Blodgett 2.10.9 Problem 5

[WS6]

Hey all, first time poster, long time reader here. I was taking a spin back through Blodgett's Design of Welded Structures recently and was reviewing the section on torsion. Problem 5 lost me on 2.10-10 calculating the total shear stress on the built-up section, hoping one of you can help clarify/confirm since there doesn't seem to be any errata published. Maybe it's just too early and I haven't had my morning coffee yet.

Starting with unit shear force along the N.A., the first moment of area for the BU box webs is calculated as 1in² x 2in, but seems like it should instead be 4in² x 2in, resulting in fb=1060 lbs/in. This would give a total unit shear force of fs=700 + 1060 = 1760 lbs/in and a total shear stress of Tau=1760/0.5 or 3520 psi compared to Blodgett's value of 4100 psi.

What am I missing here? Even if Blodgett's numbers are correct, I'm not clear where he jumps from a total unit shear force of fs=1560 lbs/in to suddenly using fs=2050 in calculating total shear stress. Any clarity would be greatly appreciated.

 

[SE2607]

You don't mention which edition you are using. I have the eighth printing, dated July, 1976. Here is the problem from that edition.

 

[WS6]

Thanks for the picture SE, I have the same edition. My first point of confusion is the bottom left of the picture, in the calculation of fb, where I believe the value of a2 should be 4in² based on figure 15 from the previous page.

My second point of confusion is in the subsequent calculations, where fs is calculated to be 1560 lbs/in. This value then changes unexplainably in the calculation of total shear stress (upper right of your picture) to 2050 lbs/in. I'm hoping someone can help me either make sense of that disconnect or confirm my numbers in the original post are correct.

 

[SE2607]

Regarding a2, I agree.

I don't know where the 2050 comes from. Sorry. It would seem to me that t = 1601/0.5 = 3202 psi.

 

[bridgebuster]

I was going to annotate my copy and noted that the same question came up in June

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Another correction from 2005

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