Calculating Shear Flow in a Hollow Beam: Understanding Mistakes in Dimensioning

[fonseh]

Homework Statement

for shear flow at D , the Q = Ay , i tried another method , i gt 30000 , instead of 29750, why the ans is different from 29750? why can't i use this method( i break the shaded area into 3 parts ( as shown in figure)

Homework Equations

The Attempt at a Solution

Q = Ay = (60x10x35) + (2x30x10x150 = 30000[/B]
http://imgur.com/a/VXDK6

 

[PhanthomJay]

The author is wrong the Q is 30 in^3 I don't know why you added those zeroes to get 30000

 

[fonseh]

The author is wrong the Q is 30 in^3 I don't know why you added those zeroes to get 30000

http://imgur.com/a/VXDK6
here' my working , Q = Ay = (60x10x35) + (2x30x10x150) = 30000 , or the author's working 29750 is correct ? which cut is correct ? My cut of section into 3 parts is different from the author's working
, sorry , i posted the wrong notes...here it is

 

[PhanthomJay]

Ok then you are correct, good work..

 

[fonseh]

Ok then you are correct, good work..

do you mean the author can't cut the beam like this ? why ?

 

[PhanthomJay]

The author cut the beam at the neutral axis to determine Q about the neutral axis, just like you did. But the author messed it up and didn't calculate the areas correctly, what more can I say you are right and they are wrong. Let it be!

 

[fonseh]

The author cut the beam at the neutral axis to determine Q about the neutral axis, just like you did. But the author messed it up and didn't calculate the areas correctly, what more can I say you are right and they are wrong. Let it be!

no , i am concern about how should the beam be cut ? i divided the beam into 3 parts like this(red) , whereas the author divided the beam into 3 parts like this (green), i gt 30000 , but the author gt 29750 , there's a difference of 250 , which is correct ?
i checked thru the author's my working using calculator , i gt 29750 based on his working

 

[PhanthomJay]

come on now I keep telling you the author made a mistake !

He/she did not use those green areas...they dimensioned incorrectly. The numbers come out to 27500, but the numbers inputted are wrong. The correct ans is 30,000 mm^3.

 

[fonseh]

come on now I keep telling you the author made a mistake !

He/she did not use those green areas...they dimensioned incorrectly. The numbers come out to 27500, but the numbers inputted are wrong. The correct ans is 30,000 mm^3.

sorry , i messed up QC and QD , question solved