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Moment in beam (caused by influence line)

  1. Jan 17, 2017 #1
    1. The problem statement, all variables and given/known data
    http://cereference.com/book/structural-analysis/influence-lines-beams [Broken]
    In the example , i dont understand the moment in the part of beyond AC . I dont understand why we dont have to consider the momenty due to unit virtual load in the part beyond AC , just like what we did for section AC
    2. Relevant equations


    3. The attempt at a solution

    Why shouldnt the moment equation beyond AC look same as the region within AC ? , which is Mc = -ax/L + x
     

    Attached Files:

    • 538.PNG
      538.PNG
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    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Jan 17, 2017 #2
    What the author did is evaluate the Moment when the virtual load is on segment CB. To do so, you can analyse segment AC or CB, and note in segment AC, it won't have the unit load.

    If you perform the same analysis on segment BC , you will have to consider the unit load, take into consideration the different member lengths for moment calculations - but ultimately, you will get the same answer.
     
  4. Jan 17, 2017 #3
    Do you mean the author is evaluating the moment when the virtual load is in segment CB , so in the segment AC , it wouldnt have to include unit load ?

    It seems contrary to what the author did , in photo 538 i uploaded earlier , I found that when it's within segment AC , the author still consider the unit load , while in CB ( segment beyond AC ) , the unity load is not considered ...
     
  5. Jan 17, 2017 #4
    How do we know we need to apply the unit load within AC or CB ?
    btw , the author asked for find the influence line for the moment at C ...
     
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