how to connect quad UP element to zeroelement

[szy123465]

I am just doing a seismic analysis of pile group in liqufiable site,i use quad UP element to model soil and zeroelement to model soil-pile interaction,the zeroelement link node of piles and the dummy node(3dofs) then I connect quad UP element nodes with the dummy node,after all are done,i begin to analysis. First ,gravity analisis of the soil,my procedure works well in this step,then pile selfweight analysis,the procedure appears a error and the analysis cann't go on. The error is as follows:
solvecurrentstep<>-the linearsysofeqn failed in solve<>
StaticAnalysis::analyze<>-the Algorithm failed at iteration:0 with domain at load factor 1
opensees> analyze failed,returned:-3 error flag
is there anybody could tell me what's wrong with my procedure?i will really appreciate you help.

[fmk]

you probably don't have enough constraints.

[szy123465]

Thank you, Frank.I have considered you suggestion seriously, i find if i restrain the 3rd dof of the nodes of quad UP element(the soil model in my procedure is 1D),then my procedure works well.But what i want to model is partial drainage,so the 3rd dof can not be restrained.Besides,if i change the soil element to quad element,my procedure works well too.So whether it is the problem that the quad UP element can not be linked with zeroelement with equlDof method?

[avytin]

Is the 3rd DOF constrained in at least one node (e.g. surface nodes should have 0 pore pressure)?

[szy123465]

Thank you,Avytin.You are quite right that the nodes on the surface of up element should be constrainted,and i do constraint these nodes,but my model still doesn't work right,so i constraint all the node s of up element,then my procedure works well.So what's problem with my model?

Avytin, I have read you Master's thesis,I think you did a very good job.Let's keep in touch with each other,I'm sure i can learn a lot from you,thanks again.

[jinlu]

can you post your tcl script?