Some More Equation Confusion

Structural Layout Vignette and Multiple Choice

Some More Equation Confusion

Postby benlink » Sun Feb 14, 2016 10:32 am

I took the Thaddeus course a few years ago but, long story short, never took the SS test: SO I have a lot of study material without the virtue of his instruction.

In the Reference folder (if any of you also have the material) on page R-15, the Section Modulus and Moment of Inertia for a 2x8 (b=1.5 d=7.25) is given.

S=1.5x7.25^2/6 = 21.39in^3 <-----what Thaddeus gets 13.14 in^3 <---------what I get.

We agree for the beam rotated = 2.72in^3

Moment of Inertia:
I= 1.5x7.25^3/12 = 98.93 in^4 <------what Thaddeus gets 47.63 in^4 <------what I get.

Maybe a fluke. Except on the next page he does the same for a simplified wide flange:

overall depth = 10.25"
overall width = 7.25"
chords and web are all 1.5" wide
leaving a 7.25" deep x 2.875" wide negative space between top and bottom chords

I = I whole - I holes (negative space)
I = 7.25x10.25^3/12 - 2(2.875*7.25^3/12) = 635.40in^4 <-----what Thaddeus gets 468.02in^4 <--------what I get.

I/C is used to get the Section Modulus, which will obviously come out the same if you use his MoI.

What am I doing wrong here?

Edit: I just remembered that, for the 2x8, the Moment of Inertia and Section Modulus should be well documented and easily found -- so I Googled it. Turns out I'm right. Thaddeus's solutions are wrong, I reckon. Let me know if you discover something I'm not seeing. I'm going to assume the same for the wide flange above.
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Re: Some More Equation Confusion

Postby gumption » Mon Feb 15, 2016 12:47 am

Why not ask Thaddeus if you could get access to his seminar again? Doesn't hurt to ask.
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Re: Some More Equation Confusion

Postby Bamla » Mon Feb 15, 2016 11:20 am

benlink wrote:I took the Thaddeus course a few years ago but, long story short, never took the SS test: SO I have a lot of study material without the virtue of his instruction.

In the Reference folder (if any of you also have the material) on page R-15, the Section Modulus and Moment of Inertia for a 2x8 (b=1.5 d=7.25) is given.

S=1.5x7.25^2/6 = 21.39in^3 <-----what Thaddeus gets 13.14 in^3 <---------what I get.
it seems that he is using nominal size.

We agree for the beam rotated = 2.72in^3

Moment of Inertia:
I= 1.5x7.25^3/12 = 98.93 in^4 <------what Thaddeus gets 47.63 in^4 <------what I get.
I don't know where the 98.93 comes from, because if nominal size is used I got 85.33in^4, unless I am doing something wrong
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Re: Some More Equation Confusion

Postby benlink » Mon Feb 15, 2016 12:02 pm

The numbers I used in the equation are the same that he used. Our answers were the only place we deviated, but I see what you mean. It works out for the Section Modulus but not for the Moment of Inertia. Even the pros make mistakes.

Thanks.
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Re: Some More Equation Confusion

Postby Coach » Mon Feb 15, 2016 2:35 pm

benlink wrote:Thaddeus's solutions are wrong, I reckon.

Yes, his answers are for a 2x10 rather than 2x8.
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Re: Some More Equation Confusion

Postby benlink » Mon Feb 15, 2016 7:01 pm

10-4. Thanks, Coach.

gumption,
Indeed, Thaddeus is a kind gentleman and his lectures are enlightening to say the least.
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Re: Some More Equation Confusion

Postby ataribeats » Mon Feb 12, 2018 8:39 pm

Ha! I was going through the references, doing the numbers myself and I'm glad I noticed the mistake and that I found this post. Makes me feel a bit better within the overall studies tragedy lol
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Re: Some More Equation Confusion

Postby lali » Mon Feb 12, 2018 9:42 pm

you are correct and he already corrected it. I have the latest document from a month ago.
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Re: Some More Equation Confusion

Postby ataribeats » Tue Feb 13, 2018 10:09 am

What? I got my vids 3 weeks ago and the reference I have shows the error :cry:
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Re: Some More Equation Confusion

Postby lali » Tue Feb 13, 2018 11:11 am

Are these the formulas we are talking about?
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Re: Some More Equation Confusion

Postby ataribeats » Tue Feb 13, 2018 7:09 pm

lali wrote:Are these the formulas we are talking about?


Those are ok! It is the page after.
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