Truss connection

[songwang]

Dear all:

Hi, I want to test a simple truss frame but encountered a problem about boundary conditions and connection condition. My model is as simple as using two truss element to restrain a node in the system. The script is as below:

#################################################################################
wipe

model BasicBuilder -ndm 2 -ndf 3

puts "model geometry"
node 1 0 1
node 2 1 1
node 3 1 0

puts "boundary condition"
fix 1 1 1 0
fix 3 1 1 0

puts "Geometry Transformation"
geomTransf Linear 1

puts "material"
uniaxialMaterial Elastic 1 1

puts "element"
element truss 1 1 2 1e-4 1
element truss 2 2 3 1e-4 1
###################################################################################

When I apply a horizontal load at node 2, the model doesn't work. When I restrain the rotational DOF at support 1 and 3 as below:
fix 1 1 1 1
fix 3 1 1 1

Still, it doesn't work. However, when I add rotational restraint at node 2 as below:
fix 2 0 0 1

The model works! I know if I build a 2D 2-DOF per node there's no problem about rotational restraint for truss element, but in case I want to contain both truss and beam-column elements in the model with 2D and 3-DOF per node, how should I deal with the connection?

Besides, in this case, if I apply a large horizontal load at node 2, there's no deformation and no axial force in the vertical truss element. I suppose this is the case in small displacement so the vertical truss just do rigid body motion, but not in this large visible displacement case. Does anyone know how to make the truss element has axial deformation if one of its end goes large displacement in transverse direction and the other end is fixed?

I'm very grateful to whoever gives me suggestions. Thanks a lot!

Song

[fmk]

of course a truss model needs the dof to be retained if there are just truss elements and you define 3dof at a node .. if you are going to combine the models (truss and beam), no constraints will be needed for all those nodes where there are beam elements framing into the node .. the model with just truss elements alone of course needs the rotational degree-of-freedom to be constrained .. just consider what the stiffness matrix of a truss element would look like with 3 dof at a node to understand why.

[songwang]

Thanks very much