Calculate the angle of the neutral axis from the x-axis

  • Thread starter vtaela
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  • #1
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Homework Statement


Calculate the angle of the neutral axis from the x-axis when the beam is loaded with a positive couple of 6kNm acting around the X-axis



Homework Equations


i have calculated:
Ixx: 2854758.6 mm^4
Iyy: 1429034.6 mm^4
Ixy: -1143476 mm^4


The Attempt at a Solution



i know that to calculate the angle:

tanθ= - (IxxMy -IxyMx)/(IyyMx-IxyMy)

since i am confused with the statement "positive couple of 6kNm acting around the X-axis"

does that mean that Mx= 6KNm?

thank you
 

Answers and Replies

  • #2
SteamKing
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Ya got any pictures of this problem?
 
  • #3
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ImageUploadedByPhysics Forums1363452818.241318.jpg


This is the the question
 
  • #4
SteamKing
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Mx is a couple which is acting as a bending moment on the cross section.
 
  • #5
nvn
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Does that mean Mx = 6KNm?
That is correct.

(1) By the way, always leave a space between a numeric value and its following unit symbol. E.g., 6 kN, not 6kN. See the international standard for writing units (ISO 31-0).

(2) The unit symbol for kilonewton is spelled kN, not KN. Capital K means kelvin.

(3) Two unit symbols multiplied together cannot be written together, and must be separated by, e.g., an asterisk or a space. E.g., kN*m or kN m, not kNm.
 
Last edited:
  • #6
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Thank u! I will have it in mind! In addition my ixy when i calculate it is positive and when i got the answer from solidworks i get a negative value! Which one is the correct?
 
  • #7
nvn
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vtaela: Although I have not tried it, it looks like Ixy perhaps should be positive. I do not know what SolidWorks is doing. Ensure your positive axes are pointing in the correct direction in SolidWorks.
 
  • #8
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Thank you very much
 
  • #9
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Homework Equations


i have calculated:
Ixx: 2854758.6 mm^4
Iyy: 1429034.6 mm^4
Ixy: -1143476 mm^4

I got this except the Ixy was plus 1143476. Is this what you got when you did it again?
 
  • #10
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Yes i calculates a positive ixy and a positive 38.66 degree
 
  • #11
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When calculating the maximum stress did you use the unsymmetric bending equation? If you did what did you use as your x and y co-ordinates to input in that equation?
 
  • #12
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X = -13.78 and y+= 38.11 getting a maximum stress of 136.7119
 
  • #13
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How did you get them? Did you draw it out or is there a way to calculate them?
 
  • #14
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Actually think i understand it.

X = Xc - bredth of rectangle 1
Y = height of rectangle 1 - Yc

That how you got them?
 
  • #15
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Yes thats right
 
  • #16
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Are you sure about your final stress value? I get 151.9855 N/mm^2 every time.

I've tried using hand calculations as well as using excel to double check the values and get that everytime.
 
  • #17
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Take a picture of what you have done to compare the solution! I will reply tmr since i am not home now
 
  • #18
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Heres my solution.
 

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  • #19
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Same procedure except i used a positive Mx
 
  • #20
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Any idea how to do question 1 of the coursework?:p
 
  • #21
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Can you stick a pic of your solution up please. I completely missed the positive couple part in the question so can see where the positive mx comes from but now i now get -117.879 - 34.1061 = -151.99! Absolute nightmare haha.

Haha no idea, think everyone is stumped with that one.
 
  • #23
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Cheers, i think you have your x and the y mixed up at the end. You have -3.09x + 2.475y when it should be -3.09y + 2.475x according to the equation?
 
  • #24
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Yes you are right thank you for that
 
  • #25
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No problem
 

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