The polar section modulus of a cross arm is a geometric property that specifies its resistance to torsional stress or twisting. This article covers the formulas that are used if the button Derive from beam shape is clicked.
Solid Rectangular Crossarm
The following formulas are used to calculate the polar section modulus for solid rectangular cross arms. Note that the series is evaluated for n = 1, 3, 5.
Below are some calculated values for k2 for common cross arms (where b/h is the ratio of the largest dimension divided by the smallest dimension). Hence for a 100mm x 200mm cross arm k2 would be 0.2459. Note these values have been evaluate using the formulas above.
b/h | k2 |
1 | 0.208193 |
1.25 | 0.221227 |
1.5 | 0.230984 |
1.75 | 0.238976 |
2 | 0.245888 |
Tubular Crossarm
The following formula is used to calculate the polar section modulus for tubular cross arms.
Neara uses the equations in ASCE 48-05 Appendix 2 to calculate the value of Max C/J for each shape.