Finding principal stress from combined bending and torsion

[frozen7]

Homework Statement

In the attachment

Homework Equations

The Attempt at a Solution

I have been solving this question for quite some time but I can't get the answer yet.
I offset all the forces to point C and I get the following values:
Moment at y-axis, M(y) = 10Nm
Moment at z-axis, M(z) = 48Nm

Stress at x-axis = -M(z) x (21mm) / I = -20MPa
Stress at z-axis = 0

Shear stress at xz plane = Tc/J
= (40Nm) (21mm) / J
= 8.33MPa
By applying the following equations to get the principle strain,
tan(2theta)=2(Shear stress at xz plane) /(Stress at x-axis - Stress at z-axis)

I can't get the correct answer.

Can anyone help me? Thanks in advance.

 

[radou]

Did you check your units?

 

[frozen7]

Yes i did...Is my way of doing correct?