Error with Simple Truss Structure

[dschmuhl]

Hello all,

I have encountered errors in a more complex model I am working on when attempting to apply prestressing force to a steel tendon which is built using truss elements. I will not go through the details of the more complex model, but in that scenario I am getting errors when trying to apply gravity analysis to what should be a stable element configuration. To debug, I have scaled my model all the way down to a very simple truss structure using 3 elements and applying a load using static analysis. However, I am still getting the same errors using this small model as the more complex model. I am wondering if my modeling approach is flawed or if something else is at play. Below I will post the code for the simple model along with the error I receive when it is run. Any feedback on what might be happening is welcome. Thanks in advance!

****CODE*****

wipe;
model BasicBuilder -ndm 2 -ndf 3;
source LibUnits.tcl

# tendon nodes and boundaries
node 1 0. 0.
node 2 0. -1.
node 3 0. -2.
node 4 1. -1.

fix 1 1 1 0
fix 3 1 1 0
fix 4 1 1 0

# elements
element truss 1 1 2 0.1 1
element truss 2 2 3 0.1 1
element truss 3 2 4 0.1 1

# loading
pattern Plain 1 Linear {
load 2 0.1 0.0 0.0;
}
constraints Transformation
numberer RCM
system UmfPack
test NormDispIncr 1.0e-8 100;
algorithm Newton;

set NstepGravity 30;
set DGravity [expr 1./$NstepGravity];
integrator LoadControl $DGravity;

analysis Static;
analyze $NstepGravity;
loadConst -time 0.0


****ERROR WHEN RUNNING****

UMD2FA: matrix is singular. Only 2 pivots found.
WARNING UmfpackGenLinSolver::solve(void)- 4 returned in factorization UMD2FA()
WARNING NewtonRaphson::solveCurrentStep() -the LinearSysOfEqn failed in solve()
StaticAnalysis::analyze() - the Algorithm failed at iteration: 0 with domain at
load factor 0.0333333

[fmk]

you need to constrain all the rotational dof or use an ndf of 2

[dschmuhl]

Frank,

Thank you for the help. constraining the rotational DOF for each node did not fix the problem but changing to an ndf of 2 did. Would you have any insight into why that may be?

Thanks!
Dan

[fmk]

you probably did not constrain the rotations at node 4.