Beam elements weights and lumped masses

[madg1]

Hi

I have a question that bothers me for a while.Please find attached sample ach file.

We use beam elements if we want to add say remote loads to a solid model.And we give them absurdly high dimension values to make sure they are stiff/rigid.Obvisouly we neglect the weight it adds to the system.

Now I got two issues.

1.

Suppose my design is weight sensitive and I do need to use beam elements, but I dont want their weights adding up to the model.We get this issue when we are using large models say the max dimennsion along length wouldbe 80ft and I need to add a beam element for some case.Now due to sheer size , aprt from applied loads, self weights also comes into the play.I

2.

Suppose I want the weight of the beams to be part of my whole structure.And the weight of the beams is not the same as asim calculates.I want to add my own calulated weight.one possible solution for this is to make use of nodal loads.While using nodal loads, it asks us to mass or weight and also asks for I values.I m not sure about this.

Coming to file   attached, the structure got all beam elements.While calculationg the weight it gives  2 warnings, which my brain can't figure out  .And suppose if I want to add weight manually to any of the beams, how is it possible?

Your help is appreciated.

[John_Holtz]

1. Edit the material properties and set the mass density to 0.

2. There are multiple solutions:

    2.a Edit the material properties and set the mass density to the required value. (Area * mass density * gravity) = weight per unit length. This would work for all analysis types.

    2.b Use a distributed load to add the additional weight per unit length. This would only work for stress analyses.

    2.c For point loads (a heavy weight sitting at a specific location in the model) use Weight or nodal Force. Weight would work for stress analayses that include gravity and would work in modal analysis. Force would work for stress analyses.

About the warnings when you ran the "Weight and Center of Gravity" calculation,

Warning 1: The nodal force in your model was converted to a mass. That is, forces do not have weight, but masses do. If you do not want the force to be included in the weight, then you should suppress it before running the Weight and Center of Gravity.

Warning 2: Your model now has a mass (the converted force), but the mass is a different value in all 3 directions. (This does not occur in reality, but we are dealing with mathematical realities which can occur in some situations.) We do not calculate the Weight in three directions (X, Y, Z); there is just the weight. So the Weight and Center of Gravity calculator chose to use the X value of the mass (the converted force) to include in the calculation. Since the nodal force is only in the Z direction, has a magnitude of 1000 pounds, and the calculated weight is only 283 pounds, the force obviously did not contribute to the calculation.

I suggest suppressing the loads before running the weight and center of gravity in order to avoid the confusion describe above. Therefore, the only thing the calculation has to calculate the weight is the mass density.  Edit the material properties and set the mass density to the required value. (You knew I was going to say that! )

[madg1]

thanks john for the reply..

so to manually add weight, put a zero in mass density field and add equivalent distributed load on the member.

This means weight and center of gravity calculator doesn't add this distributed load to the weight calculated.so if I want the total dead weight of the structure, its addition of weight given by asim and distributed load.

Then if I want the centrod of the structure(as asim  does't include distributed load in weight calculations), i can't get the true one right?

[John_Holtz]

glm,

If you want to calculate the center of gravity including the extra load (dead load I presume), edit the material properties and set the mass density to the required value. (solve this equations for mass density: Area * mass density * gravity = weight per unit length.) This would work for all analysis types, including the Weight and Center of Gravity calculation.

If the extra weight distribution is not uniform, then using nodal weights ("Setup > Loads > Weight") may be a better solution. Nodal Weights should be included in the Weight and Center of Gravity calculation.

[madg1]

John

The calculation of density you mentioned involved the area of the part.The part I am replacing with beam element is a medium size assembly.I can get area from cad model, but I will prefer setup---weights as I can directly add the weight.

As you said I tried setup---weights.......But the center of weight/gravity doesnt include those values.

Please find same sample model attached.I added setup weight for part3 at the ceter of the beams.