I-Sections vs. C-Sections

Compare the stiffness in two perpendicular directions for a given pair of equivalent sections.

Compare the weight for a given pair of equivalent sections.

"Compare the stiffness in two perpendicular directions for a given pair of equivalent sections.

Compare the weight for a given pair of equivalent sections."

This is correct, and a good starting point. You will also want to check susceptibility to buckling when loaded in various directions, and probably for anticipated events such as local impact. There may also be consideration needed for fabrication; a wider single flange on a C may be identical to that of the twin stubs of one side of an I, but presents more area for bolting and the necessary surrounding clearances. At the same time, it is almost certainly more easily damaged by local impact (twice the cantilevered distance from the web member).

You need to talk to a gynecologist about C-sections.....

For long trailer , you may opt for back to back C section . C section can be easily fitted with various other fitments , than I section.

For lesses span single C section on either with the vertical faces facing each other is a better option.

Stress=My/I=M/S. For the same cross sectional area, an I-beam has a higher Section Modulus. Therefore, for a given load, the Stress in the beam will be much lower.

There's more to it than moment of inertia and section modulus.

Two C sections back to back (with a small gap between them) allow hangers to be placed on the geometric center of the assembly.

A load placed on the web of an I or W beam will deflect it, but a load on the web of a channel will deflect and twist it. You will need to place loads a short distance from the web to correct this, and then you might need to place a stiffener between the flanges. The symmetrical shape is easier to use.

This does not stop me from using channels frequently, it's just something you need to allow for.