Hi,

I wrote a UEL for a nonlinear spring element which is supposed to act as a diagonal brace for a frame. I tested it with single spring element with the inputs from literature and was giving correct results.

I am applying lateral displacement at the top node. 

Then I moved onto model a single bay, single storey frame with diagonals as my UEL (figure attached) with inputs from the literature - it is working.

Now, I moved to model single bay two storey frame with diagonals with diagonals as my UEL (figure attached) with inputs from the literature but it is giving me errors - "Too many attempts made for this increment" or "Time increment required is less than the minimum specified". I checked the same model without my UEL, it is working fine. 

When I see the message file - it shows that - moment equilibrium equations are converged, but force equilibrium equations are not converging. All the largest residual forces are in vertical direction i.e. DOF 2 at the spring end nodes.

I am unable to understand the reason why it is working for single storey case and not working for double storey case. I tried to refine the mesh - no change. I checked the support conditions - with and without UEL are same. 

Can you please help me.