Originally Posted by lukecp
I had a course in Mechanics of Materials last semster, which is basically calculating stresses and strains in deformable bodies (beams, pipes, columns, ect), along with deformation, ect. I don't remember going over it, but I cracked open my book from the class and this is what it says:
S = I/c, where:
S = Section Modulus
I = Moment of Inertia
c = Distance from Neutral Axis (center of gravity) to edge of beam
The section modulus is used then to calculate the maximum stress in any section of a beam using the formula:
Max Stress = -Mr/S, where:
M = Resisting Moment. This is the moment (think a rotational force, like a torque applied) that is resisting the forces applied to a beam. If you had a steel beam solidly attached to a wall, where the beam is free at one end, and hung a weight from it, the resisting moment would be the torque needed at the attached end of the beam to resist the force of the weight of the beam and the added weight, and keep the beam from ripping out of the wall.
It also says that the section modulus becomes larger if the shape of the beam is altered to concentrate more of the area as far as possible from the neutral axis (center of gravity). This could include raising the height of the "C" channel, making it wider, or having a frame with thicker horizontial sections than vertical sections. I'd assume that the frame is "taller" than a C-10 frame, but every C-20 I've seen has had a frame that looks identical to my C-10. The frame could be wider, that is my guess for the difference.
So, what does all of that mean? The larger the section modulus, the smaller the maximum stress in the beam caused by the weigth of the truck and whatever cargo or trailer is being carried/towed. So, a larger section modulus causes a beam (or frame in this case) to be able to safetly carry larger loads without deforming or breaking.
In this case, increasing the section modulus from 2.7 to 3.48 would decrease the maximum stress felt in the beam/frame by 22.4%.
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