Element Mass Density for Quad Element

[lindaatik]

Hi,

I am using the PressureDependentMultiYield material with quad elements in my model for seismic analysis. I already specified the soil mass density in the nDMaterial command. Do I need to specify the mass density also in the quad element command? What is the difference between the material density and the element density?

I saw in the examples posted on the ucsd website that mostly mass density in the quad elements is specified as zero, so I am confused.

Thanks for your help.
Linda.

[bayram_aygun]

Hi,

I guess if one is not interested in the dynamical behavior of soil (Period, frequency) then one doesn't have to assign a mass (no need for density therefore). I'm more interested in how the soil affects the superstructure so I assign body forces (b1 & b2 in the element quad formulation) and automatically OpenSees calculates soil nodal forces.

Of course, these are my opinions and since I'm also beginning to get to know OpenSees soil capabilities you should definitely ask other people, too.

Take care,

[bayram_aygun]

I made progress in understanding the quad density a little bit better. Here it is:

When you assign density to the material (e.g. PDMY) I think you enable the soil to behave like a real soil (with density and all that). My guess is if you don't assign density to PDMY the soil behavior would change. BOTTOMLINE: for kinematic soil-pile interaction assign density to the material only.

If you want to model kinematic+inertial soil-pile interaction, then ASSIGN DENSITY TO BOTH MATERIAL (PDMY) and ELEMENT (quad). That way the system matrix will include soil lumped mass matrix and structural mass matrix. ALTERNATIVELY: You could try assigning explicit nodal masses to soil nodes like:

mass $soilnode1 $mass_X $mass_Y

without assigning density to quad elements (doing that would double your soil mass). Both ways yield very similar fundamental periods for my soil-bridge model.

Hope this helps,