# Added Mass in FLAC for dynamic soil-structure-water interaction

Hi All, Is there any way to apply different masses in different directions (horizontal and vertical) in FLAC to perform seismic soil-structure interaction underwater? The structure will be modelled with beams. Apparently, FLAC3D can do this easily, but not FLAC from some reading of the manual. Could someone confirm this, and advise whether there is any trick in FLAC to work around this limitation? I want to do it in 2D with a simplified model to save time.

Not sure you want to add mass to structures or soil? FLAC allows modification of the gridpoint mass to account for hydrodynamic pressure ( only for EOM calculation in X direction). Adding mass to structure nodes is not allowed in the current version(the offsets provided in the str.fin for dynamic effective mass are only for view purpose). However, if the structure is already rigidly connected to gridpoints, modifying mass of the gridpoints can indirectly affect the structure as well.

Thanks!
I want to add mass to the structure, which is modelled with beams. I guess generation of a column of elatic zones rigidly connected to each beam is the only way. However, I am not sure whether in this way, I can specify any amount of added masss. Or Does âgmscl(i,j)â represent the scaling factor to factor up the current mass? For example, if the real mass is 1kg, and I want to have added mass of 2 kg in the horizontal direction for gp (i,j), Does it mean I need to use gmscl(i,j) = 2?
Not sure whether I have missed anything? Can I plot gmscl, or I have to use extra variable to plot it?

Yes, use the real mass and make sure the unit is consistent. The mass from gmscl() will be added to the gridpoint mass. Also, the gmscl() is not working in static condition, so you need to set dyn on when using this function. To check gridpoint mass value, you can use command âprint gpmâ (note: the value from this command is inverse of gridpoint mass).

Sorry. I need to 100% clear here. what do you mean by âinverse of gridpoint massâ? Is smscl only a ratio, or an absolute value of mass in the horizontal direction?

The gmscl() stores an absolute value of the added mass, and the added mass only affect the x direction.
Inverse of the gridpoint mass means if the gridpoint mass is M, when the command print gpm is given, 1/M will be printed out.

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