Torsional Modulus Constant

The values for $C_T$ are calculated from Roark's Formulas for Stress and Strain (Section 10.7, Table 10.1).

For Solid Rectangular Section:

$$C = \frac{cb^2}{3\left\lbrack 1+ 0.6095\left(\frac{b}{c} \right) + 0.8865\left(\frac{b}{c} \right)^2 - 1.8023\left(\frac{b}{c} \right)^3 + 0.9100\left(\frac{b}{c} \right)^4\right\rbrack}$$

For Hollow Rectangular Section:

$$C = min\left\lbrack 2t_{2}\left(b_{1}-t_{2} \right)\left(b_{2}-t_{1} \right), 2t_{1}\left(b_{1}-t_{2} \right)\left(b_{2}-t_{1} \right)\right\rbrack$$