Grant Parameters in HDR bearing

[hbma]

Hi,
I have modeled a 3D bridge model with HDR isolators, using "element HDR" command. I do not know what are the Grant parameters' quantities, a1 a2 a3 b1 b2 b3 c1 c2 c3 c4. Indeed, I have found two references with different quantities regarding to Grant parameters delivering so different periods. I use SI units in my model. The two mentioned references are:
1) Grant, D. N., Fenves, G. L., and Whittaker, A. S. (2004). "Bidirectional modeling of high-damping rubber bearings." Journal of Earthquake Engineering, 8(sup001), 161-185.
2) http://opensees.berkeley.edu/wiki/image ... ravity.tcl

Thanks in advance.