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# Steel Plate Design Using AISC Section Modulus6

## Steel Plate Design Using AISC Section Modulus

(OP)
I am looked through the 14th edition of the AISC steel manual and see that there are two options for rectangle section moduli in Table 17-27. One is moment about the middle neutral axis and is typical but I also see this one, S = bd^2/3, for moment about the edge of a rectangle. What real world application is there for this later section modulus? Could it apply to a cantilevered plate?

### RE: Steel Plate Design Using AISC Section Modulus

Trent L:
Those are particular section properties of a given shape about a specific base line. If you really want to understand them, and their use, you might do well to buy a good Strength of Materials textbook and study it a bit to understand these things. We don’t normally teach first semester courses in engineering here on E-Tips, nor should we waste our time conjuring the possible uses of a given formula. You might become a better engineer if you did a little self-study on some questions like this, and then come back with a meaningful question once you have some of the basics sorted out. AISC does (used to) allow a somewhat higher allowable stress for a plate stressed in the flatwise orientation, in bending, because there are almost no stability issues, but you still use S = bd2/6 for the bending stress calc.

### RE: Steel Plate Design Using AISC Section Modulus

4
(OP)
dhengr:

I did not realize I had to have PHD level questions for someone on here to be helpful. It may be that this is a simple question, if so then it should be easy to answer. I am very into self study and look through my old textbooks often. That is not the issue here. I have also spent some time online looking for information on section moduli and applications and have not seen this equation used on anything, but I also do not want to spend a lot of time digging things up. There are lots of questions on this site that someone like yourself could say the same to, but that does not make it right to be rude. If you don't like my question move on and let someone else answer.

It would seem initially that the S = bd^2/3 would apply to two identical plates that have composite bending along their weak axis?

### RE: Steel Plate Design Using AISC Section Modulus

2
Trent - dhengr has more engineering experience in his little finger than most of us will amass in a lifetime (or two). He earned the right to be frustrated with simple questions before most of us were born.

To your question, though: it's not really useful on its own. It's just an application of the parallel axis theorem about a particular point.

### RE: Steel Plate Design Using AISC Section Modulus

Trent,

Keep in mind that you are referring to the section properties of the cross section of a beam member. If you want to apply the formulas of this particular arrangement to a NEW situation, i.e. a cantilever plate YOU are responsible for understanding how the formulas for another situation have to be modified to apply to a new and different situation.

Jim

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