Hello all!!!

In forming the transformation matrix T (to convert local forces,moments,displacements, rotations, stifness matrix into a global one.) for space frame which is in 3 dimensional, we need to have the rotation matrix (r) which is compose of the direction cosines for x, y, z axis...for x axis the direction cosine is straightforward which is rxx=Xe-Xb/L; rxy=Ye-Yb/L; rxz=Ze-Zb/L (where L=length of element, Xb,Yb,Zb=global coordinates at the beginning of element,Xe,Ye,Ze=global coordinates at the end of element, rxx,rxy,rxz are the direction cosines of the element in the x-axis).But for y and z axis, it uses the angle of roll to relate the direction cosine in terms given on the x-axis..Does anybody know how to define and get the value of the the angle of roll, given the data above?..I find it hard understanding this, just for curiosity's sake.Any help is highly appreciated...Thanks all!!!Cheers to all Stressers!!!