Pinching4 material

[amir.bozorgmehr]

Dear everyone
I am trying to model a partial restraint composite connections, using available experimental M-theta relations of such connections. The implementation of zero length rotational spring elements for my purpose sounds effective so that I defined my springs as PINCHING4 materials. Unfortunately I am not capturing the full response at early cycles since my backbone curve is larger than hysteretic results. So as to solve the problem I should decrease the strength at beginning by means of Reloading stiffness degradation at vicinity of maximum deformation demand, however NOT in usual way. the deterioration should be diminished during development of displacement (rotation) . Therefore I defined the following parameters which can not give me accurate result.
1)Is there anybody who can suggest me a way of defining these parameters in a way that for instance(referring to manual for pinching4 material) in dmaxi/defmax=0.05 I would get delta d= 10 but in dmaxi/defmax=0.6 I would get d=0.5? Can you suggest any better factors?
2)In manual defmax & defmin are defined as positive and negative deformations that define failure. where are excatly these points? Are they the last points(4th one) of response envelope that we defined as ($ePd4,$ePf4) & ($eNd4,$eNf4)?
I supplemented the figure of my hysteresis loop and experimental one in the link below?

http://www.megaupload.com/?d=VQ5CZWFK after 25 seconds hit the regular download please

PINCHING4 material details:
set MatTag 1;
set Mp [list 91 798 2455 2456 ]; #positive envelope Moments(Kip.in)
set Rp [list 0.00025 0.0030 0.0200 0.0210]; #positive envelope rotations(radian)
set Mn [list -438 -876 -1638 -2328 ]; #Negative envelope Moments(Kip.in)
set Rn [list -0.0005 -0.0010 -0.0040 -0.0200]; #Negative envelope Moments(Kip.in)
set rDisp [list 0.0 0.30]; # ratio of the deformation at which RELOADING occurs to the maximum historic deformation demand [Pos_env. Neg_env.]
set rForce [list 0.2 0.30]; # ratio of the strength at which RELOADING occurs to the strength corresponding to the maximum historic deformation demand [Pos_env. Neg_env.]
set uForce [list -0.2 0.1]; # Ratio of monotonic strength developed upon UNLOADING [Pos_env. Neg_env.]
set gammaK [list 0.0 0.0 0.0 0.0 0.9]; # Coefficients for Unloading Stiffness degradation [1(coe.)&3(pow.) for deformation 2(coe.)&4(pow.) for energy]
set gammaD [list 0.0 0.0 0.0 0.0 15]; # Coefficients for Reloading Stiffness degradation [1(coe.)&3(pow.) for deformation 2(coe.)&4(pow.) for energy]
set gammaF [list 0.0 0.0 0.0 0.0 0.9]; # Coefficients for Strength degradation [1(coe.)&3(pow.) for deformation 2(coe.)&4(pow.) for energy]
set gammaE 10; # Total energy dissipation capacity is defined as this factor multiplied by the energy dissipated under monotonic loading
set damage "energy"; # damage type (option: "energy", "cycle")
PinchingMaterial $MatTag $Mp $Mn $Rp $Rn $rDisp $rForce $uForce $gammaK $gammaD $gammaF $gammaE $damage;


But when I give the gammaD like:
set gammaD [list 1.0 0.0 -0.4 0.0 15]; # Coefficients for Reloading Stiffness degradation [1(coe.)&3(pow.) for deformation 2(coe.)&4(pow.) for energy]
I cannot get significant deterioration of strength at the beginning and on the other hand it continues to deteriorate the strength at larger displacements!
I appreciate your nice attention