modeling viscouz damper

[greatFAZ]

Hi every body.
I want to model viscous damper.I've followed 2 approaches:
1.using truss element with viscous material:it doesn't work on my model(2D) but it worked on a 3D model.I changed the area of truss on my own model(2D).the displacement of the roof is reduced when i change the area of truss but the base shear increases to about 3times!!!!!!
2.using maxwell viscouzDamper material:i want to model pure viscous damper so the periods of frame shouldn't change.but when i use this material the first period changes from 1.48s to 0.52s!!!!!!!!!I've changed the stiffness of spring but there's is no any difference in periods.it's 0.52s.besides the base shear increases here,too.
I really have no idea and i don't know what to do.How can i model viscouz damper in opensees?
PLEASE HELP ME.
thanks in advance

[vesna]

Viscous dampers in addition to adding damping to the structure they also increase the stiffness. I model viscous dampers by using zeroLength element and Maxwell material. So far it worked well for me.

[greatFAZ]

thanks for replying vesna.You said viscous damper add stiffness to the main structure,but
1.what if I want to model pure viscous damper?(the periods of the structure doesn't change so the added stiffness is very little)
2.I checked maxwell model.It consists of a Linear spring and a Nonlinear dashpot.I found a Link in SAP2000 which is very similar to maxwell material in opensees.(Damper Link in SAP2000).the documation of sap 2000 states:"If pure damping behavior is desired from the damper element(spring and dashpot in series),the effect of the spring can be made negligible by making its stiffness sufficiently stiff."
when I assigned a big stiffness in sap2000 to the damper link the periods of the main structure don't change(pure viscous damper) whereas the periods of the structure change completely when i do the same in opensees.Indeed there's no difference in the periods when i change the stiffness of the spring in maxwell model(i mean the period of the structure reduce to 0.52s from 1.48s,no matter what the stiffness of the spring is).Note that i've used both truss element and nonlinear elements. I haven't used zerolength elements,but I don't think there will be any differences if I use them.I've modeled in opensees 2.4.0 & 2.4.2
3.when i used viscous material and truss elements to model pure viscous damper the periods of the structure didn't change(added stiffness=0).Displacements and Drifts were reduced when i modeled this way,but the base shear of the structures increased(time history analysis).Note that i used different records and i got the same reasons(base shear increases).I really have no idea and i doubt if this is rational.i couldn't figure out why the base shear increases.Do you have any idea vesna,that why the base shear increases? Note that in SAP2000 the base shear reduced when i modeled pure viscous damper.
I'd be very grateful if you answered my questions.
Regards

[vesna]

Can you try zeroLenght element with Maxwell material?