1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Principal Stress and Maximum Shear Stress

  1. Mar 27, 2013 #1
    1. The problem statement, all variables and given/known data

    Hi all,

    For my CW I have a question on a simple beam, ABCD and its cross-section. Please see attachment for figures

    The material of the beam is steel, where modulus of elasticity, E = 210 GN/m^2

    I have been asked to calculate the principle stresses and the maximum shear stress at the top of the beam for the loaded system and 2 m from the left of the beam.

    2. Relevant equations

    x-bar = (A1x1 + A2x2 + A3x3)/(A1 + A2 + A3)

    y-bar = (A1y1 + A2y2 + A3y3)/(A1 + A2 + A3)

    Ixx = bd^3/12 + Ah^2

    τ = (VAy-bar)/bI

    M/I = σ/y = E/R

    3. The attempt at a solution

    I have worked out the roller support (A) = 94166.67 N and hinge support (D) = 90833.33 N

    Next I drew a shear force diagram and a bending moment diagram. From the diagrams, max shear force = 94.17 kN and max bending = 243.33 kNm

    Following on from this I concentrated on the z-bar cross-section. I divided the section into three sub-sections and calculated the area of each part and their centroids. From this I calculated x-bar and y-bar and started to calculate Ixx using Ixx = bd^3/12 + Ah^2

    I want to ask whether I am on the right lines because I don't know how to carry on from here.

    Attached Files:

    Last edited: Mar 27, 2013
  2. jcsd
  3. Mar 27, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    The next step would be to start calculating bending and shear stresses at the requested locations.

    Once you have calculated those stresses, you may use Mohr's circle to calculate the principal stresses.
  4. Mar 27, 2013 #3
    Thanks for your reply SteamKing

    Do you mean σx, σy and τxy? If so, how do I do this?
  5. Mar 27, 2013 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    You've put down the formulas. Don't you know how to use them?
  6. Mar 27, 2013 #5
    I think I understand how to get τ but I'm not so sure about σx and σy
    The equation is σ = (M/I)×y right? Is this σx or σy?

    Also I'm not sure if the method I used to calculate Ixx was correct. Is it as simple as splitting it into three smaller sections because the examples we have covered in class involved integration.
  7. Mar 31, 2013 #6
    Bump. Can anyone help please?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted