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!

Finding the right beam

  1. May 3, 2010 #1
    1. The problem statement, all variables and given/known data
    A simple beam of length L = 5m carries a uniform load of intensity q = 5.8kN/m and a concentratexd load 22.5kN. Assuming stress allow = 110MPa, calculate the required section modulus S. Then select a 200mm wide flange beam (W shape) from the table. recalculate S taking into account the weight of the beam. select a new 200mm beam if neccessary.

    2. Relevant equations

    3. The attempt at a solution

    Of first I was trying to calculate the Mmax and i made a boo boo somwhere.

    First I found the Moment about A

    0 = 5.8(5)(2.5) + 22.5(3.5) - 5RB.... RB = 30.25kN

    About B

    0 = 22.5(1.5) + 5.8(5)(2.5) - 5RA.... RA = 21.25kN

    Then so determine the Maximum Moment I was preparing the Shear & moment graphs. I took x starting from the left where x = 0 @ A


    Force sum in y = 0 = RA - qx - V.... V = 21.25 - 5.8x
    Sum of M = 0 = RAx - qx2/2 - M... M = 21.25x - 2.9x2


    Force in y = 0 = RA - qx - P -V.... V = -1.25 - 5.8x
    Sum of M = 0 = RAx - qx2/2 - 22.5(x - 3.5) - M... M = 78.75 - 1.25x - 2.9x2

    when I graph the V's, the lines do not intersect... did i do something wrong. Would it even matter as long as I did the M's correctly. Could someone just double check my work.

    Attached Files:

    • 1.pdf
      File size:
      13.2 KB
  2. jcsd
  3. May 3, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Your equations look OK. Max moment occurs at point of zero shear. Draw a shear diagram. There is an abrupt change in shear at the applied concentrated load. If you plot your 2 shear equations, they won't intersect at a common point because of the concentrated load discontinuity.
  4. May 3, 2010 #3
    ok i found V = 0 @ x = 3.664

    So pluged this into the second M equation and got Mmax = 35.7kN*m

    S = M/[tex]\sigma[/tex] = 3.25X10^-4 what happened here
  5. May 3, 2010 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Your numbers are off a bit...point of zero shear ocurs at the concentratd load at x =3.5 m. M_max at that point is about 40 kN-m. Then when you calculate S, the result is in m^3. (i don't work in SI units, so i don't have a feel for the numbers)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Finding the right beam
  1. Find reaction to beam (Replies: 14)