## Simulation Mechanical and Multiphysics

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# How to perform a Seismic Analysis with Nodal Weights?

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I have been unable to find any literature on this subject: What is the correct way of applying a nodal weight to a model and conduct a nodal frequency and a response spectrum analysis? Do I specify the nodal force as "Uniform" with the weight of the object, or the weight of the component that will act in that direction (x^2 + z^2 + y^2)^(1/2)? Or do I apply the weight of the object based off of a load case? In addition, how do I specify a response spectrum analysis? 1 in X, 1 in Y, and 1 in Z axis?

My objective was to just apply a weight to the modal and look how the structure performs under seismic conditions, but I'm confused since applying the weight isn't merely just specifying its location, but what axis and and what magnitude the mass will travel in? Any literature would be appreciated.

# Re: How to perform a Seismic Analysis with Nodal Weights?

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Forget it.

I'm just going to add beam elements and inflate the area of the beam to simlulate the weight based off of the density of the material. I just cannot **trust** anything with nodal weights, so even if someone does respond to this post, I'm just going to ingnore it.

# Re: How to perform a Seismic Analysis with Nodal Weights?

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Hi lemensk,

You can choose "Uniform" and enter the weight or mass. As you will see, this puts your value in the X, Y, Z directions because the total weight does act in all directions; it is not a component of the total weight that acts in each of the directions.

After you run the modal analysis, you will copy the design scenario and set the analysis type to Response Spectrum. Under the analysis parameters, you will specify the direction in which the spectrum applies. In your case, you will enter 1 for the X, Y,and Z directions.

I presume you made your beam element (for the mass) very short so that the rotational inertia does not affect the results.

The following is an exaggeration, but you should not trust any simulations results . At least, not until you verify that they are correct. I'm sure that Autodesk does everything it can to eliminate software bugs, but bugs are inevitable. So if a result looks suspicious, do a simple model that has known results (either a hand calculation or one of the examples from the Verification Manual).

Finally, the documentation for the nodal weights (formerly known as lumped mass) is locate here.

Mechanical Engineer

Pittsburgh, PA

16 years experience with Simulation Mechanical

# Re: How to perform a Seismic Analysis with Nodal Weights?

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I just want to summarize/Verify:

- Weight in All Directions: X = 1, Y = 1, Z = 1

- Weight in X Axis Only: X=1, Y=0, Z=0

- Weight in (+) X Axis Only: X=?

- Weight in XY Plane: X=1, Y=1, Z=0

- Half of Weight in All Direcitons: X=0.5, Y=0.5, Z=0.5

- Half of Weight in X Axis: X=0.5, Y=0, Z=0

Correct?