Joint weights are used as part of seismic weight definitions. I guess your question is, how to add these joint weights using the GUI. Here is a screenshot explaining that.
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Here are a list of steps
Go to go to the General > Load & Definition page.
From within Definition> select Seismic Definition and click on Add at the bottom of the Load & Definition dialog box.
Select the appropriate seismic code and click on Add.
Select Joint Weight, enter the appropriate joint weight value and click on Add after you enter each value.
Finally assign these joint weights to the appropriate nodes using any of the assignment methods.
