pinching4 material

[nithinvl]

Hi
I have modelled a single-storey single-bay RC bare frame and i am calibrating its monotonic and cyclic behaviour with experimental data. The material models i have used for concrete is Concrete07 and for reinforcement is Pinching4. A pinching behaviour was observed in the experimental force-displacement curve and hence I have used pinching4 material for steel (as I was unable to calibrate my model for this pinching behaviour using the available steel models in OpenSees).
My query is regarding the pinching4 material :
1) Can pinching4 material be used for modelling reinforcement (as I could not find any literature on using such model for reinforcement) ?
2) The stress values corresponding to yield of steel obtained from the recorders is lower than the yield strength which is specified as input in the material model. If I give Fy=500 MPa in the pinching4 model, I get yielding stress of around 450 MPa. Kindly explain why this is happening ?

Regards
Nithin.