NCEES Practice Exam Problem 528 (2017 3rd Print)

Can anyone explain the reason of using "4-(1/2)(3/8)" for e in the solution?  Where did the "1/2" come from? 

I doubt the leg will fail at "(1/2)(3/8)".

Thank you!

1/2 takes your moment arm to the center of the bottom of the 3/8" thick angle rather than to either extreme of that leg.

scottp62 said:

1/2 takes your moment arm to the center of the bottom of the 3/8" thick angle rather than to either extreme of that leg.

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Well and why from 1/2? One could argue the long leg would start bending/yielding after the 3/8” thickness and our moment arm should be 4”-.375”, no? After all, our bolt nut is above our short angle leg thickness. And thats our fastener to the bolt. 

In the steel manual, what is the location of the centroid along the axis in question? 

tj_PE said:

In the steel manual, what is the location of the centroid along the axis in question? 

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For L6x4, ybar = 1.93” 

I just guess the center of plastic hinge will be at the 1/2 height of the short leg. Anyone could confirm or explain further?

Nirvana said:

I just guess the center of plastic hinge will be at the 1/2 height of the short leg. Anyone could confirm or explain further?

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Speaking with my Structural Professor from long ago...  , he says it is simply a compromise between using the top of the bottom leg or the bottom surface dimension. He explained this is a problem testing engineering intuition as there is really no set got to reference for it. That being said, if you use the entire length of the vertical leg then you get 1.88 (LRFD), and if you subtract the entire bottom leg 4-.375, you get 1.71. So I guess our engineering intuition needs to be spot on for this since 1.7 is a selection of answers.