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: Finding stress in three rods

  1. Feb 16, 2007 #1
    1. The problem statement, all variables and given/known data

    See attached - not the yellow engineering paper.

    2. Relevant equations

    PL/AE = deformation

    3. The attempt at a solution

    See attached

    I can't figure out what the heck I'm doing wrong - my work is a little scattered because I was kind of thinking while doing it...

    Attached Files:

  2. jcsd
  3. Feb 16, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Looks like the outside columns equally support the portion of the load up to the point where their deflections equal 0.3 mm. Calculate what that load is for that deflection, and the stress in the outside columns (no stress or deflection in the interior column) under that load. Then the remainder of the load will be divided equally amongst the 3 columns.
  4. Feb 16, 2007 #3


    User Avatar
    Science Advisor
    Homework Helper

    Another way of doing it is to let the displacement of the top cap be x downwards.

    Then assuming all the columns take some load, the strain in the end posts is x/125 and the strain in the middle post is (x-0.3)/125. Find the force in each column corresponding to the strain, and get an equation for x by saying the forces in the 3 columns sum to the total load.

    If x comes out as less than 0.3, then the middle column did not take any load, in which case finding the load in the two end columns is easy!
  5. Feb 16, 2007 #4
    I did it the way PhantomJay said and it worked like a charm! Thanks you guys!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook