Modeling the gap between a beam and a column

[Derekrp]

Currently I'm performing some research in component based models for steel connections, and I'm having some issues with a component that is supposed to model the influence of the gap between a beam and a column. I'm using zero-lenght elements, and basically what I need this component to do, is to have zero stiffness initially until the deformation reaches the gap value, and then become infinitely rigid. I have tried using two different types of uniaxial materials (MultiLinear and ElasticPPGap), but I keep getting a LAPACK error 7, meaning that OpenSees is not recognizing the link between them.

I'm testing this spring just by simply using it to join to beam elements fixed in one side, and then loading them axially. I have tested other springs with different characteristics in this way and all of them are working well, this one is the only one still giving me errors. My guess is that because the initial stiffness is zero, at this point the two beams are separating without any restriction, and so it is being interpreted as a mechanism. When I did the trial with the MultiLinear material I managed to make it work by using an initial stiffness higher than zero (however this is not good for me, since the value at which it starts working is quite significative, i.e. very different from zero). Any suggestion or solution in order to get this working?

The extract of the code is as follows:

# ## Spring modelling gap between beam and column
#Option 1 (Using MultiLinear)
uniaxialMaterial MultiLinear 17 $g 0 $g 999999999
#Option 2 (Using ElasticPPGap)
uniaxialMaterial ElasticPPGap 17 999999999 -999999999 -$g

element nonlinearBeamColumn 1 1 3 $nc 2 1 -iter $maxIters $tol
element nonlinearBeamColumn 2 33 5 $nc 2 1 -iter $maxIters $tol

element zeroLength 8 3 33 -mat 17 -dir 1
equalDOF 3 33 2 3