Adding mass

Rahul_P1

Hi guys,

I have problem with mass estimation. My model weighs approximately 2x less than it should. I have the data: cab mass+ 'equip'= 760 kg and my model weighs around 300 kg. The problem is that CAD model I received did not have any 'equipment' parts thus I do not know clearly how the rest of the mass is distributed. I am using GRAV card image hence it is important mass matches to the real model (more mass= more inertia load).

My question is how to increase the mass if you do not know how it is distributed?

I have few ideas but I do not know how good these ideas are:

1) I am using only one material. Could I just increase the density of material until I get the final value of mass? Changing density should not affect stiffness or any other behaviour of material?

2) I concentrate the rest of the mass (400 kg out of 700 kg) to the centre of gravity?

Any other suggestions?

Thank you very much.

Rahul_P1

What is the type of analysis you are performing? if it is the dynamic response of your structure that you are interested in you can use the non structural mass, NSM is the Mass that is present in the system and will affect the dynamic response but it is not a part of the structural mass (e.g. the payload). see User's Guide > What is Altair RADIOSS? > Bulk Data Format Input > Finite Element Analysis > Elements, Super Elements and Materials:Non-structural Mass

Changing the density may not affect a simple static analysis, but with the grav card you may have to consider how this mass is actually distributed, i.e. in reality.

Option 2 looks good

From the Student guide -
If the assembly of several components is involved then only the critical parts are meshed
appropriately. Other parts are either coarse meshed or represented approximately by
1-D beams, springs, concentrated mass,

you can also create rigid elements connecting your mass. so that it is only applied to the areas connected by the rigid elements, to simulate more realistically.

Rahul_P1

Hi,
Thank you for replies.

I am doing linear static analysis with shell elements.

I noticed there are many possibilities using mass elements:

“Bulk Data Entry
CONM1 – Concentrated Mass Element Connection, General Form; Description: Defines a 6x6 mass matrix at a geometric grid point.
CONM2 – Concentrated Mass Element Connection, Rigid Body Form; Description: Defines a concentrated mass at a grid point of the structural model.
CMASS1 – Scalar Mass Connection; Description: Defines a scalar mass element.
CMASS2 – Scalar Mass Property and Connection; Description: Defines a scalar mass element without reference to a property entry.
CMASS3 – Scalar Mass Connection to Scalar Points Only; Description: Defines a scalar mass element that is connected only to scalar points.
CMASS4 – Scalar Mass Property and Connection to Scalar Points Only; Desciption: Defines a scalar mass element that is connected only to scalar points, without reference to a property entry. “

I also checked NSM option, I might use that.

Just to be clear with expressions, non-structural mass is like adding extra mass with no inertia properties- similar to CMASS cards?

On the other hand in CONM cards it is possible to define mass with + inertia properties?

Rahul_P1

hi,

the NSM applied is also considered for inertia load. Please find the results in below picture. Hope this helps.