Unknown Load Factor Error

I currently have a model of a tied arch bridge with construction stage analysis. I have some desired tensions in each hanger that I would like to have in the hangers at given construction stages. I was hoping to accomplish this by using Unknown load factor cable controls with constraints of beam forces, however I have been running into problems in that an optimized solution can not be found. 

The 'Unknown Load Factor' function is the right tool to find the tensioning required to reach target forces in the cables. However, this function does not always give the solution. This function will not find a solution unless the constraints are realistic. For the model attached herewith, here are some recommendations.

1) In the constraint table, the constraint of beam force is given as 'Equality'. It is recommended to change this as 'Inequality' and try some wide range at first, and then smaller range.

2) Try without the effects of Time Dependent Material at first, and after finding a solution, try with the Time Dependent Material. The solution depends on creep and vice versa.