov = 20 kN = 2039 kg

Sum = 20.79 kN = 2120 kg

Robot -> 

Structure gravity center coordinates: 

X =      1.000 (m)

Y =      1.000 (m)

Z =      0.000 (m)

Central moments of inertia of a structure: 

Ix =     43.241 (kg*m2)

Iy =     48.648 (kg*m2)

Iz =     92.010 (kg*m2)

Mass =     81.066 (kg) OK

Coordinates of structure centroid with static global masses considered: 

X =      1.000 (m)

Y =      1.463 (m)

Z =      0.000 (m)

Central moments of inertia of a structure with static global masses considered: 

Ix =    639.287 (kg*m2)

Iy =   1210.087 (kg*m2)

Iz =   1849.495 (kg*m2)

Mass =   2201.565 (kg) Why not 2120 kg = 20.79 kN?

Obvious a further calculation of the coordinates of the COG wil not have the same result (X=1.00m / Y=1.463m / Z=0.00m)

Thank you in advance!

I'm running ARSA Pro 2012

[Rafal.Gaweda · Accepted solution]

The cause:

[jo2ny]

You are suggesting, that the self weight is taken into consideration twice? 2039+81+81 = 2202 kg ?

That means that I shoud remove self weight from load to mass conversion to get the corect COG?

If so, what about a model with modal analysis and seismic cases, in which i would normaly include Sw into load to mass conv. + disregard density?

[Rafal.Gaweda]

 in which i would normaly include Sw into load to mass conv. + disregard density?

Your choice, but remember not to double sw for masses unless you really want this.

[jo2ny]

Ok, one more time to be sure i got it right:

Load to mass conv. of Sw and Ov + diregard density is the same with load to mass of Ov without disregard density thicked?

[Rafal.Gaweda]

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jo2ny wrote:

Ok, one more time to be sure i got it right:

Load to mass conv. of Sw and Ov + diregard density is the same with load to mass of Ov without disregard density thicked?

---

Yes.

[jo2ny]

Thank you!