Section Modulus - NCARB Sample MC

Structural Layout Vignette and Multiple Choice

Section Modulus - NCARB Sample MC

Postby annascherman » Wed Feb 14, 2018 8:16 pm

Hi all,

I am having trouble with one of the sample problems that NCARB gives regarding section modulus (see photo).

Screen Shot 2018-02-14 at 10.11.16 PM.jpg


Why can I not use the formula S=b(d^2)/6 to solve this problem? When I do, I get 46.7 in^3, rather than 57.33 in^3.

If I must use the formula, S=I/C to solve, is there a way to know c without it being given in the question? I thought it was a constant, but apparently it can be found by diving d (overall) by 2?

Thanks in advance!
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Re: Section Modulus - NCARB Sample MC

Postby lali » Wed Feb 14, 2018 8:41 pm

You cannot use bd2/6 because it is not a a solid rectangle. It is a W beam . You have to use S=I/c c= the distance from the centroid to the top axis, but you still need to find the inertia.
An easy and faster way to find the inertia is to calculate the Inertia of a 6x10 rectangle and substract the inertia of the(2)2.5x8 empty rectangles. In that way all the elements centroids have the same distance to the top axis and the calculation is easier.

I hope that helps.
I highly recommend the Thaddeus Seminar for this kind of stuff.
Last edited by lali on Wed Feb 14, 2018 8:57 pm, edited 1 time in total.
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Re: Section Modulus - NCARB Sample MC

Postby annascherman » Wed Feb 14, 2018 8:54 pm

Thanks Lali. But even when I used b(d^2)/6, separating it into 3 rectangles and subtracting out the missing pieces, I still get a different answer?
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Re: Section Modulus - NCARB Sample MC

Postby lali » Wed Feb 14, 2018 9:00 pm

I am sorry I just edited my first reply.
You are right... it will not work because you cannot add Section modulus. The only way to find the S for the beam is using S=I/c
Please see my revised answer

Lali
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Last edited by lali on Wed Feb 14, 2018 9:15 pm, edited 1 time in total.
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Re: Section Modulus - NCARB Sample MC

Postby annascherman » Wed Feb 14, 2018 9:02 pm

Ok. Thank you!
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